8900.1 CHG 225

VOLUME 4  Aircraft equipment and operational authorizations

CHAPTER 2  All weather terminal area operations

Section 1 Introduction

4-146    ORGANIZATION AND OVERVIEW OF CHAPTER 2. The description of All Weather Terminal Area Operations (AWTA) and the approval process are divided into 11 sections:

    Section 1 introduces AWTA, providing an overview of concepts and evolution of AWTA, as well as various factors affecting AWTA. This section is historical and conceptual information only. For specific authorizations and requirements, see the following sections;

    Section 2 provides the five-step process for operator approval to conduct Category (CAT) II/III operations (except small Category A aircraft);

    Section 3 provides an approval process for conducting CAT II operations in small Category A aircraft under Title 14 of the Code of Federal Regulations (14 CFR) part 91;

    Section 4 provides guidance for Surface Movement Guidance and Control Systems (SMGCS), including operator requirements and airport authorization by All Weather Operations Specialists (AWOS);

    Sections 5–7 provide respective overviews of CAT I, CAT II, and CAT III operational requirements;

    Section 8 provides specific approval guidance for 14 CFR part 129 operators;

    Section 9 provides specific approval guidance for special instrument approach procedures (IAP);

    Section 10 provides specific approval guidance for maintenance and inspection programs for low‑visibility approach landing minimums; and

    Section 11 provides an introduction to performance-based operations.

4-147    GENERAL BACKGROUND. AWTA include all terminal area operations conducted under instrument flight rules (IFR), including certain operations conducted in visual conditions. Terminal area operations conducted under visual flight rules (VFR) in visual weather conditions are not addressed in this chapter. This chapter discusses concepts, national direction, and guidance to be used by Federal Aviation Administration (FAA) inspectors when evaluating, approving, or denying requests for authorization to conduct AWTA operations. This chapter also covers operational approvals for an operator proposing to use new aircraft, AWTA operating systems, lower-than-standard takeoff minimums, and approach and landing operating minimums. The basic principle for AWTA takeoff, approach, and landing operations is that operating minimums are permitted to be reduced through improvements in operational capabilities. This principle is valid only if an acceptable alternative maneuver is maintained or if an extremely high probability of safely completing the maneuver exists. All IAPs are constructed to permit safe instrument flight to the missed approach point (MAP), followed by an instrument missed approach. The safety of conducting an instrument approach to a published minimum and executing the missed approach is not dependent on establishing visual reference with the landing surface. The criteria for constructing an instrument approach are based on the premise that an instrument missed approach will be necessary under certain circumstances. Visual reference with the landing surface, however, becomes a safety factor when the flight descends below the published IFR minimum height or altitude. The visibility or Runway Visual Range (RVR) minimum for a particular runway becomes a safety consideration in both fuel planning and selection of alternate airports.

A.    AWTA. AWTA in domestic and international operations are complex, with many variations in aircraft and ground equipment, as well as procedures and standards. FAA inspectors must evaluate proposed AWTA, giving due consideration to the following:

    Operator’s type of operation (14 CFR part 91, 91 subpart F, 91 subpart K (91K), 121, 129, and 135—fixed-wing or helicopter);

    Type of proposed AWTA (takeoff, landing, etc.);

    Type of operator’s aircraft and equipage;

    Airports proposed for use;

    Operating minimums proposed; and

    Operator’s experience, both in similar or other aircraft, and in the type of operation proposed.

B.    Specific Standards. Specific standards are provided in this chapter to evaluate operations using aircraft and equipment that have well-understood operational characteristics and limitations in specific AWTA. When an operator requests approval to conduct operations not covered by these standards, or when an operator requests to use lower operating minimums than provided by these standards, the request must be forwarded to the regional Flight Standards division (RFSD) Next Generation (NextGen) Branch (AXX-220). AXX-220 will coordinate with the Flight Technologies and Procedures Division (AFS-400) to develop any additional necessary AWTA operational concepts.

C.    Authority and Responsibility for Approval of AWTA.

1)    The complex nature of AWTA in domestic and international environments, the wide variation of airborne and ground-based equipment, and the variation in procedures and standards used in these operations, require a broad-based evaluation and approval process. Due to operational and technical complexities, it is essential for this evaluation and approval process to use a systems approach (big picture approach).
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2)    This systems approach must involve many personnel who are knowledgeable in their respective areas. When the safety of a proposed operation is being evaluated, personnel knowledgeable in such areas as aircraft certification, instrument landing system (ILS) ground equipment design and maintenance, visual aid concepts and criteria, IAP design criteria, airport design criteria, flight inspection, air traffic control (ATC) procedures, flight operational programs, and aircraft maintenance programs must be involved.
3)    This broad-based systems approach process is particularly important in the evaluation and approval of CAT II and CAT III approach and landing operations. Although approval of CAT I operations is relatively straightforward due to the high level of CAT I operational experience and international standardization, CAT II and CAT III operations must be examined and approved on a runway-by-runway and an operator‑by‑operator basis.

4-148    EVOLUTION OF AWTA. In the early years of aviation, all flight operations were conducted in visual flight conditions. During those early years, electronic ground-based Navigational Aids (NAVAID) were not available and cockpit instrumentation could not support flight in instrument meteorological conditions (IMC). The capability of AWTA slowly evolved as flight instrumentation, airborne navigation equipment, and ground‑based electronic NAVAIDs were developed and improved. The development of the gyroscope established the foundation for instrument flight. The essential information provided by the gyroscope permitted pilots to safely control aircraft during instrument flight conditions. Operating minimums were gradually reduced as overall capability for instrument flight improved. The introduction of turbojets for commercial service in 1958 provided the stimulus for further and more rapid refinement of equipment, operating procedures, and standards. For the first 3 years, turbojet operating minimums for approaches with vertical guidance (called precision approaches) were specified as a ceiling of 300 feet and visibility of statute mile (sm). These early minimums were modified to a decision height (DH) of 200 feet and a visibility of sm (RVR 4000), and became known as the “basic turbojet minimums.” Included as part of the initial concept of operating minimums was an increase in the operating minimums for air carrier pilots in command (PIC) until 100 hours of flight experience in a particular type of aircraft was obtained. This was determined by adding 100 feet to the published ceiling and  sm to the published visibility for each approach. This aspect of the concept of operating minimums is still in use today. The high-minimum PIC requirement is currently specified in parts 91K, 121, and 135 (with RVR landing minimum equivalents in the operations specification (OpSpec)).

4-149    BASIC TYPES OF AWTA APPROACH AND LANDING OPERATIONS. There are two general classes of approach and landing operations: those conducted under VFR, and those conducted under IFR. There are three basic types of IFR approach and landing operations: visual approaches, contact approaches, and instrument approaches.

A.    Visual Approaches. A visual approach can be authorized by ATC if the aircraft is being operated under IFR in visual meteorological condition (VMC) (reported weather at airport must have ceiling at or above 1000 feet and visibility of 3 miles or greater). Although a pilot conducting a visual approach is expected to proceed to the destination airport by pilotage or visual reference to another aircraft, the flight remains under an instrument flight plan. ATC retains responsibility for both traffic separation and wake/vortex separation, unless the pilot reports the preceding aircraft in sight and is instructed to follow it. ATC will provide flight-following and traffic information until the aircraft is instructed to contact the control tower. Either ATC or the pilot may initiate a request for a visual approach.

NOTE:  Charted visual flight procedures (CVFP), a subset of visual approaches, are also considered to be visual approaches.

B.    Contact Approaches. A contact approach can only be authorized by ATC when requested by the pilot. The flight must be operated clear of clouds, the pilot must have at least 1 mile of flight visibility, and can reasonably expect to be able to continue to the airport in those conditions. The pilot must be on an IFR flight plan, and the ground visibility at the destination airport must be reported to be at least 1 sm. A contact approach is an approach procedure that may be used by a pilot (with prior ATC authorization) instead of a published Standard Instrument Approach Procedure (SIAP) or special IAP. ATC will not authorize a contact approach at an airport that does not have a functioning IAP. Although ATC provides separation services to a flight during a contact approach, the pilot must assume full responsibility for obstacle clearance and navigation to the destination airport.

C.    Instrument Approaches. IAPs are provided to permit descent in instrument conditions from the en route environment to a point where a safe landing can be made at a specific airport.

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1)    The types of SIAPs include the following approaches based on International Civil Aviation Organization (ICAO) standard NAVAIDs, such as an ILS, Global Positioning System (GPS), Very high frequency (VHF) Omnidirectional Range (VOR), and non-directional radio beacon (NDB). IAPs using these NAVAIDs may require, or may be supplemented by, use of distance measuring equipment (DME).
2)    In addition to NAVAID IAPs, there are also IAPs based on ATC radar services such as airport surveillance radar (ASR) and precision approach radar (PAR). SIAPs also include Performance-based Navigation (PBN) procedures that are developed in accordance with U.S. Terminal Instrument Procedures (TERPS) or ICAO Procedures for Air Navigation Services Aircraft Operations (PANS-OPS).
3)    Area Navigation (RNAV) and Required Navigation Performance (RNP) concepts are consistent with the performance characteristics of systems such as GPS, DME/DME/Inertial Reference Units (IRU), GPS/DME/DME/IRU, or flight management system (FMS)/GPS, or FMS/GPS/IRU.

D.    Lighting System Credits. All straight-in operating minimums are based on the use of ground-based visual aids to enhance seeing-conditions during the final stages of approach and landing operations (deceleration for helicopters). These reductions are known as lighting system credits and cannot be used to reduce operating minimums for circling maneuvers due to the large area required for safe maneuvering (turn radius) at the various speeds used. Therefore, operating minimum reductions based on lighting credits can only be authorized for instrument approaches to runways that provide a straight-in landing capability. The standard minimum IFR altitudes cannot be reduced due to obstacle limitations, NAVAID signal limitations, and/or navigation system limitations. As such, reductions in operating minimums below the basic values established for each type of approach are expressed only as reductions in the visibility/RVR required to safely conduct the approach. The minimums for the various navigation systems and lighting system combinations are specified in the current edition of Order 8260.3, United States Standard for Terminal Instrument Procedures (TERPS).


A.    Achieving Current CAT I Operating Minimums. Initial steps toward achieving current CAT I operating minimums began on September 28, 1961. These original developments became the foundation for the “building block” approach leading to further reductions in operating minimums. The first air carrier operator met these requirements on May 11, 1962, enabling itself to utilize the new reduced minimums of a 200-foot ceiling and -sm visibility (RVR 2600). The minimums reductions were based on meeting all of the following requirements:

1)    Ground-based NAVAIDs:

    A complete, operational ILS.

    A maximum glideslope (GS) angle of 3 degrees.

2)    Ground-based visual aids:

    High Intensity Runway Lights (HIRL).

    Full configuration approach lights with sequenced flashing lights.

    All-weather runway marking or runway centerline (RCL) lights.

3)    Airborne equipment:

    A flight director (FD) system or an automatic approach coupler (autopilot).

    An instrument failure warning system or cockpit procedures for assuring the immediate detection of instrument failures or malfunctions.

4)    PIC experience, training, and qualification:

    One hundred hours of experience as PIC in the particular type of turbojet or turbine-powered airplane.

    Raw data approach to 200 feet.

    FD and/or autopilot approach to 100 feet.

    ILS approach (FD and/or autopilot as appropriate) to 100 feet, followed by a landing.

    Engine-out ILS approach to a landing or missed approach.

5)    Additional runway field length and crosswind component limitations:

    Fifteen percent or 1,000 feet of additional field length (whichever is greater) over normal regulatory requirements.

    Maximum crosswind component of 10 knots.

B.    Specifying the Operating Minimums. A major change in the method of specifying the operating minimums for approaches with vertical guidance evolved with the introduction of the DH and RVR concepts. These changes were finalized by the publication of U.S. TERPS criteria in 1966. This conceptual change eliminated the ceiling requirement by introducing a DH. This conceptual change was necessary because of the limitations in the methods used to observe or measure ceiling and visibility. Often ceiling and visibility observations were taken several miles from the approach end of a runway, and as a result were frequently not representative of the seeing-conditions encountered during the final stages of an approach and landing, especially in rapidly changing or marginal weather conditions. Operational use of RVR reports began in 1955, but they were not available at most major airports until the early 1960s. Since 1989, all approach and landing operations using minimums below -sm visibility have been based on RVR reports.

C.    Reduced Operating Minimums. In 1963, operating minimums were reduced further to DH 200/RVR 1800 for two- and three‑engine airplanes (usually Category B or C) and DH 200/RVR 2000 for four‑engine airplanes (usually Category D). These reductions were based on the “building block” approach established in 1961 and the added requirement for enhanced in-runway lighting systems such as high-intensity touchdown zone (TDZ) and RCL lighting. In 1964, the minimums for runways not equipped with TDZ and RCL lights were reduced to DH 200/RVR 2400. Improvements in visual aids were and remain critical in reducing landing minimums. These aids provide pilots with the necessary external visual references for manually controlling and maneuvering the aircraft during the final approach, flare, landing, and taxiing. The requirement for improvements in the overall airborne and ground-based equipment capabilities, combined with a cautious incremental reduction in operating minimums, ensured that a high level of safety was maintained.

D.    Common CAT I Operating Minimums for Aircraft Categories A–D. In 1988, CAT I operating minimums for Category D airplanes were reduced to DH 200/RVR 1800. This change established common CAT I minimums for all airplanes. The 1988 reduction was based on more than 20 years of successful experience with Category B and Category C turbojet aircraft operating to DH 200/RVR 1800, as well as research and analysis. This research has shown that the handling characteristics and seeing-conditions in existing turbojet Category D airplanes were equivalent to other turbojets.

E.    Lowest CAT I Operating Visibility Minimums.

1)    In 2006, the lowest CAT I operating visibility minimums were revised to harmonize these minimums with the European Aviation Safety Agency (EASA). The majority of harmonized visibility minimums were based on a geometric calculation using the glidepath angle (or published vertical angle), height above threshold (HATh), and length of instrument approach lighting using the following formula: RVR = (HATh  tangent glidepath angle) - length of instrument approach lighting.
2)    Standard lengths for four categories of instrument approach lighting were based on the minimum lengths of lighting systems in each category. RVRs for 200 feet HATh were calculated using a glidepath angle of 3 degrees. RVR values were restricted to a minimum of 1800 to retain operationally proven minimum RVRs. Use of visibility minimums below RVR 2400 requires operative TDZ and RCL lighting or use of an approved Head-Up Display (HUD), FD (except single-pilot), or autopilot coupled approach to DH.
3)    In 2009, FAA Order 8400.13, Procedures for the Evaluation and Approval of Facilities for Special Authorization Category I Operations and All Category II and III Operations, provided the criteria for Special Authorization (SA) CAT I approaches with a DH as low as 150 feet (HATh using radio altimeter (RA) minimums) and a visibility minimum as low as RVR 1400 at runways with reduced lighting, provided an approved CAT II or CAT III HUD is used to DH.


A.    The Concepts and Criteria. The concepts and criteria established in the early 1960s were the building blocks for all CAT II and III operations. The initial criteria for CAT II operations were issued in October 1964. These criteria resulted in a requirement for further improvements in ground-based NAVAIDs, RVR reporting capabilities, airborne equipment, maintenance standards, and pilot training and qualification. Current CAT II criteria are essentially the same as those issued in 1964, except for enhancements to provide additional flexibility and operational credit for modern flight control systems.

B.    Guidance During CAT II Operations. During CAT II operations, greater reliance must be placed on the guidance provided by the ground-based NAVAIDs. Therefore, design and maintenance criteria for airborne and ground-based equipment must ensure that better performance and higher reliability are achieved by the total system. For example, before an airport can qualify for CAT II minimums, it must be equipped with an ILS that has greater signal quality, reliability, and integrity. It is also necessary for CAT II runways to have more than one RVR reporting system to provide more accurate information concerning seeing-conditions on the runway. A purpose of these requirements is to supplement the TDZ and RCL lighting required for operations below RVR 1800. Additional airborne equipment is also required, as follows:

    Dual ILS localizer and GS receivers.

    An autocoupler (autopilot) and an FD system, or two independent FD systems.

    Equipment to identify the DH (such as an RA).

    Rain removal equipment.

    Go-around guidance.

    An autothrottle system (for certain aircraft to reduce pilot workload).

C.    Initial CAT II Criteria. Initial CAT II criteria were established to provide flexibility to operators in choosing various combinations of airborne equipment to meet CAT II requirements. An operator had to prove (demonstrate) that the performance and reliability of their selected airborne system performed, and continued to perform, at the level of precision and reliability required for CAT II operations. The pilot training and qualification program, through enhanced ground and flight training, also had to provide the pilot proficiency required. This program had to address factors such as the availability and limitations of visual cues in the CAT II environment, as well as the procedures and techniques for transitioning from non-visual to visual flight at low altitude during landing.

D.    Type Design Approval Standards for CAT II. CAT II type design approval standards had not been established during initial CAT II operations. As a result, the following methods were established to achieve airborne equipment approval:

1)    Operational Demonstration. When the operator’s airborne equipment had not been certificated (type design approved) for CAT II operations, the operator was permitted to establish an extensive operational demonstration program. The purpose of this program was to show that the required levels of performance and reliability were attained and maintained. This program consisted of numerous approaches (approximately 300). The operator was also required to show that the methods for failure and/or malfunction detection were acceptable to the Administrator.
2)    Type Design Approval. When the operator could show that the airborne equipment had been previously tested and expressly approved for CAT II operations during FAA type certification (TC) or Supplemental Type Certification (STC), the operator was not required to conduct as extensive an operational demonstration before receiving initial CAT II approval.

E.    Demonstrating That All Initial Criteria Had Been Met. When an operator had demonstrated that all of the initial criteria had been met, initial operations to DH 150/RVR 1600 were authorized. This authorization was known as an “operational approval.” Operational approvals were accomplished by the issuance of standard OpSpecs. Following this initial operational approval, the operator was required to demonstrate the ability to maintain the required levels of reliability and performance on a continuing basis in CAT II line operations. After 6 months, assuming continued satisfactory maintenance and performance of the airborne systems, the operator was issued an operational approval to operate with minimums of DH 100/RVR 1200. These basic CAT II criteria for approval are still applicable today but the lowest authorized RVR minimum currently is 1000.

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F.    CAT II Operations Other than ILS. The only types of CAT II operations that can be currently authorized for use by U.S. operators are ILS-based operations or SA for certain CAT II operations at specifically approved facilities.


A.    Initial Step in Introducing CAT III Operations. In 1966 at an ICAO Communications/Operations (COM/OPS) divisional meeting, international CAT III ground and airborne equipment standards were established that were essential to further development of ground and airborne equipment and operating concepts.

B.    Initial U.S. CAT IIIa Criteria. The initial U.S. CAT IIIa criteria (refer to the current edition of Advisory Circular (AC) 120-28, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout) were issued on September 5, 1969, to assist industry in developing a CAT IIIa capability. These criteria were based on the CAT I and CAT II building blocks, and further improvements were required in ground-based NAVAIDs, RVR reporting capabilities, airborne equipment (such as a requirement for autoland), maintenance standards, and pilot training and qualification. These initial criteria did not include definitive operational approval requirements for ground support systems, maintenance, training, and operational procedures and limitations. However, the basic concepts and the minimum airborne equipment type design requirements considered necessary for CAT IIIa operations were clearly delineated in AC 120-28. These basic concepts and equipment requirements included the following:

    Alert height (AH) concept.

    Fail passive (FP) flight control system concept.

    Fail operational (FO) CAT IIIa system concept.

    Autoland concept.

    Dual radio (radar) altimeter requirements.

    Redundant flight control system requirements.

    Enhanced missed approach instrumentation.

    Autothrottle control system requirements.

    Enhanced failure detection and warning capability.

    Type design approval criteria.

NOTE:  “Fail operational (FO)” means an airborne system with redundant operational capability down to touchdown and, if applicable, through rollout. The redundant operational systems must have no common failure modes. If one of the required systems fails below AH, the flare, touchdown, and rollout, if applicable, can be accomplished using the remaining operational system or systems. “Fail passive (FP)” means an automatic flight control system (AFCS), which, upon occurrence of any single failure, should not cause significant displacement from the approach path or altitude loss below the nominal glidepath, or (upon disconnection) involve any significant out‑of‑trim condition. In addition, any single failure should not cause any action of the flight control system that is not readily apparent to the pilot. Refer to AC 120‑28.

C.    Initial CAT IIIa Approvals. The publication of initial CAT IIIa criteria (AC 120‑28) led to the rapid development of CAT IIIa airborne and ground-based capabilities. In February 1971, the B-747 was granted the first U.S. type design approval for CAT IIIa. This type design approval was based on the use of FO automatic landing systems. CAT IIIa criteria were significantly improved in December 1971, by the publication of AC 120‑28A. This revision enhanced the type design (airworthiness certification) approval criteria, and established initial operational approval criteria. Washington-Dulles Airport received the first U.S. CAT IIIa ILS facility approval in January 1972. The type design for the L-1011 was certificated for CAT IIIa using FO autoland systems in April 1972. The first U.S. CAT IIIa operational approval was issued to Trans World Airlines on September 15, 1972, for FO CAT IIIa operations using the L-1011. All initial CAT IIIa operations were restricted to Type III ILS-equipped runways and FO CAT IIIa airborne equipment.

D.    Type II ILS-Equipped Runways and FP Airborne Equipment. The criteria initially established for CAT IIIa (AC 120-28) were based on a conservative approach for reducing operating minimums. However, with additional operational experience, it was determined that the initial criteria were unnecessarily stringent.

1)    After a thorough review of the Type II ILS equipment, the FAA determined that some Type II installations could be upgraded with minor modification to support CAT IIIa operations. Furthermore, the operational experience of Air Inter in France during extensive CAT III operations (RVR 500) using FP autoland systems indicated that under tightly controlled conditions FP CAT III operations could be safely conducted. Research efforts in the United States and Europe also supported this conclusion.
2)    In October 1976, Notice N 8400.18, Job Function Reference Guide for Air Carrier Safety Inspectors (OPERATIONS), was issued to establish approval criteria for FP CAT IIIa autoland operations using DH 50/RVR 700. In December 1976, the B-727 became the first airplane certificated by the United States for FP CAT IIIa operations. AC 120-28B, issued in December 1977, permitted CAT IIIa operations at runways equipped with suitably modified Type II ILS equipment. It also permitted FP autoland operations with aircraft having handling characteristics, physical characteristics, and seeing-conditions equivalent to the B-727 and DC-9 airplanes.
3)    A Flight Standards Service (AFS) policy decision, expressed in a letter dated June 22, 1978, authorized CAT IIIa operations to 32 runways equipped with Type II ILS equipment at 31 airports. FAA Order 8400.8, Procedures for the Approval of Facilities for FAR Part 121 and Part 135 CAT III Operations, was initially issued on September 10, 1980, to enhance the criteria and procedures for approving CAT III operation using U.S. Type II ILS facilities. These changes significantly increased the number of facilities that could support CAT IIIa operations and the number of aircraft that could potentially use these facilities.
4)    As of 2010, all systems supporting CAT II or CAT III operations (Mark 20 systems) meet the integrity requirements of a Type III system. The lowest landing minimum currently (2010) authorized for CAT IIIa by U.S. operators at any airport is RVR 700. Consideration is being given to reducing the minimum RVR to 600 in order to harmonize U.S. CAT IIIa standards with ICAO.

E.    Initial CAT IIIb Criteria. As operational experience and capability of airborne equipment increased in CAT IIIa operations, the need for CAT IIIb criteria was gradually realized. Initial U.S. CAT IIIb criteria were issued in March 1984 (AC 120-28C). This revision permitted operations with minimums as low as RVR 300. The B-767 became the first aircraft certificated (type design approval) for CAT IIIb by the United States. The B-767 was approved under a final draft version of that AC. The initial CAT IIIb criteria were based on the CAT I, CAT II, and CAT IIIa building blocks.

1)    Further enhancements were required in the CAT IIIb criteria, particularly in ground-based NAVAIDs, lighting systems, RVR reporting systems, airborne equipment, and training and qualification programs. These revisions further clarified CAT III operational concepts, system requirements, and the visual references necessary for the various CAT III operations. Another conceptual change was implemented by establishing concepts for CAT III operations with the “pilot in the active control loop.” These new concepts permitted manually-flown CAT III operations using special flight guidance and control systems such as HUDs.
2)    The first U.S. CAT IIIb operational approvals were granted to Trans World Airlines (L-1011) and Eastern Airlines (L-1011 and A300) using minimums of RVR 600. RVR 600 was the lowest minimum supported by U.S. facilities due to RVR reporting system limitations. The first CAT IIIb RVR 300 minimum approvals were granted to Delta and Eastern Airlines in September 1984, for L-1011 aircraft. Initial RVR 300 approvals were restricted to those airports equipped with CAT III taxiway RCL lights and the capability to report RVRs as low as RVR 300. The first U.S. CAT IIIb RVR 300 ILS facility approval was granted for runway 16R at Seattle-Tacoma International Airport (SeaTac) in 1987.

4-153    CURRENT CATEGORIES OF IAPs. Various categories of instrument approach operations have been established to accommodate a wide variety of airborne and ground- or space-based capabilities. These operational categories are necessary for granting credit to operators choosing to install airborne equipment with additional capabilities. These operational categories also provide the distinction between operational capabilities and ground support system configurations. CAT I, CAT II, and CAT III are the three basic categories of instrument approach operations.

A.    CAT I Operations. CAT I operations are defined as precision approach and landing operations conducted under IFR using CAT I operating minimums. CAT I operating minimums consist of a specified IFR decision altitude (DA)/DH that is not lower than the equivalent of 200 feet (60 meters) above the TDZ, and a visibility, Runway Visibility Value (RVV), or an RVR that is not lower than sm or RVR 1800, respectively.

B.    SA CAT I. The current edition of FAA Order 8400.13 authorizes SA CAT I approaches to an RA DH as low as 150 feet and a visibility minimum as low as RVR 1400 to runways that do not have TDZ or RCL lighting when the approach is flown using an aircraft with a HUD to DH.

C.    Standard CAT II Operations. CAT II operations are approach and landing operations conducted with a DH of less than 200 feet (60 meters) but not less than 100 feet (30 meters), and an RVR of not less than 1,200 feet (350 meters).

D.    CAT II RVR 1000. Order 8400.13 authorizes CAT II approaches with a DH as low as 100 feet and visibility minimums of RVR 1000 to runways that meet all CAT II equipment, performance, and lighting requirements. The operator must use either autoland or HUD to touchdown.

E.    SA CAT II. Order 8400.13 authorizes CAT II approaches with a DH as low as 100 feet and visibility minimums of RVR 1200 at runways that do not meet all of the lighting requirements (Approach Lighting System With Sequenced Flashing Lights (ALSF)-2, TDZ, RCL lights) for standard CAT II. The operator must use either autoland or HUD to touchdown.

F.    CAT III Operations. CAT III operations are separated into three subcategories: CAT IIIa, CAT IIIb, and CAT IIIc.

1)    CAT IIIa Operations. CAT IIIa is an approach and landing operation with an RVR of not less than 700 feet (200 meters) without a DH, or with a DH of less than 100 feet (30 meters), or an AH that is typically between 50 and 200 feet, depending on aircraft certification and operator preferences. Both FP and FO airborne equipment can be used in CAT IIIa operations.
2)    CAT IIIb Operations. CAT IIIb is an approach and landing operation with an RVR of less than 700 feet (200 meters) but not less than 150 feet (50 meters) and a DH of 50 feet (15 meters) or less, or an AH which is typically between 50 and 200 feet, depending on aircraft certification and operator preferences. Both FP and FO airborne equipment can be used for CAT IIIb operations.
3)    CAT IIIc Operations. CAT IIIc is an approach and landing operation without a DH and without RVR limitations (zero-zero). CAT IIIc operations are currently not authorized.

4-154    IAPs.

A.    SIAP.

1)    SIAPs that are published in accordance with 14 CFR part 97 without Authorization Required (AR) or Special Aircrew and Aircraft Certification Required (SAACR) restrictions are approved for all users of the U.S. National Airspace System (NAS) and are incorporated in the standard OpSpecs by reference.
2)    If an IAP is published in part 97 and designated as an AR or SAACR procedure, it is only available to those operators, aircraft, and aircrews that meet the special qualification requirements for that procedure and that are approved to use it. An IAP is a series of predetermined maneuvers for the orderly and safe transfer of an aircraft under instrument flight conditions, from the beginning of the initial approach to one of the following:

    An automatic landing.

    A position from which a landing can be made visually.

    A position from which a missed approach can be executed and completed if external visual references necessary to complete the landing are not established before passing DA/DH or minimum descent altitude (MDA)/MAP.

B.    IAPs and Their Operating Minimums. An instrument approach and its operating minimums are usually prescribed and approved for a specific airport and/or runway by the aviation authority (AA) that has jurisdiction over flight operations at that airport. The FAA is responsible for developing all civil IAPs and for specifying the operating minimums for all IAPs in the United States and its territories, and the U.S. Army IAPs worldwide. In the case of other military IAPs, an instrument approach and its operating minimums are prescribed and approved for a specific airport and/or runway by the authority having jurisdiction over flight operations. There are various types of IAPs that are or may be approved for use by U.S. air carriers. These types of IAPs include the following:

    IAPs published in accordance with part 97.

    IAPs authorized in OpSpecs.

    FAA-approved special IAPs (FAA Form 8260-7, Special Instrument Approach Procedure).

    Department of Defense (DOD) IAPs at U.S. military airports.

    IAPs published by a foreign country.

    IAPs developed by an air carrier in a foreign country in accordance with the current edition of FAA Order 8260.31, Foreign Terminal Instrument Procedures (FTIP).

C.    CVFPs. Even though CVFPs are available for public use by aircraft on IFR flight plans, they are not standard Instrument Flight Procedures (IFP). Except for CVFPs, it may be assumed that any SIAP charted in a U.S. Government Flight Information Publication (FLIP) is appropriately published in part 97.

4-155    OTHER IAPs. If, however, an IAP and its operating minimums are not published in accordance with part 97, other means have been established to authorize their use. In such cases, the IAP is incorporated into standard OpSpecs by reference (either with or without additional restrictions). This group of instrument procedures that are not published in part 97 includes IAPs developed by the FAA, third party developers, certain U.S. military organizations, foreign governments, and air carriers, and IAPs based on nonstandard NAVAIDs such as Tactical Air Navigational Aid (TACAN), Tactical Landing Approach Radar (TALAR), airborne radar, or commercial broadcast stations. Many of these approach procedures are not available to all users due to the location, special training, knowledge, or equipment required to safely conduct them.

A.    U.S. Military IAPs. U.S. military IAPs are approved by the local base commander and published by the DOD. Since these procedures comply with U.S. TERPS criteria, U.S. military IAPs must be used by air carriers when operating at military airports, unless the procedure is noted “Not for Civil Use” by the military. IAPs published by the DOD for U.S. military airports are incorporated into the standard OpSpecs by reference.

B.    Foreign Government IAPs. At foreign airports, the authority having jurisdiction over flight operations at the airport establishes the IAPs and their operating minimums. In general, the IAPs and operating minimums (if specified) at most foreign airports are developed in accordance with U.S. TERPS or ICAO PANS‑OPS criteria. IAPs developed by foreign authorities using TERPS or PANS-OPS are approved for use by U.S. air carriers in accordance with FAA Order 8260.31 and are incorporated in the standard OpSpecs by reference. In some cases it may be necessary to restrict certain foreign IAPs to make them equivalent to U.S. or ICAO criteria. FAA Order 8260.31 provides direction and guidance for restricting such foreign IAPs. When a restriction to a foreign IAP is required, it must be specified in OpSpec C058.

C.    IAPs Developed by an Air Carrier. At some foreign airports, an air carrier may need to develop or choose to develop an IAP. The standard OpSpecs enable an air carrier to exercise this option, provided the developed procedure meets either U.S. TERPS or ICAO PANS-OPS criteria. In such cases, the IAP developed by the air carrier may be authorized for use by listing it in OpSpec C081, provided the air carrier submits appropriate supporting information in accordance with FAA Order 8260.31. These procedures may be based on either public or private NAVAIDs.

D.    Non-Federal NAVAIDs. Non-Federal NAVAIDs can be used for public and special IAPs. Approval for the use of these NAVAIDs within the NAS is established in the current edition of Order 6700.20, Non-Federal Navigational Aids and Air Traffic Control Facilities, and 14 CFR part 171. An inspector should become familiar with these documents before issuing approval to use these IAPs. Approval to use special IAPs based on non-Federal NAVAIDs is accomplished by listing them in OpSpec C081.

E.    Commercial Broadcast Station IAPs. In the past, limited authorizations to use commercial broadcast stations have been granted in unique situations. The need for these procedures has been steadily declining because of the increased availability of standard NAVAIDs. In general, new approach procedures based on commercial broadcast stations will not be approved. In any case, AFS-400 review and concurrence must be obtained before an inspector may approve an IAP based on commercial broadcast stations.

F.    Special IAPs. Special IAPs are those procedures evaluated and approved by the FAA but not published in accordance with part 97. These special IAPs are not approved for general use due to the special training, private facilities, procedures, knowledge, and/or equipment required to safely conduct them. Due to these special requirements, the use of special IAPs must be authorized on an operator-by-operator basis. Special IAPs are issued on FAA Form 8260-7 and authorized in OpSpec C081.

G.    IAPs Outside of Controlled Airspace. Since ATC separation services are an important element of safe instrument approach operations, special consideration and evaluation are required before operations can be authorized outside of controlled airspace (no ATC separation services available). This situation occurs when conducting an IAP at an airport that is in Class G airspace (i.e., does not have an operating control tower or when a control zone is not active). The airports, at which portions of IAPs are outside of controlled airspace, must be authorized by the standard OpSpec C064.

H.    Airborne Radar Approaches (ARA). ARAs are based on the use of airborne radar. Within the United States, ARAs are classified as special IAPs and are established by the issuance of FAA Form 8260-7. Use of ARAs can be authorized through standard OpSpecs if the criteria in the current edition of AC 90-80, Approval of Offshore Standard Approach Procedures, Airborne Radar Approaches, and Helicopter En Route Descent Areas, and this order are met.

I.    Offshore Standard Approach Procedures (OSAP). OSAPs are helicopter specials that are designed for use to offshore platforms. OSAPs are based on the use of GPS and the airborne radar systems and are established and approved in accordance with the criteria in AC 90-80. These special procedures are developed for individual operators and are issued and authorized through OpSpecs, management specifications (MSpecs), or letters of authorization (LOA).

4-156    CONSIDERATIONS FOR APPROACH AND LANDING OPERATIONS. U.S. TERPS contains the established minimum criteria for standard IAPs within the U.S. NAS. PANS-OPS, Volume II, contains the established minimum criteria for IAPs in most foreign countries. These criteria allow for safe instrument approach and landing capabilities for aircraft equipped with ICAO standard NAVAIDs (ILS, GPS, VOR, VOR/DME, and NDB) and performance-based approaches based on RNP concepts. Many operators have chosen to use airborne equipment exceeding the minimum capabilities required for instrument flight. A means of granting operational credit for using equipment with these increased capabilities has been established. The standard OpSpecs provide the method to approve approach and landing operations using such airborne equipment. Examples of airborne equipment with increased capabilities include automatic landing systems (autoland) and manually flown electronic landing systems (HUD), ARA systems, and RNAV systems with RNP and RNP AR capabilities. The following subparagraphs briefly discuss these systems.

A.    Autoland.

1)    Autoland Approach. An autoland approach is an instrument approach to touchdown, and in some cases, through the landing rollout. An autoland approach is performed by the aircraft autopilot, which is receiving position information and/or steering commands from onboard navigation equipment. Autoland approaches are flown in VFR and IFR. It is a commonly accepted safe operating practice for operators to require their aircrews to fly coupled approaches and autoland approaches (if certified) on suitable runways when the weather conditions are less than approximately RVR 4000.
2)    Automatic Landing Systems. As an example of modern airborne equipment, the autoland is often standard on many new airplanes. This modern system gives the aircrew increased capabilities by enabling them to make safer instrument approaches and landings than those being done without the autoland. Autoland also refers to the landing that is accomplished with the autoland engaged. The aircrew is required to constantly monitor this system to ensure safe operation of the aircraft.
3)    General Information. Many large transport category airplanes are equipped with autoland systems and a few helicopters are equipped with automatic deceleration and hover systems. As technology evolves, the trend of using autoland systems is increasing. Autoland systems are already standard features on many new airplanes. An air carrier, however, is not authorized to use autoland systems to touchdown in parts 121 and 135 operations unless the particular flight control guidance system is authorized for autoland by the OpSpecs. Part 121, 121.579 and part 135, 135.93 prohibit the use of most autopilots below certain heights (50 feet or greater) during approach and landing operations, even during VFR weather conditions. The intent of these rules is to provide pilots with the terrain or obstacle clearance and the reaction time necessary to safely intervene if the autopilot malfunctions.
4)    Pilot Intervention. This is especially critical if the autopilot abruptly commands a hard-over, nose-down condition. Many autopilots (“single channel” autopilots) used in parts 121 and 135 operations are not designed to provide the redundancy necessary to automatically detect all failure combinations. If such failures occur, the pilot must intervene, disconnect the autopilot, and recover manually. Since an aircraft will lose altitude if a hard-over, nose-down condition occurs, the autopilot must be routinely disengaged before descending below the height above terrain specified by 121.579 or 135.93, as appropriate. Failure to disconnect the autopilot before descending below these heights could lead to ground contact during a recovery attempt if a malfunction occurred. Many aircraft are now equipped, however, with an automatic flight control guidance system (AFCGS) designed to provide the performance, redundancy, and reliability necessary to detect all significant failure combinations and to prevent the autopilot from failing in a hard-over, nose-down condition (zero height loss). With these aircraft and equipment combinations, the safety objective of 121.579 and 135.93 can be met even if the system is used to touchdown. FP and FO automatic landing systems provide this capability and can be approved for use to touchdown. The operator’s approved training curriculum must include training on autoland operations and the autoland equipment must be properly certificated and maintained. Principal operations inspectors (POI) shall authorize the use of autoland to touchdown by issuing OpSpec C061 in accordance with  121.579(c) or 135.93(d).

B.    Manually Flown Flight Control Guidance Systems Certificated for Landing Operations (HUD). Historically, pilots have not had FD systems and other instrument information that enabled safe manual control of an aircraft to touchdown in instrument conditions. The development of flight control guidance systems such as HUD provides the pilot with instrument information in a manner that enables safe manual control of the aircraft through touchdown and rollout. The flight guidance provided by these systems enables a pilot to duplicate the performance and functions of an autoland system. These systems provide flight guidance information equivalent to the performance, redundancy, reliability, and the hard-over, nose-down protection provided by autoland systems, which are approved for use to touchdown. Manually flown flight control guidance systems certified for landing operations can be approved for use to touchdown. The operator’s approved training curriculums must include training on such manually flown operations, and the equipment must be properly certificated and maintained. Use of these manually flown systems to touchdown can be authorized by the issuance of OpSpec C062 in accordance with this order.


A.    Instrument Approach Design Criteria. In many situations, instrument approach design criteria will not permit a straight-in approach to the landing runway. In these situations, a circling procedure is necessary to maneuver the aircraft to a landing on the intended runway. Circling maneuvers are usually necessary when there is an obstacle or terrain problem. Circling maneuvers are also required when a NAVAID is located in a position that precludes a straight-in approach to the intended landing runway. U.S. criteria require a circling maneuver if the inbound course is offset more than 30 degrees from the RCL. Unless specifically restricted in the procedure, a circling maneuver can be initiated from any IAP and must be conducted entirely by external visual references. Electronic course or glidepath guidance cannot be used to perform a circling maneuver.

B.    The Circling Maneuver. A circling maneuver is not an instrument maneuver. Sufficient visual references for manually maneuvering the aircraft to a landing must be maintained throughout a circling maneuver. The pilot must keep the aircraft’s position within the established maneuvering area while performing the circling maneuver. The circling MDA must be maintained until an aircraft (using normal maneuvers) is in a position from which a normal descent (less than 1,000 feet per minute (fpm)) can be made to touchdown (decelerate to air taxi or hover for helicopters) within the TDZ. It is critical for pilots to understand that the published missed approach procedure may not provide adequate obstacle clearance, especially during the initial portion of a missed approach executed during a circling maneuver. The published missed approach is designed to provide obstacle clearance only when the missed approach is executed on the published final approach course at or above the MDA, and before passing the MAP. A published missed approach may not guarantee the necessary safety margin when a missed approach is executed past the MAP and/or below the MDA. The aircraft must remain within the established circling maneuvering area until the aircraft is at or above the MDA and established on the missed approach course. The following statements summarize the basic concepts of a circling maneuver:

    A circling maneuver is a visual maneuver.

    Sufficient visual references to manually maneuver the aircraft to a landing must be maintained throughout a circling maneuver.

    The aircraft must be maintained at the MDA until it is at a position from which a safe landing can be made.

    A missed approach must be executed when external visual references are lost or sufficient visual cues to manually maneuver the aircraft cannot be maintained.

C.    Missed Approach Procedure. The traditional published missed approach procedure does not guarantee obstacle clearance during the initial phases of a missed approach if initiated during a circling maneuver after descending below MDA or after MAP. When a pilot loses visual reference while circling to land, follow the missed approach specified for the approach procedure. An initial climbing turn toward the landing runway will ensure that the aircraft remains within the circling obstruction clearance area. Continue to turn until established on the missed approach course. An immediate climb must be initiated because obstacle clearance is not guaranteed beyond the MAP.

4-158    LOOK-SEE APPROACHES. A look-see approach is not an actual type of approach, such as ILS or RNAV (GPS). Rather, it is a term used to describe the operation of commencing and continuing an instrument approach to DA/DH or MDA to determine if the seeing-conditions actually available at those points are sufficient to continue to a landing. Look-see approaches are approaches that can be started and then continued to the DA/DH or the MDA and the MAP, even when the weather conditions are reported to be below the authorized IFR landing minimums. This operation applies domestically only to part 91 operators. This operation may be conducted in certain foreign countries by part 121 operators. Upon arrival at the MDA and before passing the MAP, or upon arrival at the DA/DH, the approach may be continued below DA/DH or MDA if the seeing‑conditions required by 121.651(c) or part 91, 91.175(c) and 91.175(1) are met. A pilot can continue to land using external visual reference if the necessary seeing-conditions are established before passing DA/DH or MDA/MAP. The operational need for look-see approaches is created by wide variations among foreign countries in weather observing and weather reporting practices, and by limitations associated with manually derived and forwarded weather reports (especially during rapidly changing weather conditions). The weather observation is often taken from a location that is several miles from the landing surface, and may not be representative of seeing‑conditions encountered at DA/DH, MDA/MAP, or during landing. Part 121 operators may conduct look-see approaches at foreign airports (civil and military) unless the foreign country specifically prohibits them. Part 121 operators, however, are prohibited from conducting look-see approaches at all U.S. airports including U.S. domestic, U.S. territorial, and U.S. military airports (including U.S. military airports in foreign countries). Part 135 operators are prohibited from conducting look-see approaches at all airports, both domestic and foreign, by 135.225.


A.    DA/DH Concept. The DA/DH concept is the foundation for CAT I and CAT II approach and landing operations. It is also an essential concept in certain CAT III operations. This concept evolved after the introduction of turbojets in 1958. It was established to resolve problems created by the use of a ceiling as an element of operating minimums, especially during rapidly changing weather conditions. The use of the DA/DH concept also enhances safety of operations in degraded seeing-conditions. A DA/DH is established to require that the pilot, at the specified height, decide whether adequate visual references are available for accomplishing the following actions:

    Verifying that the aircraft is in a position that will permit a safe landing in the TDZ.

    Determining that sufficient external visual references are available to manually maneuver the aircraft (or assess autopilot maneuvering in CAT II and CAT III operations) into alignment with the RCL.

    Determining that the aircraft can be maneuvered to touchdown within the TDZ, that directional control can be maintained on the runway, and that the aircraft can be stopped within the available runway length.

    For helicopter operations, determining that sufficient visual references are available to maneuver the helicopter to align with the landing area; to decelerate to air taxi or to hover; and to maintain directional control while air taxiing.

B.    Operational Viewpoint. From an operational viewpoint, DA/DH is the limit to which a pilot can descend before having to decide to continue the approach by visual means. If the visual references required to safely continue the approach have not been established before passing DA/DH, a missed approach must be executed at DA/DH. This does not mean that a pilot waits until arriving at DA/DH before deciding to go around or to continue the approach based on visual references.

1)    The decisionmaking process begins when the approach is initiated and continues throughout the approach. A pilot must continually evaluate course and glidepath displacement information throughout the approach. Knowing that significant changes cannot occur instantaneously, a pilot begins to formulate a decision concerning the probable success of the approach long before reaching DA/DH.
2)    Although DA/DH is a specified point in space (PinS) at which a pilot must make an operational decision, the pilot accumulates the information required to make that decision throughout the approach. It is incorrect to assume that all aspects of the decisionmaking process are delayed until the critical instant the aircraft arrives at DA/DH. The visual cues, which become available during the descent to DA/DH, enhance the pilot’s formulation of the decision, which must be made at DA/DH.
3)    The operational decision to continue the approach by visual means, however, must be made before passing DA/DH. At DA/DH, a decision to continue the approach by reference to visual cues is appropriate if a pilot is satisfied that the total pattern of the visual cues provides sufficient guidance and that the aircraft is in a position and tracking so as to remain within a position from which a safe landing can be made. However, if a pilot is not satisfied that all of these conditions exist, a missed approach must be executed.

C.    Before Passing DA/DH. The decision that the pilot must make before passing DA/DH is not a commitment to land. It is a decision to continue the approach based on visual cues. This distinction is important since the possibility exists that, after passing DA/DH, visual cues may become inadequate to safely complete the landing, or the aircraft may deviate from the flightpath to a point where a safe landing cannot be assured. Since many variables are involved, the final decision to commit to a landing is the PIC’s and is primarily a judgment based on all the relevant operational factors. The PIC shall usually delay the decision to commit to a landing until the final stages of flare and landing.

1)    The following is a list of statements that describe what DA/DH is.

    DA/DH is a specified decision point.

    DA/DH is the point at which a specific action must be initiated (either continue the approach by reference to visual aids or go-around).

    DA/DH is the limit to which a pilot can descend before having to decide to continue the approach using external visual references.

2)    The following is a list of statements that describe what DA/DH is not.

    DA/DH is not a point where the decisionmaking process begins.

    DA/DH is not the latest point at which a go-around could or should be made.

    DA/DH is not a point where all aspects of the decision are instantaneously formulated.

D.    Vertical Navigation (VNAV) Approach Procedures Using DA/DH—OpSpec C073. Based on near-term safety benefits of using a continuously defined Vertical Path (VPATH) to the runway, and a long-term goal of simplifying approach training and qualification standards, users have indicated their intent to begin additional use of VNAV capability for instrument approaches. The applicable procedures, operating criteria, and revisions to the operator’s OpSpecs, if applicable, to permit additional use of VNAV capability of FMS for IAPs are contained in Volume 3, Chapter 18, Section 5.

4-160    CONCEPT OF MDA AND MAP. The MDA/MAP concept is the foundation for safe CAT I approach operations that do not have VPATH guidance (e.g., VOR or lateral navigation (LNAV)). Electronic glidepath information cannot be provided at certain locations because of obstacle or terrain problems, NAVAID sighting problems, and cost benefit factors. The MDA/MAP concept provides for safe approach operations in instrument conditions at locations that do not have VPATH guidance.

A.    MDA. An MDA is the lowest permissible height (for a Nonprecision Approach (NPA) procedure) at which an aircraft can be controlled by reference only to instrument information. After passing the final approach fix (FAF), a pilot should descend on a VPATH that will enable a stabilized approach and, if the visual conditions are adequate, a descent to the runway without any intermediate level off at the MDA. If the visual conditions are not adequate, the pilot must level off at the MDA until sufficient visual references are available to safely complete the approach and landing. For unusual approach procedures and environmental conditions (offset final course, crosswinds, icing, etc.) a pilot may descend to the MDA at an expedited rate (not to exceed 1000 fpm).

B.    Establish an MDA. An MDA is established to require that the pilot, before descending below the specified height and before passing the MAP, determines that adequate visual references are available for accomplishing the following actions:

    Verifying that the aircraft is in a position that will permit a safe landing in the TDZ.

    Determining that sufficient visual references are available to manually maneuver the aircraft to align it with the RCL, touchdown within the TDZ, and maintain directional control on the runway.

    For helicopter operations, determining that sufficient visual references are available to maneuver the helicopter to align with the landing area, decelerate to air taxi or hover, and maintain directional control while air taxiing.

1)    The following is a list of statements that describe what MDA is.

    MDA is the lowest permissible height at which an approach can be continued by reference solely to flight instruments.

    MDA is the limit to which a pilot can descend before having to decide whether or not to continue the approach by using external visual references.

    MDA is the minimum height above the surface to which the aircraft can descend, unless the pilot determines that the aircraft is in a position from which it can be safely maneuvered using normal rates of descent (less than 1,000 fpm) to a touchdown within the TDZ (decelerate to air taxi or hover for helicopters).

1)    The following is a list of statements that describe what MDA is not.

    MDA is not a specified decision point.

    MDA is not a point at which a specific action is initiated.

    MDA is not a point where the decision process begins.

    MDA is not the latest point at which a go-around could or should be made.

    MDA is not a point where all aspects of the decision are instantaneously formulated.

C.    MAP. For an approach that does not have vertical guidance, it is necessary to define a point on or near the airport where a missed approach must be executed, if adequate external visual references for safely continuing the approach are not available. This point is specified as the MAP. A MAP is a three-dimensional (3-D) airborne position where the MDA passes over a specified geographic fix.

1)    The following is a list of statements which describe what MAP is.

    MAP is a specified decision point.

    MAP is the last point at which the approach can be continued by reference solely to flight instruments. After the MAP, the instrument approach must be discontinued.

    MAP is the last point at which the published missed approach can be safely executed in instrument conditions.

2)    The following is a list of statements which describe what MAP is not.

    MAP is not always the last point at which a pilot can decide to continue the approach by external visual references. Often, the MAP is located at a point where a pilot cannot safely descend and land if the MDA is maintained until arriving at the MAP (e.g., when the MAP is located over the VOR on the airport).

    MAP is not a point where a decision or commitment to land is made.

    MAP is not a point where the decision process begins.

    MAP is not a point where all aspects of the decision are instantaneously formulated.

4-161    MINIMUM INSTRUMENT FLIGHT ALTITUDES. Except for certain CAT III operations, all instrument approach and landing operations have limitations related to obstacles, airborne instrumentation and equipment, ground-based navigation equipment, and/or visual aids. Because of these limitations, external visual information is required to safely complete instrument approaches and landings. Airborne instruments and equipment and the signals in space radiated by ground-based NAVAIDs must provide pilots adequate guidance to safely control an aircraft by reference solely to instruments until the aircraft arrives at a preestablished minimum height or altitude (DA/DH or MDA) for instrument flight. The total system (airborne and ground-based) does not provide this capability below the minimum height or altitude for instrument flight. Therefore, descent below the specified minimum height or altitude for instrument flight can only be safely accomplished when adequate external visual references are available. If adequate external visual references are not established, a pilot must execute an instrument missed approach at or before passing a preestablished MAP.

NOTE:    Descent below the specified minimum IFR altitude without adequate visual references to control and maneuver the aircraft to a landing is unsafe and prohibited. The minimum height or altitude for instrument flight for an instrument approach and landing is specified in various ways depending on the type and category of the instrument approach conducted.

A.    NPA Procedures. The minimum heights or altitudes for IAPs that do not have vertical guidance can be specified as an MDA, height above touchdown (HAT), height above airport (HAA), minimum descent height (MDH), Obstacle Clearance Altitude (OCA), Obstacle Clearance Height (OCH), or Obstacle Clearance Limit (OCL). MDA, HAT, HATh, and HAA are used by the United States and certain foreign countries that use TERPS criteria. OCA, OCH, and OCL are used in most foreign countries and are established in accordance with ICAO PANS-OPS. Although the current edition of ICAO PANS-OPS eliminated use of OCL, some countries still use OCL criteria from previous editions of PANS-OPS. Some countries, in addition to OCA and OCH, provide MDA and MDH. MDA and OCA are barometric flight altitudes referenced to mean sea level (MSL). HAT, HATh, HAA, MDH, OCH, and OCL are radio or radar altitudes referenced to either the elevation of the airport, the elevation of the TDZ, or the elevation of the landing threshold.

    MDA or OCA may be specified for any approach procedure that does not have vertical guidance.

    HAT, MDH, OCH, or OCL may be specified for straight-in approach procedures that do not have vertical guidance.

    HAA, MDH, OCH, or OCL may be specified for circling maneuvers.

B.    Precision Approach Procedures and Approach Procedures with Vertical Guidance (APV). The minimum heights or altitudes for IAP with vertical guidance can be specified as a DA, OCA, DH, OCH, or OCL. In the United States and certain foreign countries that use U.S. TERPS criteria, the minimum instrument flight altitude for precision with vertical guidance and APV is DA/DH. DA/DH is specified as a DA referenced to MSL for aircraft equipped with only barometric altimeters and as HAT or HATh (for procedures developed with harmonized visibility minimums) for aircraft equipped with radio altimeter or RAs. DA, DH, OCH, and OCL are used in most foreign countries and are established in accordance with various versions of ICAO PANS-OPS. DA and OCA are referenced to a barometric altitude (MSL). DH (in most countries), OCH, and OCL are referenced to a radio or radar height above either the elevation of the airport, the elevation of the TDZ, or the elevation of the landing threshold.

C.    Lowest Permissible Height or Altitude for Instrument Flight. The lowest permissible height or altitude for instrument flight for any approach cannot be lower than any of the following:

    Minimum height specified by the FAA-approved Aircraft Flight Manual (AFM).

    Minimum height or altitude for which the signals from ground-based or space-based navigation equipment can be relied upon for instrument flight.

    Minimum height or altitude that provides adequate obstacle clearance.

    Minimum height or altitude authorized for the flightcrew.

    Minimum height or altitude authorized for the operator for that aircraft and equipment combination.

    Minimum height or altitude permitted by the operative airborne and ground-based or space-based equipment.

    Minimum height or altitude published or otherwise established for the instrument approach.

    Minimum height or altitude authorized in OpSpecs for the operation being conducted.

4-162    OPERATING MINIMUMS. The lowest operating minimums for operations conducted under 14 CFR parts 91K, 121, 125, and 135 are specified in standard OpSpecs, MSpecs, and LOAs as appropriate. In general, an air carrier is authorized to use operating minimums specified by the following groups of IAP, provided the minimums are not lower than the lowest minimums specified in the air carrier’s OpSpec for any particular type of approach procedure.

    Part 97, IAP.

    U.S. military IAPs at U.S. military airports.

    Any IAPs approved and incorporated into the OpSpecs.

    ICAO contracting State IAPs at foreign airports.

    IAPs established by an air carrier at foreign airports, provided the procedure is accepted in accordance with the OpSpecs.

A.    Straight-In Minimums for Approaches with a DA/DH. The lowest permissible DA/DH and visibility minimums for all airplanes conducting standard straight-in IAPs other than CAT II or CAT III that have a DA/DH are HAT 200 and RVR 1800. The lowest permissible DA/DH and visibility minimums for helicopters is -sm visibility or RVR 1200. These basic DA/DH and visibility minimums are normally restricted to runways that are equipped with a lighting system consisting of TDZ and RCL lights and medium intensity approach lighting system with runway alignment indicator lights (MALSR), simplified short approach lighting system with runway alignment indicator lights (SSALR), and ALSF-1 or ALSF-2 approach lighting systems. RVR 1800 is authorized when FD, autopilot, or HUD is used in lieu of TDZ and RCL lights. Additionally, SA CAT I operations are discussed in Volume 4, Chapter 2, Section 6.

B.    Straight-In Minimums for Approaches with an MDA. The lowest permissible MDA and visibility minimums for Categories A, B, C, and D aircraft during the conduct of straight-in IAPs that have an MDA are HAT 250 and -sm visibility or RVR 2400. The lowest permissible MDA and visibility minimums for helicopters operated at 90 knots or less are HAT 250 and -sm visibility or RVR 1600. The lowest MDA and visibility minimums for helicopters operated at more than 90 knots are HAT 250 and -sm visibility or RVR 2400. These minimums are the lowest authorized for approaches that have an MDA and are restricted to runways that are equipped with MALSR, SSALR, ALSF-1, or ALSF-2 approach lighting systems, or foreign equivalents.

C.    Controlling Minimum Concept. The concept of a controlling minimum is based on reported weather conditions at the destination airport. The controlling minimum concept includes considerations for the reported weather conditions, the capabilities of the flightcrew, and the capabilities of the airborne and ground- or space-based equipment. This concept prohibits a pilot from continuing past the FAF or beginning the Final Approach Segment (FAS) of an IAP unless the reported visibility (RVV or RVR, if applicable) is equal to or greater than the authorized visibility (RVV or RVR) minimum for that IAP.

1)    Objective. The basic objective of the controlling minimum concept is to provide reasonable assurance that once the aircraft begins the FAS, the pilot will be able to safely complete the landing. The controlling minimum concept, however, permits a pilot to continue a CAT I approach to DA/DH if the visibility/RVV/RVR was reported to be at or above the controlling minimum when the pilot began the FAS, even though a later visibility/RVV/RVR report indicates a below-minimum condition. RVR reports, when available for a particular runway, are the reports (controlling reports) that must be used for controlling whether an approach to, and landing on, that runway is authorized or prohibited.
2)    Parts 91 and 91K Controlling Minimum. The controlling minimums concept as described above is not applicable to part 91 or 91K operators when determining if the pilot can continue past the FAF or begin the FAS. Parts 91 and 91K operations can begin an approach and continue to the DA/DH or the MDA and the MAP, even when the weather conditions are reported to be below the authorized IFR landing minimums. Upon arrival at the MDA and before passing the MAP, or upon arrival at the DA/DH, the approach may be continued below DA/DH or MDA to the runway if the seeing-conditions required by 91.175(c)(d) or 91.175(1) are met.
3)    Part 121 Controlling Minimum. The controlling minimum concept for operations conducted under part 121 is implemented by 121.651(b). For these operations, the controlling minimum must be used at civilian airports within the United States and its territories, and at U.S. military airports, unless the provisions of  121.651(d) are met. Section 121.651(d) permits a pilot to begin the FAS, even though the reported visibility/RVV/RVR is below the controlling minimum, if the approach procedure is an ILS and the flight is actively monitored by a PAR.
a)    Therefore, pilots are not constrained by the controlling minimum on runways with ILS and active PAR facilities, provided the provisions of 121.651(d) are met. The controlling minimum concept allows for a pilot to continue a CAT I approach to DA/DH or MDA if the visibility/RVV/RVR was reported to be at or above the controlling minimum when the pilot began the FAS, even though a later visibility RVV/RVR report indicates a below-minimum condition.
b)    Upon reaching DA/DH or MDA and before passing the MAP, the approach may be continued below DA/DH or MDA to touchdown if the requirements of 121.651(c) are met, even though the visibility/RVV/RVR is reported to be below the controlling minimum. The controlling minimum concept does not apply to part 121 operations conducted at civilian airports in many foreign countries. In foreign countries, part 121 operators may conduct look-see approaches unless the rules of a foreign country (such as the United Kingdom) prohibit look-see approaches. If the rules of the foreign country prohibit look-see approaches, the controlling minimum concept applies in that country.
4)    Parts 125 and 135 Controlling Minimum. The controlling minimum concept for parts 125 and 135 differs in application from part 121. Part 91 applies to all parts 125 and 135 operations whether they are conducted in foreign countries or the United States (see part 125, 125.23(b) and 135.3(b)). Operations conducted under parts 125 and 135 must also be in compliance with 125.381 and 135.225 (which applies to all operations within the United States, its territories, U.S. military airports, and foreign airports). For parts 125 and 135 operations, the controlling minimum concept must be used at all airports (with the exception of a part 135 “eligible on-demand” operator who is permitted to start an approach without weather reported above landing minimums (see 135.225(b)).
a)    As a consequence, 125.381(b) and 135.225(b) prohibit part 125 and 135 operators from conducting look-see approaches at any airport. The controlling minimum concept, however, allows for a pilot to continue a CAT I approach to DA/DH or MDA if the visibility/RVV/RVR was reported to be at or above the controlling minimum when the pilot began the FAS, even though a later visibility/RVV/RVR report indicates a below-minimum condition.
b)    The controlling minimum concept also allows for a pilot (upon reaching DA/DH or MDA and before passing the MAP) to continue the approach below DA/DH or MDA and to touchdown, if the requirements of 91.175 are met, even though the visibility/RVR is reported to be below the controlling minimum.


A.    Perceptual Limitations. Restricted seeing-conditions significantly affect a pilot’s ability to visually detect or perceive vertical height, sink rate (vertical velocity), and vertical acceleration. As seeing-conditions decrease, the pilot’s ability to perceive vertical height, sink rate, and vertical acceleration degrades faster than the ability to perceive lateral errors and lateral accelerations. Personnel establishing operating minimums must consider these human perceptual limitations.

B.    Aircraft Structural Limitations. According to structural design criteria, the aircraft structure must tolerate touchdown sink rates (vertical velocity) of at least 10 feet per second (600 fpm). Touchdown sink rates higher than the maximum rates evaluated during the certification of an aircraft can cause serious structural damage, including catastrophic failure. Therefore, instrument procedure design must provide for sink rates that give a pilot the capability of detecting unacceptable situations and adjusting the flightpath to achieve a safe landing, considering available visual aids and operating minimums. Visual aids and operating minimums must provide a high probability that a pilot will be able to control the aircraft adequately and adjust the vertical flightpath to achieve acceptable sink rates at touchdown and touchdown within the TDZ.

C.    Maximum Acceptable Sink Rates. Operational experience and research have shown that a sink rate of greater than approximately 1,000 fpm (16.67 feet per second) is unacceptable during the final stages of an approach (below 1,000 feet above ground level (AGL)). This is due to a human perceptual limitation that is independent of the type of airplane operated and is equally applicable to helicopters. Therefore, the IAPs and the operational practices and techniques must ensure that sink rates greater than 1,000 fpm are not required or permitted in either the instrument or visual portions of an approach and landing operation. Operating minimums and available visual aids must provide reasonable assurance that a pilot will have adequate external visual references in the visual portions of all IFP (certain CAT III operations excepted). To be considered adequate, these external visual references must permit a pilot to adequately perceive sink rates and manually maneuver the aircraft (or evaluate autopilot performance) to achieve an acceptable touchdown sink rate and touchdown point, considering the operating minimums and the available visual aids.


A.    Design of an Aircraft. The overall design of an aircraft and the design of a cockpit significantly affect seeing-conditions during the latter stage of an approach and landing and during the initial stage of a takeoff. Cockpit design has a direct effect on a pilot’s ability to determine the 3-D position of an aircraft in relation to a landing or takeoff surface and, consequently, on the ability to safely control the flightpath of the aircraft. Therefore, cockpit design is a significant factor in establishing operating minimums of a particular aircraft. Generally, aircraft with larger cockpit cutoff angles (better downward viewing angles over the nose) and shallower landing pitch attitudes provide for better seeing-conditions. Improved seeing-conditions derived from improved cockpit design can be used to justify lower operating minimums. Seeing-conditions are affected by geometric factors related to the design of an aircraft’s structure and by aerodynamic factors related to an aircraft’s pitch axis. When considering aircraft/cockpit design, it is important to note the following:

    The radio (radar) altimeter is calibrated to read the height of the landing gear above the terrain (when in the landing configuration).

    The glidepath antenna tracks down the centerline (CL) of the GS when the instruments in the cockpit indicate the aircraft is on glidepath.

    The pilot’s eyes are always higher than what is indicated on the radio (radar) altimeter.

    The pilot’s eyes are above the electronic GS in most aircraft.

B.    Aircraft and Cockpit Physical Design. The significant factors related to the physical design of an aircraft and cockpit combination that affect seeing-conditions most are as follows:

    Distance along the longitudinal axis from directly above the main landing gear to directly beneath the pilot’s eyes.

    Vertical distance from the pilot’s eyes to a position abeam the main landing gear.

    Distance along the longitudinal axis from directly beneath the GS antenna to directly beneath the pilot’s eyes.

    Vertical distance from the GS antenna to abeam the pilot’s eyes.

    Cockpit-Cutoff (CCO) angle.

C.    The CCO Angle. The CCO angle is the angle, measured downward, from the longitudinal axis of the aircraft (zero pitch reference) to the lowest (most depressed) angle that can be seen over the aircraft’s nose from the proper sitting position (eye reference position). The CCO angle in most transport category aircraft is between 15 and 25 degrees. Although many VFR helicopters have an excellent CCO angle, most IFR helicopters have CCO angles equivalent to transport category aircraft.

D.    Aircraft Aerodynamic Design. The significant factors associated with the aerodynamic design of an aircraft that affect seeing-conditions are related to pitch attitudes. The pitch attitudes necessary for final approach, flare (deceleration for rotorcraft), and landing (air taxiing for rotorcraft) have a major effect on seeing-conditions. This is because a nose-up attitude reduces the downward viewing angle relative to the horizon, which reduces seeing-conditions.

1)    For example, an aircraft with an excellent CCO angle of 21 degrees and a high final approach pitch attitude of 8 degrees would have a seeing condition comparable to a similar size aircraft having a poor CCO angle of 13 degrees and a 0 degree pitch attitude. Since the pitch attitude on final approach varies with approach speed, aircraft configuration, and gross weight, the seeing-conditions change as these operational factors change.
2)    The aircraft’s flare characteristics (deceleration for rotorcraft) can also have a significant effect on the seeing-conditions during landing. The seeing-conditions during flare decrease if any positive pitch change is required. In helicopters, the most severe degradation to the seeing-conditions occurs during deceleration to air taxi or hover. Often, the deceleration rate in a helicopter must be limited to maintain adequate seeing-conditions.
3)    For example, when a typical IFR helicopter with an 18 degree CCO angle and a 0 degree final approach attitude approaches an 18 degree pitch attitude during a maximum effort deceleration, the pilot will lose sight of the landing surface. At an 18 degree pitch attitude with an 18 degree CCO angle, the lowest downward viewing angle would be parallel with the horizon.
4)    Therefore, a deceleration pitch attitude must be maintained significantly below 18 degrees to maintain adequate visual references with the landing surface. A similar situation is encountered in turbojet airplanes during takeoff rotation and initial climb when external visual references can be lost.

E.    Eye Reference Position. Eye reference position is a critical factor in achieving optimum seeing‑conditions. A pilot’s seat must be individually adjusted so that the pilot’s eyes are located at an optimum eye reference position. When seated in this position, a pilot should be able to take advantage of the full CCO angle, maintain reference with the necessary flight instruments, and operate all necessary controls. Many aircraft have special devices that indicate proper seat adjustment. Improper seat adjustment, especially in CAT II and III operations, can prevent the pilot from acquiring adequate external visual reference upon arrival at DA/DH or MDA/MAP.

1)    The seating position commonly used for en route operations in many aircraft is too low and too far aft for the pilot to achieve optimum seeing-conditions during approach and landing operations. This lower and further aft seating position results in a reduction of the CCO angle, which degrades the seeing-conditions by reducing the segment of the approach and landing surface visible over the aircraft’s nose.
2)    A pilot maintaining this undesirable seating position during approach and landing may tend to compensate for the reduced CCO angle, and its effects, by leaning forward in an attempt to acquire the necessary external visual references. A consequence of this practice is a tendency to unintentionally reduce the pitch attitude. Since seeing-conditions improve as the nose is lowered, this tendency to reduce pitch attitude can contribute to the tendency to duck under, which has resulted in landings short of the runway.


A.    Executing a Go-Around. Most aircraft used in air transportation have the capability, in a normal approach and landing configuration, of safely executing a go-around from any point before touchdown, even when significant failures occur, such as engine, hydraulic, or autopilot failures. This aircraft performance capability for safety in go-arounds should be provided for, particularly for go-arounds caused by operational factors, such as airborne and ground-based equipment failures, ATC contingencies, loss of external visual references, and misalignment with the landing surface. This capability is required in all CAT II and CAT III operations. When establishing operating minimums for aircraft that do not have this capability, the consequences of the failures that would preclude a safe go-around must be considered. Operating minimums for aircraft without the performance capability to safely go around following engine failure must provide adequate seeing-conditions to successfully accomplish a forced landing in a preestablished location. The following factors must be considered when evaluating the safety of go-arounds from any point in the approach before touchdown.

B.    Go-Around Capability. The go-around capability is based on normal operating conditions at the lowest authorized operating minimum. Factors related to geometric limitations of the aircraft during the transition to a go-around (such as tail strike, or rotor strike) must be considered. Other factors such as the available visual cues, autopilot or FD mode switching, altitude loss in transition to go-around, and altitude loss due to autopilot malfunction must also be considered.

C.    Inadvertent Touchdown. If a go-around could result in an inadvertent touchdown, the safety of such an event must be considered. The aircraft design and/or procedures used must accommodate for relevant factors. Examples of relevant factors that must be considered include operation of engines, the operation of autothrottle, autobrakes, autospoilers, autopilot mode switching, and other systems that could be adversely affected by an inadvertent touchdown.

D.    Failure Condition in the Aircraft. If the occurrence of any failure condition in the aircraft or its associated equipment could preclude a safe go-around from low altitude, then these failure conditions must be clearly identified. In these cases, a minimum height must be specified from which a safe go-around can be initiated if the failure occurs. If the failure occurs below the specified height, pilots must be made aware of the effects or consequences of any attempt to go around.

E.    Appropriate Procedures for Low-Altitude Go-Arounds. Information must be provided to the flightcrew concerning appropriate procedures for low altitude go-arounds and the height loss expected. If the conduct of certain approach and landing operations is authorized with an engine-out, height loss information for engine-out operations must also be provided to the flightcrew.

4-166    FUNCTION OF EXTERNAL VISUAL REFERENCES. Except for certain CAT III operations, external visual information is essential for a pilot to safely take off or to complete an instrument approach and landing. This external visual information (visual cues) is necessary for a pilot when assessing the 3-D position of the aircraft, its velocity, and its acceleration or deceleration in relation to the intended landing or takeoff surface. This information is essential for a pilot when manually maneuvering (or when evaluating the autopilot’s performance in maneuvering) the aircraft into alignment with the centerline of a landing or takeoff surface. External visual references are essential for a pilot to safely touchdown (decelerate to air taxi/hover for rotorcraft) within the TDZ and for maintaining directional control to stop on the runway (maintain directional control and avoid obstacles while air taxiing for rotorcraft). In degraded seeing-conditions, the quality of external visual information can be significantly improved by use of visual aids, such as runway markings and lighting. Such visual aids are necessary to increase the conspicuousness of the landing or takeoff surface. These aids provide pilots with the necessary visual references during takeoff, the final stages of approach and landing, and ground movement. The importance of visual aids increases as seeing-conditions decrease.

A.    Lateral Position and Crosstrack Velocity or Acceleration. Approach lighting, TDZ lighting, RCL lighting, runway edge lighting, and runway markings provide visual references to pilots for assessing lateral position and crosstrack velocity or acceleration.

B.    Visual Roll References During Landing, Takeoff, Rotation, and Initial Climb. Approach lighting, threshold lighting, in-runway lighting, and runway markings provide visual roll references during landing, takeoff, rotation, and initial climb.

C.    Visual Information for a Pilot. TDZ lighting and runway markings indicate the plane of a landing surface and identify the touchdown area, thereby providing a vertical and longitudinal reference. These visual aids provide necessary visual information for a pilot to determine vertical position, sink rate, and vertical acceleration or deceleration.

D.    Adequate Alignment and Directional Control Information. The visual guidance information from in-runway lights and/or markings must be sufficient to ensure adequate alignment and directional control information during takeoff or during final stages of landing and deceleration.

E.    External Visual Aids. Reference to external visual aids is a primary requirement for controlling the aircraft’s flightpath when operating below the minimum altitude (height) published for instrument flight.

4-167    MINIMUM VISIBILITY, RVV, AND/OR RVR. Upon arrival at the minimum height or altitude for instrument flight and before passing a preestablished decision point, a pilot must establish adequate seeing‑conditions to safely complete the approach and landing.

A.    Establishing Operating Minimums. Operating minimums are expressed as visibility, RVV, or RVR. Criteria for establishing operating minimums must provide a reasonable assurance that a pilot can establish the required seeing-conditions before passing the decision point. This criterion provides this assurance if the weather conditions are reported to be at or above the landing minimum when the approach is initiated. To achieve this objective, the operating minimums specified for the procedure (visibility, RVV, RVR) must be compatible with the minimum height or altitude for instrument flight and the decision point specified for the procedure.

B.    Establishing Visual Reference. Therefore, when the reported weather conditions are at the authorized minimums, a pilot should be able to establish external visual references upon arrival at the minimum height or altitude (DA/DH or MDA), and before passing the decision point (DA/DH, MAP, or visual descent point (VDP)). At this point a pilot must be able, by external visual reference, to maneuver to a landing without exceeding a descent rate of 1,000 fpm or exceeding aircraft limitations on touchdown. For example, it would not be practical to specify a DA/DH of 200 feet (HAT 200) with an operating minimum of RVR 700 since the first visual contact in a typical aircraft would not occur until approximately 130 feet above the elevation of the TDZ.

C.    Adequate External Visual References. The specified operating minimum must also permit adequate external visual references to be established early enough for a normal descent to landing (less than 1,000 fpm). For example, it would not be reasonable to specify an MDA equivalent to a HAT of 400 feet and an operating minimum of RVR 1600 for typical turbojet airplanes. In this situation, the pilot would not establish first visual contact until the airplane is within 4,000 feet of the landing threshold and would require a descent rate much higher than 1,000 fpm to land within the TDZ.

4-168    CONCEPT OF RVR.

A.    Operating Minimums. Operating minimums are specified in terms of ground visibility, tower visibility, and RVR. The RVR concept has evolved over a long period, and its use in the United States began in 1955. As operating minimums were reduced due to improvements in airborne and ground-based equipment, it became more likely that pilots would not see the full length of the runway upon arrival at the specified decision point. Positions established for taking visibility observations were often several miles from the approach end of many runways. This resulted in reported visibility values that frequently did not represent the seeing-conditions encountered during the final stages of approach and landing. This deficiency was particularly critical when rapidly changing weather conditions within the terminal area occurred. These factors generated a need for systems such as RVR, which could rapidly and reliably provide reports of the seeing-conditions that a pilot could expect to encounter in the TDZ and along the runway.

B.    RVR Measurements. RVR measurements are taken by a system of calibrated transmissometers and account for the effects of ambient background light and the runway light intensity. Transmissometer systems are strategically located to provide RVR measurement associated with one or more of the three basic portions of a runway: the TDZ, the runway midpoint (Mid), and the rollout end of the runway (rollout).

C.    Instrumentally Derived Value. RVR is an instrumentally derived value that reflects an artificially created seeing-condition on or near the portion of the runway associated with the RVR report. This artificially created seeing‑condition is achieved by using HIRLs, as well as TDZ and RCL lights if they are installed. These lights increase the conspicuousness of the landing surface and reach out to the pilot, thereby creating a seeing‑condition that is significantly better than the reported ground visibility or tower visibility. Since RVR is based on high intensity lights, an RVR report only has meaning when associated with the seeing-conditions on or near the portion of the runway where the report was obtained (TDZ, Mid, or rollout). An RVR report has no meaning unless a pilot is also seeing the high intensity lights on which the report is based.

1)    To properly apply operating minimums, it is important to understand RVR. The following is a list of statements that describe what RVR is.

    RVR is an instrumentally derived value.

    RVR is currently measured by transmissometers located approximately 400 feet from RCL.

    RVR is related to the transmissivity (degree of opaqueness) of the atmosphere.

    RVR is an approximation of the distance a pilot should see when an aircraft is on, or slightly above, the portion of the runway associated with the report.

    RVR is calibrated by reference to runway lights and/or the contrast of objects.

    RVR is a value that varies with runway light setting.

    RVR is a value that only has meaning for the portions of the runway associated with the RVR report (TDZ, Mid, or rollout).

2)    The following is a list of statements that describe what RVR is not.

    RVR is not a measure of meteorological visibility.

    RVR is not a measure of surface visibility or tower visibility.

    RVR is not a measure of seeing-conditions on taxiways, ramps, or aprons.

    RVR is not a measure of seeing-conditions at or near MDA or DA/DH.

    In the United States, RVR is not measured or reported by a human observer.

    RVR is not “visibility.”

D.    Concept of Controlling RVR. Controlling RVR means that RVR reports are used to determine operating minimums whenever operating minimums are specified in terms of RVR, and RVR reports are available for the runway being used. All CAT I operating minimums below sm and all CAT II and III operating minimums are based on RVR. The use of visibility is prohibited because the reported visibility may not represent the seeing-conditions on the runway. All takeoff minimums below -sm visibility (RVR 1600 for airplanes and RVR 1200 for rotorcraft) are predicated on RVR and use of visibility is prohibited. For example, if the takeoff minimum for a particular operation is TDZ RVR 1200/rollout RVR 1000, RVR reports are controlling and a takeoff is prohibited unless the TDZ RVR report is at or above RVR 1200 and the rollout RVR report is at or above RVR 1000. In this example, a takeoff cannot be based on visibility if the RVR system is operative, even if the reported visibility is greater than 1 sm.


A.    Identifying Contrast Levels. A primary factor in the identification of objects, such as landing surfaces, depends on a pilot’s ability to see contrasts between the object and the surrounding background. The ability to see and recognize contrasts in the brightness or color of an object is much greater than the ability to determine the actual level of illumination of an object. For example, a 100-watt light bulb seems to be much brighter at night than during daylight conditions, even though the actual level of illumination is the same.

B.    Increasing Contrast Levels. The contrast between a 100-watt light and a dark night background is much greater than it is in a daylight background. The presence of airborne particles or water droplets causes the available light to diffuse or scatter. This scattering effect raises the overall illumination of the background that, in turn, reduces the level of contrast between an object and its background. This is the primary reason why seeing‑conditions decrease when landing into the sun on a hazy or foggy day or when the landing lights of an aircraft are turned on in snow or fog conditions. Reduced levels of contrast increase the difficulty of identifying objects such as snow-covered runways or runways located in heavily lighted urban areas. As a result, contrast levels must be increased to provide the seeing-conditions necessary for the safe conduct of operations with reduced operating minimums.

1)    Seeing-conditions can be improved by using visual aids and by enhancing the level of contrast within the runway environment. For example, the difference in the level of contrast between a landing or takeoff surface and the surrounding area can be improved through good airport maintenance practices. Such practices as planting and maintaining grass around a runway and between a runway and a taxiway, and plowing snow-covered runways, improve levels of contrast. The most effective way to improve the contrast of a landing or takeoff surface, however, is to use visual aids because they are effective in a variety of weather conditions.
2)    Visual aids such as approach lights, runway lights, and runway markings significantly improve the contrast between a landing or takeoff surface and the immediate surrounding area. The improved contrast provided by approach and runway lighting significantly improves seeing-conditions in both night and daylight operations. Approach lighting and runway lighting are essential elements of all landing operations conducted in weather conditions below RVR 4000 and all takeoff operations below RVR 1600.
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4-170    THRESHOLD CROSSING HEIGHT (TCH) CONCEPT. Many complex technical factors must be considered during the installation of ILS equipment to support approach and landing operations at any particular runway. The signals in space radiated by the facility must meet required flight inspection requirements (accuracy and course structure) for the particular category of operation to be supported. Design of ground support systems must be such that there is an extremely small probability of losing electronic guidance during actual operations (continuity of service). The design must also provide for an extremely high probability of providing continuously reliable electronic guidance (integrity). The ILS accuracy and course structure, continuity of service, and integrity must meet established standards for the category of operation authorized at that facility. Another critical factor in installing and siting these systems is the TCH. The following discussion addresses significant factors that must be considered when establishing acceptable TCHs.

A.    Aircraft GS/Elevation Antenna Location. The GS/elevation receiver of the aircraft detects vertical movement (displacement) of the GS/elevation antenna in relation to the centerline of an electronic GS/elevation radiated from a ground facility. As a result, the location of the GS/elevation antenna on the aircraft directly relates to terrain and obstacle clearance during the final stages of an approach and landing.

1)    The physical dimensions and aerodynamic characteristics of the aircraft (especially pitch attitude) are important factors in the determination of the proper location of a GS reception antenna. In conventional aircraft, the GS/elevation antenna is located above the height of the main landing gear. Since an aircraft is maneuvered so that its antenna tracks the centerline of the electronic glidepath, the main landing gear will track below the glidepath.
2)    For example, if the antenna of an aircraft is located 40 feet above the landing gear and the electronic glidepath crosses 30 feet above the runway threshold, the main landing gear will touch down short of the runway since the antenna, not the landing gear, flies the glidepath. This example illustrates the important relationship between the aircraft antenna location and the electronic glidepath TCH.
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3)    This situation can be resolved by siting the ILS to achieve a specified TCH and by requiring proper location of the GS/elevation antenna on the aircraft. Similar problems are encountered when using visual vertical guidance systems such as Visual Approach Slope Indicator (VASI) or precision approach path indicator (PAPI), since the pilot’s eyes track the visual glidepath and the gear follows a lower path. The need to maintain certain landing gear crossing heights at the threshold establishes the minimum safe TCH for a particular aircraft. The current minimum TCH requirements are based on the DC-10 that has, in landing configuration, the greatest vertical displacement between the antenna location and the landing gear.
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B.    Barometric VNAV (baro-VNAV) TCHs. The most significant factor in determining the threshold wheel crossing height for aircraft using baro-VNAV for vertical guidance during the FAS is the vertical distance between the static ports and the bottom of the main landing gear when the aircraft is in its normal approach attitude. The minimum and maximum acceptable TCHs for these aircraft are determined in a manner similar to ILS-equipped aircraft using the static ports and the main landing gear height, instead of the GS/elevation antenna to landing gear height.

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C.    Acceptable TCHs. Siting ILS equipment to achieve a particular TCH can be a complex task. Operational experience with siting these systems has shown a need to establish a range of acceptable TCHs. The types of aircraft likely to use a particular facility must be considered. Another consideration in establishing the range of acceptable TCHs is the pilot’s ability to detect (by external visual references) deviations from the proper glidepath and to make the necessary flightpath adjustments for adequate landing gear clearance at the threshold. Proper TCHs in CAT II and especially CAT III operations are more critical because of the limited visual cues available and the use of automatic landing systems.

D.    Minimum and Maximum Acceptable TCHs in the United States. The minimum acceptable TCH at a particular runway is determined by the most TCH-critical aircraft likely to be used at that facility. The maximum acceptable TCH also depends upon the types of aircraft likely to be used at the facility. The instrument approach and landing system must be sited so that all aircraft have a high probability of a safe touchdown (deceleration to air taxi or hover for rotorcraft) in the TDZ. Landing performance is based on the assumption that touchdown will occur in the TDZ. Very high TCHs will not permit some aircraft to safely touchdown within the TDZ, therefore maximum acceptable TCHs must also be established.

E.    TCHs at Foreign Airports. GS TCHs at foreign airports may not be equivalent to U.S. criteria. It is important for pilots and operators using foreign airports to understand the significance of TCH and to know the minimum TCHs that can be safely used by their aircraft. Operations should not be conducted to runways with TCHs below minimum acceptable TCHs for any particular aircraft, unless special limitations are placed on the conduct of the operation. These special limitations must be such that a pilot can safely and consistently touchdown within the TDZ and safely complete the rollout on the available runway length.

4-171    AIRPORT FACILITIES AND SERVICES. The varied seeing-conditions encountered in AWTAs require pilots to rely heavily on visual aids, electronic guidance from ground-based facilities, and other facilities and services provided by the airport. Therefore, basic VFR airport facilities and services must be enhanced before safe operations can be conducted in instrument flight conditions. Runways and taxiways must meet more stringent criteria with respect to width, length, marking, and lighting. Instrument approach aids and IAPs are required. Visual aids are needed to assist a flightcrew during transition from instrument to visual flight and during ground movement. Meteorological observation and measurement equipment must be available to provide real‑time information on weather conditions. Equipment and procedures must be established to provide aeronautical information on runway surface conditions and the status of airport facilities and services.

A.    VFR Airport Facilities and Services. Enhancements to basic VFR airport facilities and services necessary to support instrument flight operations include the following general factors:

    Physical characteristics of the runway environment, including approach, departure, and pre‑threshold terrain characteristics.

    Obstacles and the obstacle limitation assessment surfaces.

    Visual aids.

    Electronic aids.

    Secondary (standby) power supplies.

B.    Physical Characteristics. Physical characteristics of a runway environment become increasingly important as seeing-conditions deteriorate. Excessive runway or approach light gradients can create undesirable visual illusions and can cause hard or long landings. Longer runway lengths are necessary for reasons such as the tendency to land further down the runway because of visual illusions and the increased difficulty in controlling the aircraft’s flightpath. The topography in the approach and pre-threshold areas should be regular and preferably level to ensure proper operation of radio (radar) altimeters, FD systems, and automatic landing systems.

1)    The operation of automatic landing systems and other systems that provide flight guidance during flare and landing (such as HUD) is dependent on input from RAs. As a result, the flare profile, touchdown sink rate, and touchdown point can be adversely affected by the profile of the pre-threshold terrain. Where the pre-threshold terrain for a particular runway could affect safe operations (e.g., Seattle-Tacoma International Airport (SEA) 16R, Cincinnati/Northern Kentucky International Airport (CVG) 36C, Minneapolis-St. Paul International/Wold-Chamberlain Airport (MSP) 30L, and Pittsburgh International Airport (PIT) 10L), an in-flight demonstration must be made to determine that the flight control system of a particular aircraft is not adversely affected by the pre-threshold terrain profile.
2)    Additionally, the pre-threshold terrain at certain runways (e.g., MSP 30L and PIT 10L) may not permit an RA to be used to define DH for CAT II or AH/DH for CAT III operations for certain aircraft. In certain situations, an inner marker (IM) can be used to define the CAT II DH or the CAT III AH.

C.    Obstacles and Obstacle Limitation Assessment Surfaces. Degraded seeing-conditions decrease a pilot’s ability to see and avoid obstacles. Therefore, it is essential that obstacle protection is provided along the approach paths, missed approach and departure flightpaths, and in areas on or near runways used for takeoffs and landings. Obstacle protection criteria for different categories of operations and the various phases of an approach, landing, missed approach, takeoff, and departure are specified in U.S. TERPS, ICAO PANS-OPS, and applicable ACs.

1)    In certain situations, obstacles may prevent the conduct of CAT II or III operations. In other situations, a higher-than-normal DH for CAT I or II operations may be required to guarantee obstacle clearance upon the execution of a missed approach. During operations using approaches with vertical guidance, it is essential to provide obstacle protection in Runway Safety Areas (RSA) and obstacle-free zones. An RSA is an area adjacent to the runway that must be free from fixed or mobile “nonbreakable” obstructions. RSAs reduce the potential for catastrophic accidents if portions of the aircraft structure (such as a wingtip) extend beyond the runway edge, or if an aircraft departs the runway during a landing or takeoff roll.
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2)    An obstacle-free zone is a 3-D area including portions of the landing surface that provides obstacle clearance during landings or during rejected landings, including missed approaches after touchdown. The only fixed obstructions permitted in RSAs or obstacle-free zones are frangible objects or obstructions that are fixed by their functional purpose. “Fixed by their functional purpose” means that the installation of the object in those areas is essential to the safe conduct of operations on the runway; there are no alternative locations (examples include such objects as runway lights, GS/elevation antennas, and RVR reporting systems). Mobile obstructions (such as aircraft and/or vehicles) are not permitted within RSAs or obstacle-free zones while aircraft are using the runway. Aircraft, vehicles, and other objects that could disturb ILS electronic guidance are not permitted in ILS critical areas when other aircraft are critically dependent on this type of guidance.
3)    Since protection of these areas or zones is critical to safe operations (particularly during degraded seeing-conditions), visual aids (such as signs, markings, or lighting) must be provided for identifying the boundaries of these areas to pilots and operators of other vehicular traffic. ATC procedures and ground movement restrictions must be provided to ensure that these areas are protected.

D.    Visual Aids. Visual aids are essential for most AWTAs. Visual aids are also important for the safe and expeditious guidance and control of taxiing aircraft. These aids include signs, markings, and lights that identify holding points or indicate directions, and the marking or lighting of the taxiway centerline and edges. The conspicuousness of runway and taxiway markings deteriorates rapidly, especially at busy airports. These markings must be frequently inspected and maintained, particularly for CAT II or III operations.

1)    All lighting systems should be monitored by ATC so that timely information on system failures or malfunctions can be provided to pilots. Regular visual inspections of all sections of the lighting systems are normally used to determine the status of individual lights.
2)    Therefore, it is usually only necessary for ATC to remotely monitor lighting circuits to determine whether the proper amount of power is being demanded by, and delivered to, the lighting systems. Remote monitoring of approach, runway edge, and in-runway lighting is essential during CAT II and CAT III operations, unless frequent visual inspections (every 2 hours) or timely pilot reports indicate that the lights are serviceable for the operations in progress.

E.    Nonvisual (Electronic) Aids. Ground- or space-based systems that provide electronic guidance must provide the quality of guidance (flight-inspected course structure), integrity (degree of trust that can be placed on the accuracy of the guidance), and continuity of service (protection against loss of signal) appropriate to the category of the operation being conducted (CAT I/CAT II/CAT II). Systems used operations using approaches with vertical guidance must provide acceptable flightpath angles and acceptable TCHs. A classification system has been established through ICAO for ground-based electronic systems used for approaches with vertical guidance.

Indicates new/changed information.
1)    This classification system reflects the ground-based system configuration, course quality, integrity, and continuity of service capabilities. Since the electronic aids provide such a critical function, pilots conducting takeoff or landing operations must be notified immediately of any changes in system status or of any malfunctions or failures. To meet this requirement, all facilities associated with ILS ground equipment must be constantly monitored by ATC or other appropriate personnel.
2)    The required levels of reliability, integrity, and continuity of service for these facilities are usually provided by automatic electronic monitoring systems, online standby equipment (backup transmitters), duplication of key functions, and secondary power supplies.

F.    Secondary Power Supplies. Secondary power sources (standby power supplies) are essential for ensuring that visual aids, electronic aids, meteorological reporting systems, and communication facilities continue to function, even if the main source of power is interrupted. Loss of power to these systems becomes more critical as seeing-conditions deteriorate. Therefore, as conditions change from CAT I to CAT II or CAT III, the levels of required redundancy increase, and standby power switchover times decrease. Secondary power supply requirements are established in ICAO Annexes 10 and 14, and in various FAA orders and ACs.

RESERVED. Paragraphs 4-172 through 4-186.