VOLUME 4 AIRCRAFT EQUIPMENT AND OPERATIONAL AUTHORIZATION
CHAPTER 2 ALL WEATHER TERMINAL AREA OPERATIONS
Section 7 Safety Assurance System: Category III Operations
4-331 OVERVIEW. This section contains concepts, direction, and
guidance to be used by principal inspectors (PI) for evaluating and approving
or denying requests for authorization to conduct all Category (CAT) III operations.
This includes all CAT III operations at airports and runways new to an operator/program
manager, even though previously approved aircraft, airborne equipment, ground-based
equipment, concepts, and procedures are being used in these operations. Additional information may be found in
Volume 4, Chapter 2, Section 1.
4-332 APPLICABILITY. The intent is that the PI will use the general
information provided in this section, along with the specific information provided in
Volume 4, Chapter 2, Section 2 for
domestic operators and
Volume 4, Chapter 2, Section 8 for
Title 14 of the Code of Federal Regulations (14 CFR) part
air carriers when evaluating requests for CAT III operations. This process is only applicable for operator requests for instrument
landing system (ILS)-based CAT III operations. Requests for other than ILS-based
CAT III should be forwarded to the regional Flight Standards division (RFSD) Next Generation (NextGen) Branch (AXX-220).
4-333 REPORTING SYSTEMS.
A. Safety Assurance System (SAS). The PI will utilize SAS Data
Collection Tools (DCT) 2.2.2, Category II & III Ops, and 2.3.1, Appropriate Operational Equipment.
B. Program Tracking and Reporting Subsystem (PTRS). The PI will utilize PTRS activity codes.
· CAT II/III ILS Operations Phase I Approval: 1430.
· CAT II/III ILS Operations Phase II Approval: 1431.
· CAT II/III ILS Operations Phase III Approval: 1432.
· CAT II/III ILS Operations Phase IV Approval: 1433.
· CAT II/III ILS Operations Phase V Approval: 1434.
4-334 REFERENCES, FORMS, AND JOB AIDS.
A. References (current editions):
· Title 14 CFR Parts
· FAA Order
Landing System and Ancillary Electronic Component Configuration and Performance Requirements.
· FAA Order JO
· FAA Order JO
· FAA Order
Offset Instrument Approach (SOIA).
· FAA Order
for the Evaluation and Approval of Facilities for Special Authorization Category I Operations and All Category II and III Operations.
· Advisory Circular (AC)
Test Guide for Certification of Transport Category Airplanes.
Visual Range (RVR).
for Approval of CAT III Weather Minima for Takeoff, Landing and Rollout.
for Approval of Category I and Category II Weather Minima for Approach.
Movement Guidance and Control System.
Operating Procedures for Flight Deck Crewmembers.
· Technical Standard Orders (TSO).
· U.S. Flight Information Publications (FLIP).
B. Forms. None.
C. Job Aids. PIs of parts
will utilize DCTs 2.2.2, Category II & III Ops, and 2.3.1, Appropriate Operational Equipment.
A. Category (CAT) III Operations. CAT III operations are separated
into three subcategories: CAT IIIa, CAT IIIb, and CAT IIIc.
B. CAT IIIa Operations. CAT IIIa is an approach and landing operation
with a Runway Visual Range (RVR) of not less than 700 feet (200 meters) without
a decision height (DH), or with a DH of less than 100 feet (30 meters), or an
alert height (AH), which is typically between 50 and 200 feet, depending on
aircraft certification and operator preferences. Both fail passive (FP) and fail operational (FO) airborne equipment can be used in CAT IIIa operations.
C. 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. Presently,
the lowest FAA-approved minimum for any CAT III operations is RVR 300 (75 meters)
due to difficulties in accomplishing safe ground movement for both aircraft
and ground safety services, such as Aircraft Rescue and Fire Fighting (ARFF).
D. CAT IIIc Operations. CAT IIIc is an approach and operation
landing without a DH and without RVR limitations (zero-zero). CAT IIIc operations are currently not authorized.
4-336 APPROVAL METHOD.
91 and Part
of Deviation Authority (LODA) Holder. A part
91 (including part
91 subpart F) or
holder operator is issued a letter of authorization (LOA) for all CAT III operations using the C060 in Web-based Operations Safety System (WebOPSS).
91 Subpart K (Part
91K). A part
is issued a management specification (MSpec) for all CAT III operations using C060 in WebOPSS.
are issued operations specification (OpSpec) C060 for all CAT III operations (or H109 for helicopter operations) in WebOPSS.
A. Concepts, Direction, and Guidance. This section contains concepts,
direction, and guidance to be used by inspectors for evaluating and approving
or denying requests for authorization to conduct CAT III All Weather Terminal
Area Operations (AWTA). All CAT III operations using aircraft, airborne equipment,
ground‑based equipment, or concepts/procedures, which are new to a particular
operator, require approval. In addition, all CAT III operations at airports
and runways new to a particular operator require approval even though previously
approved aircraft, airborne equipment, ground-based equipment, concepts, and
procedures are used in those operations. This section contains an amplification
of the general concepts, policies, direction, and guidance covered in previous
sections of this chapter. Specific standards are provided for inspectors evaluating
CAT III AWTAs with airborne and ground-based equipment, which have well understood
operational characteristics and limitations. In cases where an operator requests
approval to conduct CAT III AWTAs using equipment, concepts, and/or procedures
not addressed in these standards, a request for policy and guidance must be
forwarded through the appropriate RFSD NextGen Branch (AXX-220) to the Flight Technologies and Procedures Division (AFS‑400).
B. Types of CAT III Operations. The only types of CAT III operations that
can be currently authorized (as of 2010) for use by U.S. operators are ILS based.
CAT III operations using Global Positioning System (GPS) Landing Systems (GLS),
which requires the use of a Ground Based Augmentation System (GBAS) or Local Area Augmentation System (LAAS), may also be approved in the near future.
C. Objective of CAT III Operations. The essential difference
between CAT III AWTAs and CAT I and II operations is that a CAT III operation
places a greater reliance on the guidance provided by the airborne and ground-based
guidance equipment. The guidance provided by the equipment must continue through
touchdown in CAT IIIa operations and through touchdown and rollout to a safe
taxi speed in CAT IIIb operations. In contrast to other types of operations,
CAT III operations do not ensure sufficient external visual cues for the pilot
to manually control the aircraft during flare and landing. The primary objective
of CAT III operations is to provide a level of safety equivalent to CAT I and
CAT II operations without the use of these visual cues. The desired level of safety for CAT III operations is achieved by the following enhancements:
1) The airborne equipment and ground-based equipment must ensure
increased precision in flightpath control. The increased reliability and precision
of flightpath control (as compared to CAT I systems) is achieved through highly
reliable and precise ground-based equipment and airborne systems. These systems
are capable of guiding the aircraft with significantly increased precision to touchdown or through rollout, as appropriate.
2) The aircraft performance and equipment requirements associated
with a missed approach from very low altitudes are enhanced to ensure that these
operations can be safely conducted even if a momentary touchdown occurs on the
runway after the go-around is initiated. Special criteria assures obstacle and terrain clearance to accommodate missed approaches from very low altitudes.
3) More stringent criteria are specified for the profile of the
pre-threshold terrain to ensure that the flight guidance and control systems function properly during the final stages of approach, flare, and landing.
4) Surface Movement Guidance and Control Systems (SMGCS) are
established to ensure the landing runway is protected from potential incursions and ensure the overall safety of the operation.
4-338 CAT III OPERATIONAL CONCEPTS. The weather and environmental conditions encountered in CAT III AWTAs
severely restrict seeing conditions. External visual reference is not acquired until the aircraft reaches a very low altitude. Typically,
external visual references begin to become available below 100 feet. Even though external visual references are usually available before touchdown,
the seeing conditions are not sufficient for the pilot to consistently perform a safe, manual landing. Therefore, the aircraft must be controlled by
instruments and special equipment throughout the approach, flare, and touchdown (deceleration for rotorcraft) in weather conditions as low as RVR 700
and through rollout to a safe taxi speed (air taxi or hover for rotorcraft) in weather conditions below RVR 700. Due to the reduced seeing conditions
and the hazards associated with a pilot’s attempts to manually maneuver the aircraft to landing in those seeing conditions, the precision of the flight
guidance and control system and the overall precision of flightpath control must provide these capabilities.
A. DH and AH. There are substantial differences between the DH
and AH concepts. All FP operations are conducted in accordance with the DH and
RVR concepts. All FO operations are normally conducted in accordance with the
AH and RVR concepts. DHs are only used with FO systems in very unique situations (refer to AC
and AH are never used together in any operation since the DH requires that external visual reference be established before passing a specified
point and AH does not. The very limited seeing-conditions available in CAT III
operations require additional criteria to ensure that an adequate level of safety is achieved and maintained when operating in this environment.
1) DH Operations. For FP operations using a DH, the pilot must
arrive at a decision before passing the DH to either permit the flight guidance
and control system to be used to touchdown or to execute a missed approach.
For a DH operation, the external visual information, the instrument information,
and the airborne system information must be integrated so the flightcrew can
make a definitive decision no later than arrival at the DH. The decision to
permit the flight guidance and control system to continue to direct the aircraft
to touchdown must be based on an assessment that the airborne systems have not
degraded in a way that affects FP capabilities, and the external visual cues
(touchdown zone (TDZ) lighting) confirm that the aircraft will touchdown within
the TDZ. Since all DHs used in CAT III operations must be 50 feet or less, the
inner marker (IM) cannot be used to define the CAT III DH. Since DH 50 is located
very near the runway threshold, the terrain profile within these areas is adequate
for CAT III radio altimeter (RA)-based DHs. CAT III operations are normally
conducted using a DH of 50 feet; however, a different DH may be operationally
approved in special circumstances, such as the use of a hybrid CAT III landing system.
2) AH Operations. For operations with an AH, the pilot must also
arrive at a decision, before passing the AH, to either permit the flight guidance
and control system to be used to touchdown or to execute a missed approach.
In direct contrast to operations with a DH, however, this decision must be based
on an assessment that the airborne systems have not degraded in a way that affects
FO capabilities. FO systems permit certain system failures below AH without
requiring a missed approach, because the aircraft is still capable of continuing
to a safe landing and rollout. External visual references are not required before touchdown in operations based on the AH concept.
B. Kinds of CAT III Operations. There are two different and distinct
kinds of CAT III operations: FP operations and FO operations.
1) FP operations use a DH of 50 feet (15 meters) and a TDZ RVR
no lower than RVR 600 (175 meters). In the event of a failure when using a FP
CAT III system, the flight guidance and control system will not disturb the
aircraft’s flightpath when it fails, and the flightcrew immediately receives
an aural and visual warning of system failure. Therefore, a DH must be used
to ensure that, before passing 50 feet above ground level (AGL), the flightcrew
establishes external visual reference with the TDZ to determine that the flight
guidance and control system is functioning properly and to ensure that the aircraft
will land within the TDZ. A go-around is mandatory if the flight guidance and
control system fails before touchdown during FP operations in CAT III weather conditions.
2) FO operations usually use an AH instead of a DH (refer to AC
the name implies, critical CAT III systems remain operational
even if some failures occur. FO systems are designed so that the system remains
fully operational following any failure or combination of failures that are
likely to occur after the aircraft passes AH. Therefore, there is no requirement
to establish external visual reference before touchdown to confirm that the
aircraft will land safely. The lowest minimum that may currently be approved
for any AWTA by a U.S. operator is RVR 300 (90 meters). This restriction is
based on the difficulties associated with aircraft movement on the taxiways,
ramps, and other maneuvering areas on the airport and the difficulties related to providing timely safety facilities and services (such as ARFF).
C. CAT III Operating Minimums. CAT III operating minimums are based on two criteria and are always the higher of the
1) Published ILS Minimums. The part
Instrument Approach Procedure (SIAP) criteria is based on the
integrity and reliability of the ILS ground system. When the ILS supports operation
to touchdown but not rollout, RVR 700 is the lowest authorized minimum. When
the ILS supports both touchdown and rollout operations, RVR 600 minimums are
typically published. If the ILS supports both touchdown and rollout operations
and the Localizer (LOC) is monitored to an even higher standard of reliability, RVR 300 minimums may also be published.
2) CAT III OpSpec/MSpec/LOA C060 Minimums. CAT III minimums in
C060 are based on the certification statement in the Aircraft Flight Manual
(AFM), and based on the airworthiness requirements contained in the appropriate revision of AC
3) AC 120-28C-Authorized Aircraft. Aircraft authorized under AC 120-28C and
earlier typically contain a statement authorizing CAT IIIa or CAT IIIb automatic approach and landing operations.
a) Aircraft authorized for CAT IIIa are currently limited to RVR 700 for
TDZ and RVR 700 for Mid Runway Rollout (MID) by regulation, unless the AFM also
contain statements authorizing FP landing systems. The aircraft may then be authorized RVR 600 for TDZ, RVR 600 for MID, and RVR 300 for rollout.
b) Aircraft authorized for CAT IIIb with a rollout control system that meets the criteria in AC 120‑28, Appendix
2 (FP rollout system) may be authorized minimums as low as RVR 400 for TDZ, RVR 400 for MID, and RVR 300 for rollout.
c) Aircraft authorized for CAT IIIb with a rollout control system that meets the criteria in AC 120‑28, Appendix
3 (FO rollout system) may be authorized minimums as low as RVR 300 for TDZ, RVR 300 for MID, and RVR 300 for rollout.
Aircraft. Aircraft authorized under AC
subsequent revisions contain a statement authorizing FP or FO landing and/or rollout control systems.
e) FP Landing System without Rollout System. Aircraft with an FP landing
system without a rollout system may be authorized minimums as low as RVR 600 for TDZ, RVR 600 for MID, and RVR 300 for rollout.
f) FP Landing System with Any Rollout System. Aircraft with a FP landing
and rollout system may be authorized minimums as low as RVR 600 for TDZ, RVR 400 for MID, and RVR 300 for rollout.
g) FO Landing System with a FP Rollout System. Aircraft with an FO landing
system and FP rollout systems may be authorized minimums as low as RVR 400 for TDZ, RVR 400 for MID, and RVR 300 for rollout RVR.
h) FO Landing System with a FO Rollout System. Aircraft with FO landing
and rollout systems may be authorized minimums as low as RVR 300 for TDZ, RVR 300 for MID, and RVR 300 for rollout RVR.
4-339 RA AND PRE-THRESHOLD TERRAIN.
A. Operation. The operation of almost all CAT III landing systems
is dependent on RA information during the latter stages of the landing. The
flare profile, the rate of descent at touchdown, and the distance of the touchdown
point from the threshold can be adversely affected by the profile of the terrain
immediately before the landing threshold. The terrain, which is most critical,
lies in an area approximately 200 feet on either side of the runway centerline
(RCL) extended from the threshold into the approach area to a distance of approximately 1,000 feet before the landing threshold.
B. Pre-threshold Terrain. At runways where the terrain beneath
the approach flightpath is not approximately level, abnormal autopilot (AP)
and/or flight director (FD) behavior may result from erroneous RA signals. Irregularities
in the pre-threshold terrain can have a major effect on the performance of the
landing systems required for CAT III operations (autoland or Head-Up Display
(HUD) to touchdown). A special operational test and evaluation program is required
before approving any CAT III operations for any aircraft on these runways. This
test program is essential to ensure that CAT III operations can be safely conducted
on these runways with a particular CAT III aircraft type (aircraft with similar
flight characteristics and similar flight guidance and control systems). This special test and evaluation must be accomplished in nonrevenue service.
C. Requests for Evaluation. All requests to conduct this evaluation
must be sent to the RFSD NextGen Branch (AXX-220) for approval. Inspectors shall
not, under any circumstances, authorize any CAT III operations with any aircraft
to these runways unless that particular CAT III type aircraft has been specifically evaluated in accordance with AC
specifically approved for that operation by the RFSD. The Flight Operations Branch (AFS-410) maintains a list of these special terrain
runways which must be authorized in OpSpec/MSpec/LOA C059 and/or C060 to utilize
any CAT II/III minimums that require the use of autoland or HUD to touchdown.
This document lists all CAT II/III special terrain runways in the United States,
as well as all approved aircraft make, model, and series (M/M/S) for each runway. This list is available on the AFS-410 public Web site at
Volume 4, Chapter 2, Section 2 and AC
more information about authorizing CAT II/III operations at special terrain runways.
D. Runways. A few CAT II runways currently have restrictions
(i.e., Radio Altitude minimums Not Authorized (RA NA)) due to the pre-threshold
terrain profile, which prohibits the use of RA information to determine the
CAT II DH. Standard CAT II operations (DH 100 and RVR 1200) can still be conducted
by using the IM to determine arrival at the DH. Although the RA cannot be used
to accurately and reliably determine arrival at the CAT II DH (height above
touchdown (HAT) 100), it may still be possible to safely conduct CAT III operations
to these runways if the pre-threshold terrain does not adversely affect the
performance of the CAT III landing system. For example, Pittsburgh International
Airport (KPIT) runway 10L has a CAT II approach with “RA NA” minimums (RA DH
not authorized), but certain aircraft have been approved for CAT III autoland operations.
4-340 FUTURE REDUCTIONS TO LANDING MINIMUMS. The lowest landing
minimum currently authorized for CAT III operations by U.S. operators at any
airport and by foreign air carriers in the United States is RVR 300. This restriction
is related primarily to problems associated with aircraft taxi operations and
the difficulty of providing adequate safety services (such as crash, fire, rescue,
and collision prevention) when operating in seeing-conditions less than RVR
300. The lowest minimum currently authorized for foreign air carriers outside
the United States is 75 meters (RVR 300) and is based on an operational determination
similar to the U.S. RVR 300 decision. Future reductions in landing minimums
are unlikely until technology permits the development of economically viable
capabilities to adequately resolve these limitations. Presently potential solutions
appear to be enhancements in airborne equipment such as forward-looking infrared or millimeter wave radar technologies.
4-341 FOREIGN CAT III INSTRUMENT APPROACH PROCEDURES (IAP).
A. Operations. All CAT III operations by U.S. operators at foreign
airports must be authorized in accordance with the guidance and direction in
this order. All foreign runways approved for CAT III operations by U.S. operators
must also meet the requirements of this order and the current edition of
8260.31, Foreign Terminal Instrument Procedures.
B. Foreign Equivalence. Although it is recognized that the CAT
III ground-based systems and procedures at foreign airports may not be in exact
accordance with U.S. standards, it is critical for the foreign airports to provide
the information and functions that are necessary for CAT III operations in a manner consistent with the intent of U.S. CAT III standards.
C. General. The FAA region, which has surveillance responsibility
for a particular foreign airport, has the responsibility for evaluating any
runway at that airport which supports CAT III operations. The controlling region
is also responsible for evaluating these CAT III runways in accordance with Order
for recommending approval or disapproval of CAT III operations to those
runways. The controlling region’s recommendations must be forwarded to AFS-400 for further evaluation and final approval or disapproval.
Note: Foreign runways that have been determined to be equivalent to
U.S. CAT III standards and approval for CAT III operators are identified in
the list of runways maintained by AFS-400. Operators desiring CAT III approvals
at foreign airports that are not on this approved list should submit a request
for evaluation and approval through their principal operations inspector (POI)
to the RFSD NextGen Branch. The controlling region’s NextGen Branch will conduct
an evaluation. Upon successful completion of the evaluation, the NextGen Branch will notify AFS-400 for final approval and addition to the approved list.
D. Determining Equivalence. The major factor that is considered
by the controlling region in approving foreign runways in accordance with Order
CAT III operations by U.S. operators is the degree of equivalence with U.S. CAT III standards. This determination evaluates the equivalence of:
· High-intensity approach lights;
· High-intensity runway edge lights;
· TDZ and RCL lights;
· Taxiway edge lights;
· High-intensity taxiway centerline lights;
· Runway markings;
· The quality and integrity of the approach and landing
ground-based guidance system;
· The RVR reporting capabilities and procedures;
· ILS critical areas, including signs and markings;
· Obstacle clearance protection in the approach and missed
approach, including the obstacle-free zone;
· Airport surface traffic control;
· Terminal area air traffic control (ATC); and
· Procedures for regulating the ground movement of aircraft and
vehicles during CAT III operations.
E. Authorizing Foreign CAT III Runways. All CAT III operations
conducted in foreign countries must be conducted in accordance with the operator’s
CAT III OpSpecs. The only means of approving these operations is through the
issuance of OpSpecs, which specify the foreign runways and minimums authorized
for each aircraft used by the operator. OpSpecs C060 and H109 specify the CAT
III IAPs, runways, operating minimums, limitations, and aircraft authorized
for CAT III operations for a particular operator. These paragraphs also specify
the CAT III airborne equipment, RVR equipment, pilot qualification, and missed
approach requirements that apply to the operator’s CAT III operation. If the
flightcrew is properly qualified and the aircraft is properly equipped and maintained,
an operator can, in general, be authorized to conduct CAT III AWTAs to any foreign
runway approved for CAT III operations. The exception to this are those runways specified as restricted CAT III runways. The criteria in AC
this order establish the lowest approach and landing minimums
that can be authorized, under any circumstances, for CAT III operations. The
lowest minimums for each kind of CAT III operation for a particular operator
are specified in OpSpecs C060 and H109, as appropriate. FAA inspectors shall
not authorize an operator to use minimums lower than these values. Additionally,
inspectors shall not authorize the use of CAT III minimums for any foreign runway unless the provision of this order and Order
8260.31 are met.
4-342 CAT I AUTOLAND OR HUD TO TOUCHDOWN OPERATIONS. Autoland or
HUD to touchdown operations are required for all CAT III operations, and many
operators use autoland or HUD for CAT II, CAT I, and visual flight rules (VFR) operations as well. Part
125.329(d); and part
the use of autoland or HUD to touchdown in any operation unless the operator is specifically authorized via OpSpecs. OpSpec C059 and
C060 authorize autoland or HUD to touchdown in CAT II and CAT III operations,
respectively. OpSpec C061 or H110 authorizes autoland operations in other than
CAT II/III operations and OpSpec C062 or H111 authorizes HUD to touchdown in other than CAT II/III operations.
A. ILS Category Classification. The ILS category classification
system provides a more comprehensive method of describing ILS performance than
the simple CAT I/II/III classification. ILS facility classification is described in Order
facility’s classification is defined by using two characters (I/C, II/D, III/E,
etc.). facility’s classification is defined by using two characters (I/C, II/D, III/E, etc.).
1) The first character indicates conformance to the facility
performance category standards. This character indicates if the ground equipment is classified as a CAT I, CAT II, or CAT III ILS.
2) The second character defines the ILS point (Figure 4-21A,
Localizer Course and Glidepath Bend Amplitude Limits) to which the LOC conforms
to the Facility Performance CAT III course structure tolerances. These classifications
indicate ILS conformance to a physical location on the approach or runway as follows:
· A: 4 NM before the threshold.
· B: 3,500 feet before the threshold (CAT I decision point).
· C: Glidepath altitude of 100 feet height above threshold (HATh)
(CAT II decision point).
· T: Threshold.
· D: 3,000 feet beyond the threshold (CAT III requirement only).
· E: 2,000 feet before the runway end (CAT III requirement only).
Figure 4-21A. Localizer Course and Glidepath Bend Amplitude Limits
B. Use of Autoland at U.S. CAT I Facilities or Equivalent. For
CAT I, autoland may typically be used at runways with facilities other than
those with published CAT II or III IAPs. This is to aid pilots in achieving
stabilized approaches and reliable touchdown performance to improve landing
safety in adverse weather; for CAT II or III training; to exercise the airborne
system to ensure suitable performance; for maintenance checks; or for other
such reasons. Use of this capability may be particularly important for pilot
workload relief in stressful conditions of fatigue after long international
flights; night approaches; crosswinds or turbulence; when there may be other
aircraft non-normal conditions being addressed; or to aid safe landing performance
in otherwise adverse weather, in restricted visibility, or with cluttered runways.
This is true even though reported visibility may be well above minimums (e.g.,
heavy rain distorting view out the windshield, snow covered runways where markings are not easily visible, etc.).
C. ILS Classification and CAT I Autoland Operations. ILS classification
is being added to the Airport/Facility Directory (A/FD), but not all ILS runways
are included as of August 2010. To support autoland or HUD to touchdown operations,
CAT III course structure tolerances to at least point D are preferred (e.g.,
I/D, II/D, III/D). In 2010, all published CAT II and III approaches in the United
States meet this standard. CAT II approaches that do not support autoland operations
will note this limitation via a chart note or Notice to Airmen (NOTAM). When
conducting autoland operations on a CAT I runway/ILS, runways with an I/D or
I/E classification are the most suitable and are preferred. Practice autoland
or HUD to touchdown operations may be conducted at CAT I runways in CAT I or
better weather conditions where the facility classification is unknown, because
the flightcrew is monitoring system performance, visually verifying the position
of the aircraft, and can determine whether to continue to a landing or execute
a missed approach in CAT I or better weather conditions. Though it is not recommended,
flightcrew monitoring and increased visibility also permits practice autoland
or HUD to touchdown operations using an ILS classified as I/A, I/B, I/C, or
I/T, provided the operation is performed in VFR conditions. The FAA maintains an information-only list of ILS classification status at
D. Pre-Threshold Terrain.
1) AFS-410 maintains a list of CAT II/III runways with special
terrain which may affect autoland or HUD to touchdown operations, such as irregular pre-threshold terrain or TDZ slope. This list is available at
operator and aircraft must be approved for each special terrain runway to conduct any CAT II or III operations using autoland or HUD to touchdown.
Volume 4, Chapter 2, Section 2 contains
more information about how to authorize CAT II/III operations at special terrain runways.
2) The FAA does not analyze CAT I runways to determine if any
irregular pre-threshold terrain will impact autoland or HUD to touchdown operations.
Practice autoland or HUD to touchdown operations may be conducted at CAT I runways
where the pre-threshold terrain has not been analyzed because the flightcrew
is monitoring system performance, visually verifying the position of the aircraft,
and can determine whether to continue to a landing or execute a missed approach
in VFR conditions. All operators approved to use autoland- or HUD-equipped aircraft
should be encouraged to routinely use these systems at suitably equipped runways
during operations in VFR and in CAT I instrument flight rules (IFR) conditions.
Flightcrew training should emphasize the importance of monitoring equipment performance on all practice autolands.
E. Maintenance Return to Service and Required Practice Autolands.
An aircraft manufacturer and certification requirements may require that a practice
autoland or HUD to touchdown be performed on a published CAT II or III approach.
If so, the operator should adhere to these requirements. If autoland is not
required to be performed on a CAT II ILS, it is important to note that an unsatisfactory
approach is extremely difficult to attribute to small errors in ILS ground equipment.
Even CAT I ILS facilities that meet CAT III signal standards are not monitored
to the same tolerances as CAT II/III facilities. An unsatisfactory approach
due to a critical error incursion is something that may be identified, but an
unsatisfactory approach due to a signal or monitor error cannot be detected by the flightcrew or maintenance.
F. Flightcrew Training. In addition to other training requirements,
flightcrew training should emphasize the importance of:
1) Monitoring equipment performance and visual verification of aircraft position on all practice autolands.
2) Verifying that the CAT I approach does not have any charted
restrictions, which would prohibit autoland or HUD to touchdown operations (“ILS
unusable within 0.5 DME,” “autopilot coupled approach not authorized below XXX feet”).
3) Requesting that ATC protect the critical area for all practice
autolands. ATC will protect the ILS critical areas if the ceiling is less than 800 feet and/or the visibility is less than 2 miles.
4) Performing maintenance return to service as required by either
the manufacturer, certification, or the operator, as described in subparagraph 4-342E.
4-343 CAT II/III ILS OUTAGES AND CONTINUITY OF SERVICE (CoS) REQUIREMENTS.
A. Purpose. CAT II/III operations rely on high-integrity LOC and glideslope (GS) signals with a low probability radiated signal
interruption (CoS). ILS design and approval criteria address required system integrity. The current edition of FAA Order JO
Landing System Continuity of Service Requirements and Procedures, contains the policy and requirements for Technical Operations (Tech Ops) to meet
CoS requirements for CAT II/III ILS facilities, including CAT II/III ILS downgrade and reinstatement criteria. The RFSD NextGen Branch (AXX-220)
is the Flight Standards Service (AFS) point of contact (POC) for ILS CoS issues. AFS-400 is responsible for ILS CoS policy, guidance, and support.
This section summarizes ILS CoS requirements relevant to inspectors and summarizes some of the relevant Tech Ops requirements. Refer to Order JO
current ILS CoS criteria, Order
6750.24, and Order
8400.13 for current AFS ILS facility requirements.
1) Continuity of Service (CoS). The quality that relates to the
rarity of radiated signal interruptions. The level of CoS of the LOC or the
glidepath is expressed in terms of the probability of not losing the radiated guidance signals.
2) CoS Outage. Any reportable LOC or GS unscheduled interruption
for which there was an unexpected cessation of the radiated signal, regardless
of cause, or any failure of a LOC or GS to properly initiate radiation, such as when an ILS fails to radiate upon a runway interlock selection.
3) CoS Mean Time Between Outages (MTBO). Average number of hours between countable CoS outages, considering only the
last five in-service periods. This MTBO calculation does not include consideration
of all interruptions as is typical in National Airspace Performance Reporting
System (NAPRS) reporting. Rather, it represents the MTBO outlined in International Civil Aviation Organization (ICAO) Annex 10.
4) Instrument Landing System (ILS) CoS Office of Primary Responsibility
(OPR). A single, functional entity within the technical services organization
of each service area, consisting of one or more designated ILS experts. The
ILS CoS OPR is responsible for analyzing and monitoring ILS CoS throughout the
entire service area and recommending actions to best ensure that MTBO requirements are met.
5) Predicted MTBO. The CoS MTBO if an outage occurred at the time of the calculation.
6) Projected MTBO. The CoS MTBO projected out 3 months from the predicted MTBO.
C. ILS Outage and Downgrade Requirements. Tech Ops maintains
a comprehensive database, which contains the outage history and up-to-date MTBO for all ILS facilities in the United States. Order JO
an ILS facility to be downgraded based on the number of outages in the preceding 6
months. As a result, Tech Ops and Navigation Services escalate their priorities
and involvement in repairs at facilities that accrue multiple outages over a short period of time.
1) When Tech Ops downgrades a facility, the corresponding ILS
landing minimum is also taken out of service via NOTAM:
a) Fourth outage in the preceding 6 months: CAT III RVR 300 minimum not authorized.
b) Fifth outage in the preceding 6 months: All CAT III minimums not authorized.
c) Sixth outage in the preceding 6 months: All CAT II and CAT III minimums not authorized.
2) This outage count applies to the separate LOC and GS systems,
not the sum of the outages for each system. For example, a LOC which has its
fifth outage in the preceding 6 months will lose all CAT III minimums. An ILS
with three LOC outages and three GS outages is not downgraded, because neither
the LOC nor the GS individually have four or more outages in the preceding 6 months. (See Table 4-8, Outage Consequences.)
3) Order JO
clear that downgrades are required as a result of excessive outages in the preceding 6 months. Tech Ops will notify
the appropriate NextGen Branch (AXX-220) of a downgrade due to outages, and
will not waive any ILS downgrade requirements except as allowed in Order JO
to the strong emphasis on system reliability to support CAT II/III
landing operations, the RFSD NextGen Branch should not consider waivers to ILS downgrades as a result of excessive outages.
D. ILS Reinstatement After a Downgrade. To reinstate an ILS landing
minimum, Tech Ops repairs the ILS facility, verifies CAT II or III capability
(as applicable), and first restores CAT I landing minimums. After the system
demonstrates sufficient reliability by operating outage free for a fixed period
of time, lower landing minimums are then reinstated as authorized in Order JO
any downgrade, the ILS must operate without an outage for 4 weeks to restore the next lower landing minimum. To restore each additional lower
landing minimum, the system must operate outage free for an additional 2 weeks
for each minimum to reinstate. See Table 4-8A, Instrument Landing System Reinstatement Periods, for examples of ILS reinstatement after a downgrade.
E. MTBO Requirements. Unlike downgrades as a result of excessive outages in the preceding 6 months, Order JO
not require a downgrade if a LOC or GS MTBO falls below the hours required in Order
6750.24 and Order
a downgrade is at the discretion of AFS, in coordination with Tech Ops. This is because MTBO is very useful as a trend analysis
number, but averaging the time periods between outages can be misleading in specific circumstances.
1) The calculation of MTBO is the average number of hours between
countable CoS outages, considering only the last five in-service periods, as shown in Figure 4-22, Active Periods Between Outages.
2) While MTBO provides a very useful tool for trend analysis,
very large or very small numbers can skew the entire MTBO up or down respectively.
Table 4-8B, Mean Time Between Outages—False Positive, and Table 4-8C, Mean Time
Between Outages—False Negative, show an example of MTBO depicting a false positive and false negative when considering the reliability of an ILS component.
a) In Table 4-8B, a very large outage-free Period 5 raises the MTBO on a
system which has incurred four outages in the preceding 500 hours of operation.
Four outages in 500 hours clearly shows faltering reliability, but the MTBO
figure itself shows a system suitable for CAT III RVR 600 operations. If this
system has one more outage, the 10,000-hour period would no longer be in the preceding five periods, and the MTBO will drop substantially.
b) In Table 4-8C, the MTBO is decreased by four short periods caused by
four outages over a 300-hour period. However, the system has operated for 3,700
hours without an outage, showing a strong indication that those problems have
been repaired and increased system reliability. To get an MTBO of 4,000 hours
in Table 4-8C, the system would have to operate for an additional 16,000 hours without an outage (20,000 total hours divided by five outage periods).
3) Therefore, when Tech Ops informs the appropriate NextGen Branch
(AXX-220) of predicted or projected MTBO that do not meet the requirements in Order
may require a downgrade, it is important to discuss the facility’s history,
factors which may be affecting MTBO, outages which were attributed to a common
cause, repair history, trends affecting the MTBO, and the technician’s confidence
in both the repairs and the reliability of the system. When in doubt, the All
Weather Operations Specialist (AWOS) should consult AFS-410 before making a decision to downgrade the ILS.
F. CoS Analyze. Navigation Services created and maintains the
CoS Analyze program, which tracks all ILS facilities (LOC and GS) in the United
States, including outage history, MTBO, and repair information. CoS Analyze is available to AWOS at
Analyze can be used by AFS inspectors to track ILS outages and MTBO for their specific region. The ILS CoS OPRs have usage rights within the CoS
Analyze software to mitigate or exclude outages, add comments, and provide updates
to system downgrade status. All of this information is available to other users on a read-only basis.
Figure 4-22. Active Periods Between Outages
Table 4-8. Outage Consequences
CAT III RVR 300
CAT III RVR 300 N/A
CAT III N/A
CAT II/III N/A
CAT III RVR 700/600
CAT III N/A
CAT II/III N/A
CAT II N/A
Table 4-8A. Instrument Landing System Reinstatement Periods
CAT III RVR 300
2 weeks (6 total)
2 weeks (8 total)
CAT III RVR 300
2 weeks (6 total)
CAT III RVR 300
CAT III RVR 700 or 600
CAT III RVR 700 or 600
2 weeks (6 total)
CAT III RVR 700 or 600
Table 4-8B. Mean Time Between Outages—False Positive
Total Hours =
Table 4-8C. Mean Time Between Outages—False Negative
Total Hours =
RESERVED. Paragraphs 4-344 through 4-360.