Federal Aviation
Administration

 

MMEL Policy Letter (PL) 54, Revision 10 GC

 

Date:

October 31, 2005

 

To:

All Region Flight Standards Division Managers
All Aircraft Evaluation Group Managers

 

From:

Manager, Air Transportation Division, AFS-200

 

Reply To
Attn Of:

Manager, Technical Programs Branch, AFS-260

 

MMEL GLOBAL CHANGE (GC)

PL-54 is designated as GC-139

This GC is an approved addendum to all existing MMEL documents. Operators may seek use of the specific relief contained in this PL by revising their Minimum Equipment List (MEL). In doing so, each applicable sample proviso stating the relief in this PL must be copied verbatim in the operator’s MEL. Approval of a revised MEL is gained utilizing established procedures, through the Operator’s assigned Principal Operations Inspector (POI).

 

Subject:

Terrain Awareness and Warning System (TAWS)

 

MMEL CODE:

34 (NAVIGATION)

 

REFERENCE:

 

Revision 9, dated May 26, 2005

Revision 8, dated March 10, 2005

Revision 7, dated October 15, 2001

Revision 6, dated January 19, 2001

Revision 5, dated September 29, 1999

Revision 4, Subj: GPWS, dated January 12, 1998

Revision 3, Subj: GPWS, dated August 15, 1997

Revision 2, Subj: GPWS, dated April 1, 1993

Revision 1, Subj: GPWS, dated July 27, 1992

Original, Subj: GPWS, dated April 10, 1991

 

 

PURPOSE:

To provide policy for Ground Proximity Warning System (GPWS) and Terrain Awareness and Warning System (TAWS) Master Minimum Equipment List (MMEL) requirements.

DISCUSSION:

Revision 10 added limited relief for the GPWS Windshear Mode inoperative for Class A TAWS equipment, revised sub item numbers for the Terrain functions/displays and revised the titles used for Class C TAWS equipment.

Revision 9 clarified that the TAWS relief allowed is for the Class of TAWS equipment required, not the Class of TAWS equipment installed. References to TSO C151 were deleted.

Revision 8 was based on changes to the FARs for TAWS that became effective 29 March 2005. Relief was reorganized under the different Classes of TAWS equipment required by 14 CFR. Added “Reactive” and “Predictive” to windshear systems for clarification and added relief for optional Runway Awareness & Advisory System (RAAS) feature. Moved revision highlights that were under “Policy” to the “Discussion” and added historical highlights for revisions 2, 3 and 4.

Revision 7 provides guidance for aircraft that are not required by 14CFR to have GPWS/TAWS. For aircraft equipped with GPWS/TAWS not required by 14 CFR, the repair interval category C will be assigned for system/systems.

Revision 6 revised windshear mode provisions as defined in PL-67.

Revision 5 introduced TAWS language as acknowledged by the FAA and replaces Enhanced EGPWS references with TAWS. Information added for GPWS and TAWS provides system background and relates system input and output requirements to MMEL provisions.

Revision 4: The previous policy for an inoperable Ground Proximity Warning System (GPWS) provided relief for the loss of certain modes of the GPWS. However, it did not address relief for the entire GPWS inoperative. This revision clarifies the intended relief when all GPWS modes are inoperative. Policy remains a category A, two-day repair period for the GPWS modes 1-4 and the test mode. Glideslope Deviation (mode 5) remains a category B item. Altitude advisory/callouts, and windshear mode remain category C items. Enhanced GPWS is designated category C.

Revision 3 retains a category A, two-day repair period for entire GPWS, the GPWS modes 1-4, and the test mode. Glideslope Deviation (mode 5) remains a category B item. Altitude advisory/callouts, and windshear mode remain category C items. Enhanced GPWS designated time limit is category C.

Revision 2: The revised GPWS policy would require a category A repair interval for the GPWS modes 1-4 and the test mode, with a specified two-flight day period. Glideslope Deviation (mode 5) would be a category B item. The remaining modes (altitude advisory/callouts, and windshear mode) would be category C items.

Controlled flight into the terrain (CFIT) has historically been a major contributor to aircraft hull loss. The introduction of GPWS, a reactive system providing visual and aural warnings when the aircraft enters an unsafe flight path has reduced CFIT events.

Conditions GPWS recognize and provides reactive warnings for are:

- excessive rate of descent,

- aircraft approaching rising terrain,

- takeoff altitude loss,

- unsafe terrain clearance,

- deviation below glideslope,

- descent below decision height, and

- windshear

TAWS was introduced in 1997 to further reduce CFIT events and is anticipated to be adopted as required equipment. It provides Terrain Clearance Floor and Terrain Awareness Alerting and Display functions. GPWS and TAWS are housed in the same unit under current manufacturer design; however, they function independently of each other. TAWS functions provides pilots a display of the terrain to which they are approaching relative to their altitude by utilizing a worldwide runway and terrain database, as well as aircraft position (from Flight Management Systems or Global Positioning System). The Electronic Flight Information Systems or Weather Radar displays are used to show terrain to which the aircraft is approaching. Different intensities (light, medium, and heavy) are used on the display depending on distance in relative altitude to the aircraft. In addition, if alerts are provided to the crew and no evasive action is indicated, displays change from green dotted to solid bright yellow to solid bright red.

 

 



 

POLICY:   .

The standardized MMEL format for GPWS and TAWS is provided below. The title for TAWS should be customized to reflect the manufacturer’s title for the particular equipment (e.g., Collins title is Enhanced GPWS).

Flight Operations Evaluation Board (FOEB) chairmen and principal operation inspectors (POI) should consider subordinate components (switches, lights, etc.) to be listed as part of the MMEL (or operator's MEL relief under the authority of the system deferral).

The POI will ensure an operator's alternate procedures are comprehensive and appropriate for dispatch with Windshear modes or functions of TAWS inoperative. An operator's alternate procedures and preflight briefings must include and emphasize:

Use of established procedures to assess and minimize the probability of encountering windshear during takeoff/departure and approach/landing.

Use of established procedures (windshear escape/recovery maneuvers) to minimize the effects of unexpected windshear encounter during takeoff/departure and approach/landing.

 

 

ATA 34 NAVIGATION

 

 

 

 

 

Class A TAWS Equipment Required

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1)   GPWS                 A          1          0          (O) May be inoperative provided:

                                              a)   Alternate procedures are

                                                                                                   established and used, and

                                                                                             b)   Repairs are made within

                                                                                                   two flight days .

 

    a)   Modes 1-4                          A          4          0          (O) May be inoperative provided:

                                                                                             a)   Alternate procedures are

                                                                                                   established and used,

                                                                                             b)   Repairs are made within

                                                                                                   two flight days.

 

    b)   Test Mode                          A          1          0          May be inoperative provided:

                                                                                             a)   GPWS is considered

                                                                                                   inoperative, and

                                                                                             b)   Repairs are made within

                                                                                                   two flight days.

 

    c)   Glideslope                          C          -           1

          Deviation(s)

          Mode 5)                             B          -           0

 

    d)   Advisory                             B          -           0          (O) May be inoperative provided

          Callouts                                                                 alternate procedures are

                                                                                       established and used.

 

                                                   C          -           0          (O) May be inoperative provided:

                                                                                             a)   Advisory callout not

                                                                                                   required by FAR, and

                                                                                             b)   Alternate procedures are

                                                                                                   established and used.

    e)   Windshear Mode                B          1          0          (O) May be inoperative provided

          (Reactive)                                                              alternate procedures are

          ***                                                                         established and used.

 

                                                                                       NOTE: Operator's alternate

                                                                                       procedures should include

                                                                                       reviewing windshear

                                                                                       avoidance and windshear

                                                                                       recovery procedures.

 

                                                   C          1          0          (O) May be inoperative provided:

                                                                                             a)   Alternate procedures are

                                                                                                   established and used, and

                                                                                             b)   Windshear Detection and

                                                                                                   Avoidance System

                                                                                                   (Predictive) operates normally.

 

2)   Terrain System -                     B          1          0          (O) May be inoperative provided

      Forward Looking                                                         alternate procedures are established

      Terrain Avoidance                                                        and used.

      (FLTA) And

      Premature Descent

      Alert (PDA)

      Functions

 

3)   Terrain Displays                      C          -           1

 

                                                   B          -           0

 

4)   Runway Awareness                C          1          0

      & Advisory System

      (RAAS)

      ***

 

Class B TAWS Equipment Required

 

1)   GPWS                                   A          1          0          (O) May be inoperative provided:

                                                                                             a)   Alternate procedures are

                                                                                                   established and used, and

                                                                                             b)   Repairs are made within two flight days.

 

    a)   Modes 1 & 3                      A          2          0          (O) May be inoperative provided:

                                                                                             a)   Alternate procedures are

                                                                                                   established and used, and

                                                                                             b)   Repairs are made within two flight days.

 

    b)   Test Mode                          A          1          0          May be inoperative provided:

                                                                                             a)   GPWS is considered

                                                                                                   inoperative, and

                                                                                             b)   Repairs are made within

                                                                                                   two flight days.

 

    c)   Modes 2, 4 & 5                   C          3          0

          ***

    d)   Advisory Callouts                B          -           0          (O) May be inoperative provided

                                                                                       alternate procedures are

                                                                                       established and used.

 

                                                   C          -           0          (O) May be inoperative provided:

                                                                                             a)   Advisory callout not

                                                                                                   required by FAR, and

                                                                                             b)   Alternate procedures are

                                                                                                   established and used.

 

    e)   Windshear Mode                C          1          0          (O) May be inoperative provided

          (Reactive)                                                              alternate procedures are

          ***                                                                         established and used.

 

2)   Terrain System -                     B          1          0

      Forward Looking

      Terrain Avoidance

      (FLTA) And Premature

      Descent Alert (PDA)

      Function

 

3)   Terrain Displays                      C          -           0

      ***

 

4)   Runway Awareness                C          1          0

      & Advisory System

      (RAAS

      ***

 

Class C TAWS Equipment

TAWS/GPWS                              C          1          0          (O) May be inoperative provided

***                                                                                   alternate procedures are

                                                                                       established and used.

 

                                                                                       Note: Any mode that operates normally

                                                                                       may be used.

 

 

 

 

 

Thomas K. Toula for

Manager, Air Transport Division