Federal Aviation
Administration

MMEL Policy Letter (PL) 67, Revision 4

Date:

January 15, 2012

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

Subject:

Windshear Warning and flight Guidance System (RWS) Windshear Detection and Avoidance System (PWS)

MMEL CODE:

34 (NAVIGATION) or 22 (AUTO FLIGHT)

REFERENCE:

PL-67, Revision 3, dated December 5, 2005

PL-67, Revision 2, dated November 20, 2000

PL-67, Revision 1, dated August 15, 1997

PL-67, Original, dated December 23, 1993

14 CFR 121.358

PURPOSE:

To establish standardized Master Minimum Equipment List (MMEL) relief for RWS and PWS Systems.

DISCUSSION:

Revision 4 omits the Global Change (GC) designation for this PL.  Refer to PL-59 for post-GC policy.

Revision 3 adds limited relief when the PWS and the Ground Proximity Warning System (GPWS) Windshear Mode (Reactive) are inoperative at the same time.  Revision 3 also clarifies that the windshear systems may not be part of the GPWS or the Weather Radar system.

Revision 2 combined information and policy for approved airborne detection and avoidance systems (Predictive Windshear).

Revision 1 reformatted the original PL- 67 with no change to policy.

Title 14 of the Code of Federal Regulations (14 CFR) 121.358, sets forth requirements for part 121 operators to have their aircraft equipped with either an approved airborne RWS (reactive system), an approved airborne PWS (predictive system), or an approved combination of these systems.  This PL provides minimum equipment list relief for low-altitude windshear system on air carrier aircraft.

Some RWS (Reactive) have been designed to be a function of the GPWS.  While some PWS (Predictive) have been designed to be a function of the Weather Radar System, not all windshear system designs are a function of the GPWS or Weather Radar system.  The Flight Operations Evaluation Board (FOEB) Chairmen should determine the appropriate ATA chapter location for the MMEL provisions.

Windshear has contributed to aircraft accidents.  Windshear accidents peaked in the mid-1970's and early 1980's.  In the late 1980's, the FAA took steps to reduce the rate of windshear accidents by mandating windshear training and the implementation of RWS.  Although the combined effect of the training and RWS usage has significantly reduced the rate of windshear accidents, RWS does not provide visibility prior to entering windshear.  RWS provides flight crews with a way to positively identify a windshear after entering it, but does not provide a way to avoid it.

Ground based systems that can detect windshear before an aircraft enters it, such as Low Level Windshear Avoidance (LLWAS) and Terminal Doppler Weather Radar (TDWR), are limited to airports where they are installed.  Recent technology improvements have led to the development and introduction of airborne Doppler weather radar systems known as PWS that can detect windshear and provide warnings (aural and visual) before the airplane enters it.  Predictive windshear is expected to further reduce accidents attributed to windshear.

POLICY:

FOEB chairmen should provide MMEL relief for those aircraft that have low-altitude windshear system equipment installed in accordance with 14 CFR 121.358.  They should review the various windshear system designs applicable to the airplane model and list the MMEL provisions as appropriate.   If the RWS (Reactive) is a function of the GPWS (Windshear Mode), refer to MMEL Policy Letter 54.

The Principal Operations Inspector (POI) will ensure an operator's alternate procedures are comprehensive and appropriate for dispatch with Windshear Systems inoperative.  An operator's alternate procedures and preflight briefings must include and emphasize:

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

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

The following is the standard proviso assigned for the windshear system for Reactive and/or predictive systems:

WIND SHEAR DETECTION, GUIDANCE AND AVOIDANCE SYSTEM

 

34 (NAVIGATION) or

22 (AUTO FLIGHT)

Repair Interval

Number Installed

Number Required for Dispatch

Remarks or Exceptions

 

 

 

 

 

XX-X Windshear Warning and Flight Guidance System (Reactive)

***

B

  -

   0

(O) May be inoperative provided alternate procedures are established and used.

NOTE: Operator's alternate procedures should include reviewing windshear avoidance and windshear recovery procedures.

 

 

     C

    -

   0

(O) May be inoperative provided:

a) Alternate procedures are established and used, and

b) Windshear Detection and Avoidance system (Predictive) operates normally.

 

 

34 (NAVIGATION) or

22 (AUTO FLIGHT)

Repair Interval

Number Installed

Number Required for Dispatch

Remarks or Exceptions

 


 

 

 

 

 

 

XX-X Windshear Detection and Avoidance System (Predictive)

***

     B

    -

    0

(O) May be inoperative provided alternate procedures are established and used.

NOTE: Operator's alternate procedures should include reviewing windshear avoidance and windshear recovery procedures.

 

 

 

 

 

 

 

    C

    -

    0

(O) May be inoperative provided:

a) Alternate procedures are established and used, and

b) Windshear Warning and Guidance System (Reactive) operates normally.

 

 

 

 

 

INSTALLATIONS NOT REQUIRED BY CFR

 

 

 

 

 

XX-X Windshear Warning and Flight Guidance System (Reactive)

***

C

-

0

(O) May be inoperative provided alternate procedures are established and used.

 

 

 

 

 

 

 

XX-X Windshear Detection and Avoidance System (Predictive)

***

C

-

0

(O) May be inoperative provided alternate procedures are established and used.

 

 

 

 

 

 

Each FOEB Chairman should apply this Policy to affected MMELs through the normal FOEB process.

 

 

 

 

 

John S. Duncan

Manager, Air Transportation Division