ATA 49 Auxiliary Power


Procedures Required

(O) – An operational procedure
(M) or (M#) – A maintenance procedure
(O)(M) or (O)(M#) – Both operational and maintenance procedures

Repair Interval Categories

Category A

Items in this category shall be repaired within the time interval specified in the "Remarks or Exceptions" column of the air operator's approved MEL. Whenever the proviso in the "Remarks or Exceptions" column of the MMEL states cycles or flight time, the time interval begins with the next flight. Whenever the time interval is listed as flight days, the time interval begins on the flight day following the day of discovery.

Time Limited Dispatch - Some MMEL's for aircraft that are equipped with FADEC engines have relief that is subject to time limited dispatch expressed as a specific number of engine hours, and will start in accordance with the times established by the engine manufacturer or as indicated in the remarks column of the MMEL. Time limited relief cannot be extended.

Category B

Items in this category shall be repaired within 3 consecutive calendar days excluding the day of discovery.

Category C

Items in this category shall be repaired within 10 consecutive calendar days, excluding the day of discovery.

Category D

Items in this category shall be repaired within 120 consecutive calendar days, excluding the day of discovery. To be considered for placement in Category D, the item must be of an optional nature, or excess equipment which an air operator may, at his/her discretion, deactivate, remove from or install on an aircraft.

To be approved for Category D, the item must meet the following criteria:

  1. the absence of the item does not adversely affect crew member's workload;
  2. the crew members do not rely on the function of that item on a routine or continuous basis; and,
  3. the crew members' training, subsequent habit patterns and procedures do not rely on the use of that item.

Category D relief will generally not be approved for equipment which is considered to increase the level of safety, even if that equipment is of an optional nature.


System and Sequence No. Item Repair
Remarks or Exceptions
Auxiliary Power Unit (APU) C/D 1 0 May be inoperative. (See Discussion for ETOPS considerations)


References:  TP 6327, FAA PL 40 (Rev. 1, 15 Aug. 1997)

Because of the many variables to be considered in granting relief for an APU, it may be appropriate to include a NOTE in the MMEL to the effect that: Based on specific operational considerations the MEL may be more restrictive.

Assignment of a category C or D must be made in the context of:

1. optional/standard installation,
2. air and/or ground use, and
3. crew dependency.

Relief may be prohibited for ETOPS if the APU has been determined to be essential equipment during the ETOPS approval process (e.g. A 310).

Other considerations might be the need to de-ice with the engines off and the need for an APU to subsequently start the engines.

It is intended that foreign MMEL relief will be accepted.

Relief for APU generated electrics, air, etc. should be the same category as the APU.  It may also be necessary to review relief for APU fire protection in Section 26.

FAA Differences:  FAA relief is identical to TCCA.

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