ATA 77 Engine Indicating

LEGEND

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.

ITEM:  77-00-1  ENGINE INSTRUMENTS

System and Sequence No. Item Repair
Interval
Category
Number
Installed
Number
required
for
dispatch
Procedures
required
Remarks or Exceptions
Engine Instruments

NOTE:
See 77-10-1 PRIMARY POWER SETTING INSTRUMENTS

C - -

For primary (NH (engine core), NP (propeller), ITT, etc.) and secondary (Oil Temp, Oil Press, etc.) engine indicators which have both analogue and digital readouts, the digital displays may be inoperative.

For the secondary indicators which have both analogue and digital readouts, the analogue displays may be inoperative provided the digital display uses some method such as colour changes to incorporate limitations.

C - - The N2 indicator, of a three spool engine, may be inoperative when the speed performs no control function and is effectively limited by the basic engine control and operation.

DISCUSSION:

References:  FAA PL 6 (Rev. 1, 15 Aug. 1997), PL 13 (Rev. 1, 15 Aug. 1997), PL 38 (Rev. 1, 15 Aug. 1997)

Analogue indications provide the best rate information to assist in not exceeding limitations.

From a human factors perspective it is also easier to match analogue indicators (reduced work load) rather than having to interpret digital information.  Some aircraft such as the RJ have only a digital presentation of oil pressure and oil temperature.

The A320 only presents oil pressure information to the pilot during the start, and subsequently, if some limitation is reached.

The information is available on a system synoptic page.

FAA Differences:  FAA relief is similar to TCCA.

ITEM:  77-10-1  PRIMARY POWER SETTING INSTRUMENTS (THREE/FOUR ENGINE AIRCRAFT)

System and Sequence No. Item Repair
Interval
Category
Number
Installed
Number
required
for
dispatch
Procedures
required
Remarks or Exceptions
Primary Power Setting Indicators B - - Relief for a primary power setting indicator (TQ (torque), N1 (fan), N2 (engine core), EPR (engine pressure ratio)) can be permitted (three/four engine aircraft) provided alternate means of setting power are certified.

DISCUSSION:

References:  AWM 525.1305(d)(1), FAA PL 38 (Rev. 1, 15 Aug. 1997)

Any relief would only be permitted if another instrument on the affected engine is also certified as a means of complying with the design standard, that is, the MMEL relief would be contingent on an airworthiness certification.

FAA Differences: FAA relief is similar to TCCA.

ITEM:  77-30-1  ENGINE VIBRATION MONITORS

System and Sequence No. Item Repair
Interval
Category
Number
Installed
Number
required
for
dispatch
Procedures
required
Remarks or Exceptions
Engine Vibration Monitors
If required by the certification basis C - 0 (M) May be inoperative provided an approved maintenance reliability program (which includes engine vibration monitoring) is in place.
For ETOPS A - 0 (M) May be inoperative for one flight provided an approved maintenance reliability program (which includes engine vibration monitoring) is in place.
If not required by the certification basis D - 0

DISCUSSION:

References:  AWM 525.1305(d)(3)

EVMs were introduced as an engine trend monitoring tool, and the loss of a single data point (during ETOPS dispatch) is not considered significant.  However, it is not considered appropriate to allow the same relief as for non-ETOPS due to the increased need to be able to confirm engine health prior to an extended range departure.

Based on specific design considerations or adverse service experience a more restrictive category could be assigned.

The intent of the (M) procedure is for maintenance to verify that recent history of engine parameters shows no adverse trends.

FAA Differences:  FAA relief is slightly different than TCCA.  As an example, the FAA permits EVM relief as a cat C on the B767 with no ETOPS mention.

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