31 July 2011 C1 CIVIL AVIATION DEPARTMENT HONG KONG ...

CIVIL AVIATION DEPARTMENTHONG KONG, CHINAAirworthiness NoticeGAS TURBINE ENGINE PARTS SUBJECT TO RETIREMENTOR ULTIMATE (SCRAP) LIVESNo. 44Issue 431 January 20021 The design of gas turbine engines in service is such that certain critical parts, notablycompressor and turbine discs, experience cyclic variations of stress as a result ofmechanical and thermal effects which are of sufficient magnitude to result in fatiguedamage. The failure of these parts can result in damage to the aircraft since underoperating conditions they may possess more energy than can be absorbed by thesurrounding engine structure. It is therefore necessary to limit the life of all criticalparts in order to prevent fatigue damage developing into complete failure. As fatiguedamage is not detectable by current inspection techniques until cracking has begun, andbecause crack propagation to the point of failure can be unacceptably rapid, a safe lifefor each critical part will have been established and approved as part of thecertification procedure.2 These safe lives, also referred to as retirement lives, ultimate lives, scrap lives andlow cycle fatigue (LCF) lives, are mandatory limits which must never be exceeded.They are required by HKAR-1 Sub-section 1.5-3 to be published, in the EngineManuals, for all engines. Manufacturers also publish this information variously inService Bulletins, Service Memoranda, Notices to Operators, Maintenance Manuals,etc., for the benefit of operators and engine overhaul agencies. It may be possible toextend the published lives as a result of further testing, and this is normally indicated inthe publications as an aid to spares provisioning, but such amendments must beapproved.3 The Inspection and Test Certificate of an engine issued by a manufacturer or overhaulagency is required to include reference to a certified statement in which is recorded thelife consumed, up to the time of release, by each of the life-limited parts fitted in theengine. This statement is normally included in the engine log book, but may beincluded in any other document which has been approved as an alternative for aparticular operator.4 Each operator is responsible for ensuring that parts fitted to the engines being operateddo not exceed the published lives. Therefore accurate up-to-date records of the lifeconsumed by each engine are required to be maintained, and this may involve recordingflying hours, number of landings, 'touch and go' landings and take-offs, air re-starts, etc.,dependent upon each manufacturer's definition of a unit of life. In order to preservecontinuity of the records, an up-to-date statement of the life consumed since last releasemust accompany each engine when despatched by an operator to an overhaul agency forAN-44 P.131 January 2002