Downloaded from https://www.everyspec.com ADS-79D-HDBK G.7.4.3.3 Diagnostics. The objective of a turboshaft engine diagnostic system is to identify faults before they lead to catastrophic failures and to aid maintenance personnel in taking corrective action. This prevents costly damage to engine components and can reduce costs due to premature or past-due maintenance. 43 Diagnostic algorithms should isolate faults to specific components with high confidence. Without specific isolation of a fault through the calculation of an accurate CI and corresponding HI, mechanics may begin replacing suspect components until the fault clears (the engine returns to normal operational limits).
This process unnecessarily increases maintenance costs and aircraft down time. Current generation turbine engines have built in tests that can provide the degree of fault isolation required if the algorithms are designed to use all the available evidence. While condition monitoring systems can reduce maintenance costs and aircraft downtime, the systems themselves can be expensive. The more sophisticated the system is, the more it will tend to cost to develop and implement. This cost should be balanced against the gains made in maintenance and logistics costs. As a reference point, the key performance parameters for the CH-47 Aviation Turbine Engine Diagnostic System (ATEDS) in 2004 were 44 :
a. Detect and isolate engine faults from airframe faults and provide fault detection and isolation (threshold) 96%, (objective) 100% b. Verify proper operation of engine control units, electronic sequencing units, Line replaceable unit/Line replaceable module (LRUs/LRMs) engine sensors, and engine wiring harness 96% (T), 100% (O) c. Verify proper operation of engine-to-airframe interface (electrical) at 96% (T), 100% (O) d. Mean time between Essential Function Failure should be at least 360 hours (T), 820 hours (O). Appendix D contains guidance related to developing CIs and HIs for the CBM system.
While EMS and EMHS have often been developed independently of aircraft or vehicle health monitoring systems (VHMS), the intent should be for the aircraft and propulsion systems to integrate and consolidate system resources whenever possible to save weight, cost and complexity. G.7.4.4 Prognostics. In propulsion systems, there are typically two different categories of remaining useful life (RUL) which are based on either:
1) the existence of a confirmed fault or 2) the established life usage indicators for the rotating components which have a defined design life (turbine blades, disks, etc) based on exposure to thermal stresses or high centrifugal loading (fault mechanisms that are always present). The estimates of RUL, which influence maintenance actions to remove either the engine or subcomponents, are based on the existence of faults detected by CIs and follow the process of this ADS and its Appendices D and E. 43 Litt, J.; D. Simon, S. Garg; T-H Guo, C. Mercer, A. Behbahani, A. Bajwa, and D. Jensen.
―A Survey of Intelligent Control and Health Management Technologies for Aircraft Propulsion Systems,‖ NASA Tm-2005-213622, May 2005, pg. 10. 44 Stramiello, A., J. Moffatt, G. Kacprzynski, and J. Hoffman. ―Aviation Turbine Engine Diagnostic System (ATEDS) for the CH-47 Helicopter,‖ 2008. 134