Downloaded from https://www.everyspec.com ADS-79D-HDBK The consumed life should be available after each flight or at the end of the day‘s flight operations. The damage computation may be made available to the maintainer in real time using an onboard computer or very soon after the aircraft lands using a ground-based computer. Prior to introduction to operational service, the life usage monitoring algorithms should be verified and validated. Verification ensures that the reduced order algorithms produce the same results that the full analyses models would produce.
One method of verification is to compare the output of the reduced order model to the full analysis model used for life determination for a given set of flight profiles. The reduced model is considered equivalent to the full model when the mean values of the computed damage are the same and the maximum difference is below a defined limit. To ensure the equivalence between the design model and the reduced-order models, all the verification procedures of the reduced-order damage model should be conducted for each material on a wide range of flight profiles. Typically there are differences between the design model and the reduced order model.
First, the material data base incorporated into the reduced model is a simplified derivation of the design material data base. Second, the thermo-mechanical history of a real engine is much more erratic than the conventional flight profiles used in design. The lifing model and the material data base should accommodate 41 : a. Numerous small cycles of real flight b. Very rapid loading which occurs during rotor acceleration and deceleration c. Constant loading can lead to some concerns in real time creep and fatigue modeling The verification of the accuracy of the structural model should be performed on a specified variety of flights to ensure the operational temperatures have been validated.
Algorithm validation requires a large database of service experience to establish accurate correlations. Initial information in support of this objective can be acquired through Accelerated Simulated Mission Endurance Tests (ASMET). Information generated by the reduced order damage models should be kept during the whole fleet life of perhaps 30 years, considering the age of current aircraft fleets. If the reduced model is revised, then the relevant parts of the verification and validation process should be repeated.
When a component is replaced during servicing by another with the same life characteristics, this need only be recorded in the life database, unless the monitoring system needs to be reinitialized for the life consumed. If a component is redesigned or a modification to other components alters the behavior of the engine, the rate of damage accumulation for the monitored parts may change. The reduced order algorithms may have to be altered, which will require repeating the verification and validation process. If the engine starts being used on a different flight profile than already experienced, the algorithms should be checked for accuracy using the new flight profile.
G.7.4 Condition Monitoring. A condition monitoring system consists of the following four functions 42 : 41 RTO TR 28, ―Recommended Practices for Monitoring Gas Turbine Engine Life Consumption‖, North Atlantic Treaty Organization, Research and Technology Organization, April 2000, pg. 7-7. 42 SAE AIR 1873 Guide to Engine Monitoring Systems for Aircraft Gas Turbine Engines, pg. 7. 130