Downloaded from https://www.everyspec.com ADS-79D-HDBK FIGURE G-4. Progression of turbine engine monitoring technology G.6.1 Life Usage Monitoring. An engine part is life-limited if it is likely to fail through extended usage under normal design operating conditions. 28 The part‘s life is determined by evaluating the damage accumulated under various operating conditions. The engine‘s mission profile determines the frequency and duration of the various operating conditions. The traditional approach to calculating a part‘s life usage is to assume a standard mission profile for the engine application and sum the damage that occurs over time in normal operation.
The overhaul interval is then set to ensure that the part is removed before its life has been used up. For safety purposes, some conservatism is used in setting the time between overhaul (TBO) intervals. This statistics-based approach may result in some parts being removed because they are damaged, while others, with life remaining, are removed because of the probability that they could fail. The traditional approach may be quite suitable for commercial airline applications where mission profiles are well defined. For military applications, however, mission profiles can vary widely.
If the normal mission profile for the engine is less stressful than the assumed standard profile, engine parts will be removed while they still have some remaining useful life. By monitoring the engine‘s actual usage, it is possible to better determine the accumulated damage and calculate the remaining useful life. This ensures life-limited components are not removed prematurely. Another function, related to usage monitoring, is exceedance recording. As long as the engine is operated within its normal envelop, the damage accumulation can be based on the standard profile. 28 SAE AIR 1872A Guide to Life Usage Monitoring and Parts Management for Aircraft Gas Turbine Engines, pg. 7 118