Downloaded from https://www.everyspec.com ADS-79D-HDBK Rainflow cycle-counting method being the most widely accepted and successful method. 30 Using the actual cycle count history, the remaining life of the part can be estimated much more accurately than if estimating remaining life by assuming the same usage spectrum for a fleet of aircraft. Because complete life analyses require extensive use of complex, comprehensive models and data bases, reduced order models are used to calculate the life usage and remaining life in ―real time‖, either on the aircraft or at a ground station. G.6.2 Condition Monitoring.
The focus of condition monitoring is to look for engine behaviors that are indicative of degradation or an incipient fault that could lead to engine failure or performance changes. The techniques that are employed include performance trending, fault detection and isolation (diagnosis), and prognosis. Engines monitored with these techniques are not removed at predetermined TBO intervals, but are instead allowed to operate until performance or life limits are reached. 31 Trending is a technique of comparing the engine‘s operating characteristics over time to a baseline set of characteristics.
Measurements are usually corrected to a set of standard or reference conditions before being compared to the baseline. Over time, trend data can reveal the rate of performance degradation in the engine. Gradual degradation represents the normal wear and tear on the engine. A rapid change in the rate of degradation can be indicative of a component fault or failure. Fault detection methods are based on monitoring engine parameters such as temperatures/pressures along the gas path, vibration, fluids for oil debris, and fuel contamination. Throttle commands and actuator commands are also monitored. The signals are analyzed to search for anomalous behavior.
If such behavior is detected, diagnostic algorithms are used to isolate the fault to a specific component. Much of the basis for the diagnostic algorithms can come from the engine‘s FMECA. The diagnostic methods that may be used include artificial neural networks (ANN), Bayesian Belief Networks, Genetic Algorithms, Fuzzy Logic, and Case Based Reasoning, among others. The goal of prognosis is to determine where a fault is leading to a required maintenance action and in what time frame. 32 The time frame for further degradation is used to determine when action should be taken.
Can the aircraft perform its next mission and should the component be removed before the engine‘s next scheduled overhaul? Before the development of automated analyses, the prognoses for failures were built into the inspection schedule. For example, the time between borescope inspections would be based on the time for an undetectable crack in a fan blade to grow to the point of failure. Accurate prognostic predictions can reduce the reliance on regular manual checks / inspections and increase the time between scheduled inspections. G.6.3 Lubrication Condition, Debris, and Filter Monitoring.
Because of the high rotational speeds in typical turboshaft engines, lubrication is essential to operation. The high thermal stresses in a gas turbine engine pose challenges to maintaining effective lubrication and condition during extended operation. The operating conditions inside the engine as well as environmental factors (dust, humidity) can introduce debris into the lube oil. If the debris is large enough, it can cause premature wear in the high tolerance bearings and other surfaces of the engine, causing failure.
Normal wear of bearing material, shafts, and gears within the engine can also introduce very small particulates in the oil that can change its quality, as well as serving as early indication of failure in the parts from which the material came. As a result, turboshaft EMS/EMHS systems should also monitor the condition of the oil filter to 30 SAE AIR 1872A Guide to Life Usage Monitoring and Parts Management for Aircraft Gas Turbine Engines, pg. 14 31 SAE AIR 1873 Guide to Engine Monitoring Systems for Aircraft Gas Turbine Engines, pg. 4 32 SAE ARP 1587B, Aircraft Gas Turbine Engine Health Management System Guide, pg. 9 120