On-Condition Maintenance for Nonmodular Jet Engines: An Experience

Using updated knowledge and gained experience in engine control and maintenance, a specific on-condition maintenance concept of RD-33 engines installed on MiG-29s was developed. The engines had several built-in limitations: number of starts, number of hours at the maximum power and reheat, number of hours at the special regime of elevated temperatures, and time between overhauls (TBOs), that is, number of flight hours. During field data collection and analysis, it was found that engines worked with different working loads and different levels of life consumption. Hence, it was concluded that the limitation of TBO, expressed in terms of flight hours, do not represent actual engine health condition and that a new way of monitoring actual load needs to be introduced. An analysis of all flight profiles was carried out and a specific relation between flight hours and total accumulated cycles (TACs) was established. For this purpose, a distributed expert system in relation-operation unit—Air Force Technical Institute—overhaul depot was introduced. Each of the three participants has its own level of responsibility in the engine health monitoring, engine maintenance, and engine health condition decision-making process. Nondestructive inspection, remote visual inspection, spectral oil analysis, fault tolerant control techniques of hot engine parts, engine electronic control unit, airplane information-display system, engine performance trend monitoring, vibration monitoring, and postflight data analysis play key roles in the concept. It has been applied in practice since 1994; all faults were discovered right in time, and there were not any critical situations in flight. Detected faults were isolated and assessed for severity, so that the remaining useful life could be estimated. The original TBO was safely extended on the basis of TAC of up to more than 50% of the originally prescribed TBO hours, while maintaining the same safe margin.