A Prognostic and Diagnostic Approach to Engine Health Management

A serious operational cost trend threatens the future technical preeminence of the United States DoD. Increasing readiness costs are severely impacting acquisition of new aircraft, which translates to an increase in the average age of the United States Air Force, Navy and Army aircraft fleet. As time marches on, this undesirable trend will become more and more difficult to overcome. It would be unwise to expect congress to increase the defense budget in the near future to overcome this dilemma. Hence, as the current aircraft fleets continue to age this problem will only get worse. A revolutionary paradigm shift must take place to reverse the aircraft sustainment demand for funding. Prognosis based asset management can go a long way towards reversing the operating cost trend. When applied to aircraft engines, prognosis based asset management may allow the services to reach cost of ownership entitlement as well as achieve significant safety and readiness improvements. This revolutionary change in engine management will employ condition (or state) based component lifing and inspections (verses the current hard time inspections limits). Instead of operating to fixed intervals, based on engine health, the component will dictate when the optimal inspection should occur. In other words, a sensor will determine when the engine needs to be inspected. This includes all nondestructive evaluation, borescope activities, component replacement and depot maintenance work. The concept of engine health management (EHM) has been an interesting topic for several years. The Navy explored prognosis and mechanical diagnostics in the early 70’s for the F-8 and A-7 applications (1). Various limitations such as engine controller, storage, limited computing capacity / capabilities have prevented this from moving forward. Significant advances in both computing power and sensor technology now make it possible to obtain real time engine information and to make EHM a reality on an engine-by-engine basis. Obtaining flight-by-flight usage parameter information will provide the foundation for robust diagnostics as well as engine prognostics and allow real time fault tree analysis and near real time damage accumulation calculations. Once this information is available, engine prognosis can provide predictive capability for the health of engine components, appropriate inspection intervals and maintenance activities providing a substantial long-range cost avoidance opportunity for the DoD sustainment budget. Current fleet management capability is constrained by uncertainty in the current state of the individual aircraft engines. The ability to sense or measure the damage state of an individual part is limited at best. Further, specific part operational severity is not captured with the current lifing process, hence many components are not operating to their life entitlement because the life is based on fleet weighted average missions. Unlike the fixed interval inspections currently being performed, precise assessment is required for condition-based lifing. The key considerations in this new assessment process are 1) the fidelity of the analysis tools and 2) the definition of the boundary conditions (or environmental conditions used by the analysis tools) 3) improved understanding of diagnostics and engine faults and a better troubleshooting tool.