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A diagnostic and prognostic system for aircraft flight control actuators supports the furthering of two important initiatives for the US Navy:
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Replacement of a Time Before Overhaul (TBO) philosophy with a Conditioned Base Maintenance (CBM) approach, with an estimated savings of $150 million to the fleet by reducing false alarms by 50 percent.
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The “electrification” of aircraft systems.
As a demonstration of the benefit of diagnostics and prognostics in aircraft flight control actuators, Foster-Miller is currently developing a system for the Electrohydraulic Servo Valve (EHV) of the F/A-18 stabilator. The system monitors the commanded and actual positions of the EHV, as well as voltage applied to the stabilator actuators (two EHVs per actuator). Using this information, the system diagnoses problems and forward predicts when maintenance is required. Diagnostics and prognostics of EHVs through our nonlinear analysis techniques support a CBM approach. Service is applied only when required, with prognostics providing a capability to schedule maintenance ahead of time based on the ability to predict a component’s remaining life. This diagnostic and prognostic technique is applicable to a wide range of aircraft flight control actuators.
 We are currently working on techniques to recognize stabilator operating conditions that are conducive to discriminating normal EHVs from faulted ones through the analysis of commanded position and the valve response signal (both garnered from the flight controller signals). Once recognized, the commanded position “time window” in which the fault occurs can be weighed according to a confidence index to provide a maximally accurate indication of valve condition at all times. As the historical database of EHV faults is built up, the algorithm will be further refined to provide prognosticating ability. The Boeing Company conservatively estimates that the Navy could save $229 million through the life of the fleet at a payback ratio of 11:1 with the actuator prognostics system.
We have been able to successfully diagnose the degree of failure of an EHV with test data provided by Boeing from their F/A-18 A-D stabilator test rig. The test rig is comprised of an actual flight controller and stabilator hardware from the F/A-18, which can be mechanically loaded with flight-realistic force histories. A series of tests were performed on the test rig with valves of various degrees of failure along with normally functioning valves. Aerodynamic loads were simulated as the flight controller moved the actuator through a set of flight regimes.
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