Use of the naval air test center manned flight simulator for handling qualities evaluation of F/A-18 digital flight control system degraded modes

The F/A-18 aircraft incorporates a digital "fly-by-wire" flight control system with four channel redundancy. In the event of a like failure in three or more of these channels, some control function is lost, and the system automatically reverts to a degraded (back-up) mode of operation. Degraded flight control modes of the F/A-18 are designed to enable the pilot to safely exit the combat arena, return to base, and land. In 1988, an F/A-18 aircraft was lost after the flight controls degraded to a mode which had never been flight tested. Analysis of that mishap forced Navy testers to reevaluate the need to flight test severely degraded flight control modes, and raised the question of whether results obtained from simulator testing could be used in lieu of high risk airplane tests. This thesis compares degraded mode handling qualities of the F/A-18 Manned Flight Simulator (MFS) with those experienced in the actual aircraft during the approach and landing phases. The objective is to evaluate the accuracy of the simulator in predicting degraded mode handling qualities and to provide future testers with recommendations on conduct of handling qualities testing in the simulator. Results show that the MFS could be used to predict aircraft degraded flight control mode handling qualities, provided that the test engineer and test pilot understand and manage simulator characteristics, pilot aggressiveness and proficiency. Incorporation of motion cueing into the MFS would probably enhance the accuracy of simulator test results. The simulator was not accurate in predicting Pilot Induced Oscillation (PIG) tendency, and in particular, did not predict the tendency for lateral PIG during landing approach seen in actual flight test. Test pilots and engineers should continually be alert for the potential of dangerous PIG which may not be predicted due to subtle differences between the simulator and airplane.