F/A-18A testing of flight control system reversion to mechanical backup

The F/A-18 incorporates a digital fly-by-wire primary flight control system (FCS). An emergency mechanical backup/linkage to the horizontal stabilizers (stabs) is provided in the event of a major FCS failure. Unfortunately, mechanical backup (MECH) mode reversion incidents have increased dramatically in recent years. A flight test program was performed to identify MECH reversion trigger mechanisms and identify a minimum single engine power setting (RPM) above which, reversion was improbable. To do so required forcing in-flight MECH reversions while operating single engine. This was achieved by inducing substantial hydraulic demand through cycling the control stick and/or ancillary equipment such as flaps and landing gear. Risks associated with attempting to force the aircraft into an emergency mode with vigorous flight control inputs while single engine are addressed in this paper along with the risk mitigation, test method, conclusions and lessons learned.