F/A-18A-D Flight Control computer OFP versions 10.6.1 and 10.7 developmental flight testing out-of-controlled flight test program yields reduced Falling Leaf departure susceptibility and enhanced aircraft maneuverability /

by Mitchell, Eric John.

This thesis analyzes the recent Version 10.7 Operational Flight Program (v10.7 OFP) Flight Control System upgrade to the F/A-18A-D (legacy) Hornet fighter aircraft. This developmental program endeavored to improve high angle-of-attack (AOA) maneuverability while vastly reducing the aircraft’s susceptibility to sustained out-ofcontrolled flight events. Although the original F/A-18 Hornet, designated F/A-18A through F/A-18D, has been acclaimed for its departure resistance as well as its exceptional maneuverability as a fighter aircraft, the model, in actuality, has suffered from significant losses due to out-ofcontrolled flight (OCF) mishaps. Since its development in the early 1980s, eighteen Hornets have been lost to a particular OCF mode called “Falling Leaf”, including eight aircraft crashed since 1999. With no improvements, 10 additional aircraft, at a cost of $40 million each, were forecast to be lost. Two-seat aircraft are lost at a higher rate per flight hour than the more common single-seat version. Analysis of flight test data indicates that more two-seat aircraft sustain Falling Leaf mode due to their increased departure susceptibility. Additionally, it is apparent that the increased sprung mass of the control system, due to the addition of the rear cockpit control stick, may delay or inhibit recovery from a sustained Falling Leaf departure. This may be caused by uncommanded Flight Control System inputs from iv lateral control stick inertial motion induced by high sideforces encountered during a Falling Leaf. The v10.7 OFP test effort conducted a complete out-of-control flight test program without the benefit of having an attached spin-recovery parachute during testing. The specific test method and risk mitigation techniques used during this test program are reviewed and documented in this thesis to provide a historical record for future testing. By using the lessons learned from the development of the F/A-18E/F Super Hornet testing conducted a few years earlier, the v10.7 Team was able to complete the test at a large cost and schedule savings. The author concludes that the test program is an exceptional success. The new low airspeed and high AOA maneuvering capabilities inherent with the v10.7 software revolutionize how pilot aircrew will fight the aircraft. Further, the extremely enhanced resistance to sustained departure modes during out-of-controlled flight events will substantially reduce the frequency of aircraft mishaps and the associated loss of training and assets. v