Fly-By-Wire: Fulfilling Promise and Navigating Around Nuance

As designers and flightcrews became more comfortable with electronic flight control systems and the systems became more reliable, the idea of removing the extra weight of the pilot’s mechanical control system began to emerge. Pilots resisted the idea because electrical systems do fail, and the pilots (especially military pilots) wanted a "get-me-home” capabil­ity. One flight-test program received little attention but contributed a great deal to the acceptance of fly-by-wire technology. The Air Force ini­tiated a program to demonstrate that a properly designed fly-by-wire control system could be more reliable and survivable than a mechani­cal system. The F-4 Survivable Flight Control System (SFCS) program was initiated in the early 1970s. Many of the then-current accepted prac­tices for flight control installations were revised to improve survivabil­ity. Four independent analog computer systems provided fail-op, fail-op (FOFO) redundancy. A self-adaptive gain changer was also included in the control logic (similar to the MH-96 in the X-15). Redundant com­puters, gyros, and accelerometers were eventually mounted in separate locations in the airplane, as were power supplies. Flight control system wire bundles for redundant channels were separated and routed through different parts of the airplane. Individual surface actuators (one aileron for example) could be operated to continue to maintain control when the opposite control surface was inoperative. The result was a flight control system that was lighter yet more robust than a mechanical sys­tem (which could be disabled by a single failure of a pushrod or cable). After development flight-testing of the SFCS airplane was completed, the standard F-4 mechanical backup system was removed, and the air­plane was flown in a completely fly-by-wire configuration.[700]

The first production fly-by-wire airplane was the YF-16. It used four redundant analog computers with FOFO capability. The airplane was not only the first production aircraft to use FBW control, it was also the first airplane intentionally designed to be unstable in the pitch axis while

flying at subsonic speeds ("relaxed static stability”). The YF-16 proto­type test program allowed the Air Force and General Dynamics to iron out the quirks of the FBW control system as well as the airplane aero­dynamics before entering the full scale development of the F-16A/B. The high gains required for flying the unstable airplane resulted in some structural resonance and limit-cycle problems. The addition of exter­nal stores (tanks, bombs, and rockets) altered the structural mode fre­quencies and required fine-tuning of the control laws. Researchers and designers learned that flight control system design and aircraft inter­actions in the emergent FBW era were clearly far more complex and nuanced than control system design in the era of direct mechanical feedback and the augmented hydromechanical era that had followed.[701]