X-31

The X-31 was all about enhanced--even extreme--maneuverability for fighter aircraft. Even so, it improved flight safety because, in NASA's words, "it was fully controllable and flyable in the post-stall region, unlike other fighter aircraft without thrust vectoring." (The thrust vectoring involved three paddles, made of an advanced carbon fiber composite, on the engine nozzle at the rear of the aircraft, which could be moved to control the exhaust flow and thus allow adjustments in pitch and yaw.)

Built by Rockwell Aerospace, North American Aircraft, and Deutsche Aerospace, the two X-31 aircraft together made 555 flights in the first half of the 1990s. The fly-by-wire system used four digital flight control computers, but no analog or mechanical backup. "Three synchronous main computers drove the flight control surfaces," NASA says. "The fourth computer served as a tiebreaker, in case the three main computers produced conflicting commands."