The Advent of Digital Stability Augmentation

Airplane digital fly-by-wire flight control systems, which make possible digital stability augmentation, go back to the 1970s. Priority is difficult to establish, since many organizations were doing this work at about the same time. One early application was at the NASA Dryden Flight Research Center, using digital flight hardware from the Apollo program. Although overdesigned in many ways for the airplane application, it made possible an early demonstration of the possibilities of airplane digital augmentation.

That program used a Vought F-8C airplane (Jarvis, 1975). The first step was to fly single-channel digital flight control systems on the F-8C, with backup analog controls in case of failure. The next step was a big one from the standpoint of system complexity: the development of a triplex digital system, using redundancy management and data bus concepts. The subsequent routine use in modern airplanes of redundant, fail-operational digital flight control and stability augmentation is at least partially the result of this early NASA effort.

Another early application was the quadruplex redundant digital fly-by-wire system flown in the BAe FBW Jaguar. Design commenced in the late 1970s, and it flew between 1981 and 1984 in configurations ranging from normal to highly unstable. The BAe FBW Jaguar technology led to the EAP (Experimental Aircraft Programme) and ultimately to the Eurofighter.