Software reliability evaluation of the X-29A flight control system

Abstract

Flight qualification of digital avionics systems involves extensive simulation and testing of both hardware and software components. The testing of flight control software occurs as part of the full-up systems tests, performed on simulators as well as in actual flight tests. The X-29A forward-swept wing technology demonstrator, manufactured by Grumman Aerospace Corporation, was flight qualified at NASA's Dryden Flight Research Facility in a four year program from 1984 to 1988. The X-29A ship 1 flew 242 research flights, logging 178 hours of flight time. Over 2800 hours of simulation time was logged on the flight control computers during flight testing at Dryden. Software reliability measurements of safety critical software systems are not well understood. In particular, since the testing of flight control software for high performance aircraft is performed as part of the full-up systems tests, the applicability of reliability models developed for pure software systems is unknown. In this analysis, data from flight tests of the X-29A are studied using the Statistical Modeling and Estimation of Reliability Functions for Software (SMERFS) modeling package. Results from these models are compared to guidelines which were used in the flight test program to determine software readiness.