Avionics software problem occurrence rates

This paper discusses a feasibility study which developed estimates of the occurrence rate for significant avionics software problems. Two FAA databases, airworthiness directives (ADs) and service difficulty reports (SDRs) were used. A study of the AD database for large aircraft (1984-1994) revealed 33 avionics ADs, 13 of which were software related. Estimates were made of the operational hours for the fleet of commercial aircraft with computer avionics and the number of problem occurrences. Minimum, maximum and average occurrence rates were established. The average occurrence rate for the 6 resulting data sets was 0.15 per million operating hours. The nonoccurrence of ADs for the remaining avionics was "bounded on the average"; yielding less than 0.02 occurrences/per million hrs. The significant problem occurrence rate for the TCAS system (collision avoidance) software has motivated others to apply proof of correctness techniques to the specification and design of this software (Craigen et al., 1994).