In-flight performance monitoring of aircraft has become a valuable time- and cost-saving tool for the airlines over the last few years. This paper describes the development of such techniques on TWA's DC-9, 747, and L-1011 aircraft. The aircraft integrated data system (AIDS) has been the basis of TWA's maintenance monitoring. Initially installed in 1966 on the DC-9, the system was later expanded to the jumbo jets. This paper describes the parameters monitored, the computer interlocks, and maintenance scheduling which results. Specific examples solved via these techniques are given.