The aircraft engines represent from 40-60% of the total direct maintenance cost of a commercial airliner. The engines, along with tires and brakes, represent the “consumable” components on the airplane. The engine's requirement for high reliability, coupled with its approximately 50% share of the aircraft maintenance cost, has required development of numerous reliability, performance monitoring, and inspection tools and programs to assure its good health at minimum cost.Engine performance monitoring is one of the programs that has been developed. The scope of in-flight engine performance monitoring ranges from a slide rule in the cockpit to a completely instrumented engine monitored by an onboard computer. This paper will review United Air Lines' engine monitoring program, which is a compromise between the slide rule and the onboard computer. The flight log monitoring program is reviewed and UAL's decisions not to install AIDS are explained.