The first generation of jet aircraft had a crash rate that would be considered highly alarming today, and both the Federal Aviation Administration (FAA) and the airlines' senior management felt strong pressure to improve matters. In the early 1960s, with FAA approval the airlines began to conduct a series of intensive engineering studies on in-service aircraft. The studies proved that the fundamental assumption of design engineers and maintenance planners—that every aircraft and every major component there of (such as its engines) had a specific "lifetime" of reliable service, after which it had to be replaced (or ) in order to prevent failures—was wrong in nearly every specific example in a complex modern jet airliner.