(FORTUNE Magazine) – I just flew in from the Coast, and gee, are my wings tired. They should be: Many of America's 4,000 flying commercial planes are over two decades old. The average age of TWA's fleet is a hoary 18.8 years (see table). The airline industry insists age isn't a worry. ''The kind of maintenance an aircraft gets is significantly different than what an automobile gets,'' says Air Transport Association senior economist David Swierenga. Still queasy? Be reassured that after graying out at an average age per plane of 13 years in 1990, the U.S. fleet is gradually getting younger, for regulatory and economic reasons. Noisy workhorse aircraft such as the Boeing 727 and McDonnell Douglas DC-9 aren't up to new federal quiet standards and must be hangared for good by decade's end. Besides, the domestic airline industry, which has lost $8 billion in the past three years, needs the efficiency of newer planes. They get better fuel mileage, cost less to maintain, and use less labor -- a cockpit crew of only two. The third person required on older aircraft, a flight engineer, has been automated out of existence. That must be what happened to passenger legroom too.

CHART: NOT AVAILABLE CREDIT: KEEHAN FOR FORTUNE/SOURCE: AIRCLAIMS/TURBINE AIRLINER FLEET SURVEY CAPTION: JUST PLANE OLD