Summary: GAO discussed federal oversight of the maintenance and repair of aging aircraft, focusing on Federal Aviation Administration (FAA) rules which give the airline industry about 4 years to guard against structural fatigue in older aircraft by strengthening airframe structures. GAO noted that: (1) some airlines could have difficulty complying with the FAA aging aircraft rules, since FAA underestimated the demand for airframe maintenance during the 1990-1994 compliance period; (2) as of April 1991, repairs have been completed on only 28 of the over 1,300 aging aircraft operated by 17 U.S. air carriers surveyed; (3) many airlines are making plans to repair their aging aircraft, although some may be deferring the work until late in the compliance period, primarily due to financial trouble and difficult economic conditions; (4) if many airlines defer repairs to their aging aircraft, the repair station industry may not be able to accommodate an overabundance of demand late in the compliance period; and (5) the repair industry was planning to increase the number of airframe mechanics by 35 percent and the amount of hangar space by almost 50 percent by fiscal year 1994, but the recession could cause repair stations to defer plans to expand capacity. GAO believes that FAA needs to play a more active role in monitoring the maintenance status of the U.S. fleet.