Date: 19 Feb 97 02:46:19 From: "P. Wezeman" <email@example.com> Organization: The University of Iowa References: 1
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Airliners are designed for a certain lifetime, nominally in hours but really more affected by the number the number and severity of stress and unstress cycles in the structure, which is subject to metal fatigue. This lifetime is a conservative estimate, and if inspections show that the structure is still sound after this time, the plane may remain in service as long as certain regulations are followed. I believe that under the present regulations, which were made after the fatigue failure of the pressure cabin of an Alaho Air B737 several years ago, no airliner may remain in passenger service for more than half the demonstrated safe life of the aircraft as measured both in flying hours and in operating cycles. For example, if you had a DC-8 with 30,000 hours and 8,000 presurization-depressurization cycles, you would not be allowed to fly it with paying passengers unless, somewhere in the world, a DC-8 of the same variety had safely flown 60,000 hours and 16,000 cycles. If your plane was a stretch version I think the other plane must be a stretch version as well. For the most part, I think that this requirement is satisfied by aircraft manufacturer's prototypes which are used to test new equipment and modifications and accumulate a lot of flights and hours. If it ever got to the point that a lot of airplanes were about to be grounded, the airlines could assemble two or three shifts of flight crews and keep a plane in the air 20 hours a day until they had a reserve of flight hours. This is what I recall from reading Aviation Week and such. Others who work in the field please correct or update. Peter Wezeman, anti-social Darwinist "Carpe Cyprinidae"