Re: Is the 747-100 really "too" old ?

Date:         19 Feb 97 02:46:19 
From:         "P. Wezeman" <>
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"