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

Date:         01 Mar 97 02:44:59 
From:         shafer@ferhino.dfrc.nasa.gov (Mary Shafer)
Organization: NASA Dryden Flight Research Center, Edwards CA
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NASA Johnson Space Center acquired 747-100s to be Shuttle Carrier
Aircraft (SCA).  The first SCA, 905, acquired in the mid-70s, was
purchased from American Airlines.  The reason that we all heard for
AA's willingness to sell was that the plane was sufficiently more
expensive than subsequent models that AA was disposing of all of their
-100s.  The second SCA, 911, was acquired a few years ago.  Although I
don't remember the source (an airline is all I remember), the expense
issue was again bruited about as the reason NASA got it for such a
reasonable cost.  In both cases, the aircraft were perfectly fine
mechanically and no where near the end of their structural lives.
They were, however, near the end of their economic lives as airliners.

Economy or efficiency is the most important factor in an airline's
decision to operate an airplane.  In an industry where the change from
glass to plastic bottles for the liquor singles can make a substantial
difference in operating costs, airplanes that are less efficient to
operate than others will be retired, no matter what the condition of
their airframes, etc.

However, the FAA does recognize that some airplanes, particularly
those operated in hostile environments (i.e. salt and humidity in
Hawaii) are more prone to certain problems such as corrosion.  Flying
a lot of short legs increases the number of cycles on the pressure
vessel, making fatigue failures more likely.  And so on.  As a result,
there has been a lot of study of "elderly" and otherwise affected
aircraft and the corresponding maintainence problems, so that such
aircraft require more and different inspections than do brand new
aircraft.

The military does retire aircraft based on airframe cycles or flight
time, but only in special cases.  For example, the buffet of the F-18
verticals means that the empennage has only a limited lifespan and
once that lifespan is used up, the planes go to Davis-Monthan.

NASA is equally cautious in many cases.  We follow the same schedule
the Navy does for our F-18s, for example.  The X-29 vertical had a
limited lifespan due to buffet and we retired the aircraft when we got
to that limit.  In fact, we discovered that inspection of the vertical
was likely to reduce the lifespan enough that we stopped inspecting
and went to using only flight time, with a _big_ safety factor built
in.


--
Mary Shafer               NASA Dryden Flight Research Center, Edwards, CA
SR-71 Flying Qualities Lead Engineer     Of course I don't speak for NASA
shafer@ferhino.dfrc.nasa.gov                               DoD #362 KotFR
URL http://www.dfrc.nasa.gov/People/Shafer/mary.html
For personal messages, please use shafer@ursa-major.spdcc.com