Re: 747-400 Magnetic Detectors

From:         Reid Fairburn <cr_king@cr_king.seanet.com>
Date:         17 Sep 96 02:24:42 
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At 03:03 AM 9/13/96, you wrote:
>Many have mentioned that the pilots must give the INS system time to
>align after they have entered the lat-long values at the gate and that
>during this time, there must not be any movement so that the INS can
>sense the earth's rotation and align itself.
>
>With the shorter and shorter plane turn arounds, when do pilots get to
>do this ? Can this be done while passengers are entering the plane ?
>Can this be done while cargo is being loaded ?
>
>On a narrowbody plane, wouldn't the entry of two rather heavy passengers
>cause a bit of a movement (vertical) of the plane ? (Or more evidently,
>the loading of a heavy cargo pallet) ?

==========Early on, there was in fact a problem with the normal movement of
the airplane as passenger loading, cargo loading, food service truck
bumping, and tow tractor hook up was taking place.  The IRSs would blow the
alignment and the pilots would have to start all over.  It got to be such a
problem that the tolerances for movement had to be increased to allow
alignment  during these processes.  Also, in the case of large or really
significant movement the alignment process will automatically start over and
tell the pilot that it has done this.  For these reasons, pilots tend to not
do realignments between short flight turnarounds but just do what is called
a fast or quick alignment which removes some of the errors that accumulated
on the last flight.  I think that INS systems must be realigned completely
between flights but I am not sure...anyway the airplanes that use this type
of equipment have a lot more time on the ground during turnarounds.

Reid Fairburn
Creative Kingdom, Inc.
cr_king@cr_king.seanet.com
206-946-4815