Re: 747-400 Magnetic Detectors

From:         Art Intemann <ajintemann@earthlink.net>
Organization: Earthlink Network, Inc.
Date:         07 Sep 96 17:09:11 
References:   1 2
Followups:    1 2
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C. Marin Faure wrote:

> The IRS requires an EXACT position input and some time to stabilize before
> it can be used.  That's why some airports paint the lat/long position of
> each gate on the pavement or on a placard where it can be seen from the
> cockpit.  The IRS is turned on early in the pre-flight cockpit procedures
> to give the system time to stabilize, at which point the plane's exact
> position is entered.  At least, that's my understanding.  I'm sure there
> are pilots in this group who can respond with the exact procedure.

The old double and triple inertial unit setups (*INS*) required lat/long
input as close as possible to actual aircraft position.  Thus coordinates
are often painted on  buildings for each gate, in plain view from the
cockpit (or on light poles in remote areas).  Coordinates for each parking
position are often (though not always) listed in the Jepps for that
airport.

When the IRS came along, some divergence in airline operating procedure
showed up.  Some were still doing it the same old INS way.  Some, like
UAL began to "line select" a stored gate position, or a stored airport
position from the FMS database during initialization to avoid the potential
of mis-typing the coords.

With the 747-400, engaging the autothrottle on takeoff roll tells the FMS
that the end of the runway entered in the computer is 600 feet or so behind
you.  At 50' the LNAV locks on to ground stations to begin updating.  In
that airplane, exact coordinate entry is not as important as in the past.

Just a driver,

Art