Re: 747-400 Magnetic Detectors

From:         faurecm@halcyon.com (C. Marin Faure)
Organization: Northwest Nexus Inc.
Date:         18 Aug 96 20:13:36 
References:   1
Followups:    1 2 3
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In article <airliners.1996.1649@ohare.Chicago.COM>, Tim Hills
<Tim@cordle.demon.co.uk> wrote:

> Does anyone know if an EFIS equiped aircraft has any magnetic compass
> system other than the E2 standby compass?
>
> In a recent flight deck discussion, on a 400, I said that I thought that
> an IRS system needed an approximate heading reference for its initial
> alignment. If it doesn't of course, then there is only ONE magnetic
> compass in the entire aircraft. Answers from Boeing experts welcome.

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.

C. Marin Faure
   author, Flying a Floatplane