Re: 747-400 Magnetic Detectors

From:         Cameron Hines <camhines@ozemail.com.au>
Organization: OzEmail Pty Ltd - Australia
Date:         18 Aug 96 20:13:35 
References:   1
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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.

As far as I am aware the IRS's can work out True North. During the
alignment the IRS's sense earth rotation and find true north. As an actual
position is then entered (a known starting point is required) the magnetic
variation is stored in the FMC data base and so magnetic north is then
known.

Hope this helps.