Re: 747-400 Magnetic Detectors

From:         Ron Adams <>
Organization: Random Access Inc. +1 (800) 910-1190
Date:         07 Sep 96 17:09:11 
References:   1 2
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C. Marin Faure wrote:
> In article <airliners.1996.1649@ohare.Chicago.COM>, Tim Hills
> <> 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.

Yes, the IRS/INS needs lat/long down to .1 minute to align.  The older
Carousel IV's take about 15 minutes to align from cold start while the
new ring laser gyros need only about half that...and only about 2
minutes to realign already running ones. Usually, the present position
is loaded into IRSs throught the FMS (Flight Management System) by
selecting the airport and gate, then line-selecting the lat/long
information and line-selecting the boxes for IRS position update.  This
means one types in the 4 letter ICAO identifier for the airport, then
below that, the gate.  The lat/long appears from the database (assuming
it's stored and the typest hasn't goofed :-) ).  There are keys along
the sides of the CRT called line-select keys that will copy iformation
from that line into the scratchpad area at the bottom of the screen so
it can be entered elsewhere all at once, making 7 or 8 buttons to push
instead of 16 (i.e. N40030.8W08043.9 )

I don't know where the whiskey compass is in the whale (747) but in the
757/767 it's in the center top portion of the front windscreen area as I
recall.  It's the only truely magnetic compass.


Ron Adams
MD88 (soon to be B-767/757) Captain