Re: 747-400 Magnetic Detectors

From:         Norman Ovens <nlovens1@crems.rockwell.com>
Organization: Rockwell International
Date:         18 Aug 96 20:13:36 
References:   1
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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.

EFIS is the cockpit interface. The heading can be derived in a number of
ways.

An IRS (Typically a Laser Gyro) does not require a magnetic heading input.
An IRS senses the inertial rate and computes true North.

During alignment on the ground with the aircraft stationary. The IRS senses
the angular rate of the environment it is strapped down to. As the aircraft
is stationary the IRS measure the angular rotation of the Earth. The IRS
uses this angular rate to determine the direction of true North. The
magnitude of the rotation vector allows the IRS to compute the Latitude
of the aircraft (this is then compared with the operator's input).

IRS derived magnetic heading is calculated using magnetic variation maps
of the Earth circa 1985. This is not a very precise output in comparison
to the true North computation.

Most operators do not notice that there is a difference between the IRS
generated magnetic heading and magnetic Flux sensed heading unless they
compare it closely with a reversionary attitude and heading source such
as an AHRS,which is slaved to a magnetic flux detector.

An AHRS uses magnetic flux detectors (typically wing mounted). Slaves the
heading output to magnetic North.