Re: 747-400 Magnetic Detectors

From:         "Stuart Law" <stuart@slaw.com>
Organization: Stuart W. Law Co
Date:         07 Sep 96 17:09:11 
References:   1 2
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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.

You have an orange / apple comparison.  Electronic Flight Instrument
Systems refer to the display of information ... nominally the equivalent
of a Flight Director and Horizontal Situation Indicator displays in
"classic" aircraft.  What drives the HSI portion is external to the EFIS
and is usually either a "remote" stabilized magnetic heading sensor or
the magnetic heading output of a inertial reference/navigation unit.

As to the number of "Direction Indicators" on a Part 25 (Air Transport)
airplane, there are at least three:
    1) A stabilized system for the Left Seat
    2) A independent stabilized system for the Right Seat
    3) A compass that does not require anything but the earth's magnetic
       field for operation (traditional float or Hamilton-style eddy-
       current compass).
The rules that drive this selection include FARs 25.1303, 25.1309, 91.205,
and 121.305.

Inertial reference/navigation systems use the rotation of the earth to
determine where True North is located during alignment and then use a
model of the magnetic field variation to approximate the magnetic heading.
These often differ by several degrees to the float compass but are not
significant except for dead-reckoning navigation ... an art not generally
practiced by owners of inertial systems!

During the inertial alignment process, the processor assumes that it has
been given the correct lattitude and that it is motionless (or knows its
motion from somewhere else in the case of aircraft carrier operation,
et.al.).  From the latitude it computes the rate at which the surface of
the earth is rotating (360 degrees in 24 hours at the equator, 0 at the
north pole).  It then "levels itself" to local gravity, takes a wild guess
at North, and compensates for the earth rate movement.  After a few seconds
it sees any non-vertical accelerations as an error in its earth rate
compensation and attributes them to not knowning exactly where True North
is, re-estimates true north and tires again.  At the end of the alignment
(hopefully) it has found true north.  Errors in lattitude drive it nuts
since no true north angle will keep the earth rate in sync with its
predictions over an extended period of time!  Errors in longitude don't
affect the solution and just ride as offsets to the final solution
accuracy.

Note that since there is no earth rateat the (true) north pole, inertial
navigators can't be aligned there!  That is why you will see a limit in
their specs as to how far North/Sourth they can be initialized.  Once
operating, its no problem to fly over the poles.