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And I can confirm that still today, on very complex airplanes as arliners,
the magnetic compass is still carried aboard and taken care of trough
periodical checks by the maintenance personnel, and is part of the ck-list
too on Boeing 767.
Gennaro Sammarco
----- Original Message -----
From: "Paul Hirose" 
To: 
Sent: Wednesday, February 02, 2005 11:41 PM
Subject: aircraft magnetic compasses


> George Huxtable wrote:
> >
> > I remember reading, can't recall where, some time in the last 15 years
or
> > so, that it was still a requrement for modern civil aircraft to carry
some
> > sort of magnetic compass, to offer some sort of guidance to the pilot if
> > everything else failed, and that every 747 had such a compass tucked
away
> > at the corner of the windscreen. Can anyone confirm (or refute) this?
Would
> > a modern pilot have any idea how to apply magnetic variation?
>
> To pass the U.S. private pilot written test you need to know how
> to measure true course on a chart, then apply wind correction angle,
> variation, and deviation to obtain compass heading.
>
> Also, the effect of acceleration and bank angle on the mag compass
> must be understood. Students learn mnemonics such as ANDS: accelerate
> north, decelerate south (referring to the false indications those
> actions induce on a horizontal card compass).
>
> http://williams.best.vwh.net/compass/node3.html
>
> The heading instruments in a typical light plane are definitely
> "traditional navigation". You'll find a magnetic compass up where the
> rear view mirror would be located in a car. A directional gyro is
> installed in the instrument panel. It's not north seeking, so every
> few minutes you have to tweak the DG's knob to re-sync it with the mag
> compass. (One complaint about an older release of Microsoft Flight
> Simulator was the too-rapid drift of the directional gyro. That DG
> was ready for overhaul.)
>
> This company's page shows some typical heading instruments:
>

http://www.chiefaircraft.com/cgi-bin/air/hazel.cgi?action=serve&item=/Aircraft/FlightInstruments/Compasses.html
>
> A deluxe light plane might have a gyro stabilized flux valve heading
> system. But an old fashioned magnetic compass ("whiskey compass") will
> also be present, even in advanced military aircraft. The B-2 bomber
> has one in the same place as a small Cessna's compass. Although you
> can't tell from the following picture, it also looks just like a light
> plane compass, which isn't surprising since the manufacturer is
> Airpath.
>
> http://www.airforce-technology.com/projects/b2/b27.html
>
> There was a navigation bug in one of the B-2 software releases. The
> aircraft had to be parked on a heading of true north, plus or minus 5
> degrees, or the nav system wouldn't wake up properly. That problem
> fell into my lap. For a while the flight test program relied on my
> personal Mini Morin "hockey puck" compass (purchased from Celestaire)
> to align the aircraft on the parking ramp before flight.
>
> We couldn't use the whiskey compass for that purpose because it had
> never been swung. My recollection is that when I left the B-2 program
> in '97, it still hadn't been swung.
>
> However, in the regular Air Force (in contrast to us in flight test),
> the standby compass would have been adjusted on the four cardinal
> directions, by means of N-S and E-W screws under a cover on the front
> of the compass. Then a deviation card would be prepared and mounted
> near the compass. Heading systems were not in my job description, but
> I heard this was often accomplished by the pilots in flight. The
> extremely accurate heading from an inertial nav system and an
> autopilot with heading hold mode make it easy.
>
> Unfortunately, flight time is expensive, and there has to be enough
> airspace to fly all those different headings. Often you have to do the
> compass swing on the ground. Even the simplified procedure for a
> standby compass is tedious on a large aircraft, enough to make time
> saving gadgets worthwhile. Here's one:
>
> http://www.firstmarkaerospace.com/standbycompass.asp
>
> The procedure is described in a 410 kbyte PDF you can download on this
> page ("Standby Compass Calibrator Overview"):
>
> http://www.firstmarkaerospace.com/literature.asp
>
> This company also makes the much more elaborate Mark 3 system for
> swinging a flux valve heading system or calibrating the compass rose
> at an airfield.
>
>
> Aviation is strongly oriented to magnetic directions. Air traffic
> controllers give magnetic headings to fly. VOR and TACAN stations are
> aligned to magnetic north. (There's sometimes a little discrepancy; I
> think it's because variation has changed since the station was
> established.) In the B-2 bomber, the heading displays default to
> magnetic, though you can select true. Either way, heading comes from
> gyros, so it originates as true heading, then is converted to mag
> heading if necessary. The standby compass is the only device in a B-2
> that senses Earth's magnetic flux.
>
>
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>



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