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    Aircraft magnetic compasses
    From: Paul Hirose
    Date: 2005 Feb 2, 14:41 -0800

    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).
    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:
    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
    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:
    The procedure is described in a 410 kbyte PDF you can download on this
    page ("Standby Compass Calibrator Overview"):
    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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