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    Magnetic anomaly incident at London City Airport
    From: Paul Hirose
    Date: 2008 Jan 25, 20:23 -0800

    The UK Air Accidents Investigation Branch recently released a bulletin
    on the airplane that returned to London City Airport shortly after
    takeoff due to large discrepancies on its heading displays. There's a
    history of heading system troubles observed by crews after takeoff here.
    In addition, planes often wander off the correct departure course for no
    apparent reason.
    Investigators found a powerful magnetic anomaly on a taxiway. It's
    supported on pilings made from sections of a former oil pipeline. In
    addition, the area used to be a dock. Massive bases for the old cast
    iron bollards are still under the ground. All this underground metal
    generates a disturbance powerful enough to drive a gyro stabilized
    magnetic compass system tens of degrees from the correct heading, if a
    plane waits there for a few minutes.
    The AAIB report mentions similar troubles at other airports. For
    example, at Houston, Texas, a runway became magnetized by a cleaning
    process which blasted the pavement with steel shot. Planes experienced
    heading anomalies of 40 to 90 degrees. With a combination of degaussing
    (not completely successful), pilot awareness, and the passage of time,
    the problem faded away.
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