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Paul Hirose wrote-
|
| 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.
|
|
http://www.aaib.gov.uk/publications/bulletins/january_2008/raytheon_hawker_800xp__cs_drq.cfm

==================

Comment from George-

I hadn't heard about that one. But I can see how it might happen. London
City Airport was recently constructed by infilling the basins several of
London's enormous dock system. No doubt, when the warehouses and cranes came
down, the old iron and steel would be taken for scrap, and the remaining
rubble used for infill.

I am unfamiliar with London's docks, but I spent quite a bit of my boyhood
around Liverpool's dock system, which was built in  the same era, much of it
before 1900. Dock machinery was powered by high-pressure water power, stored
in a lofty reservoir, and distributed via massive iron pipes all around the
dock system.

For example, at each quay.head, there would be a rotating bollard, over
which a few turns of warp line would be dropped, to tow a vessel into
position. These were powered from immense hydraulic engines, rather like
steam engines, buried below the quay to allow free passage for the warps.
Completely smooth and almost silent in operation they were, controlled by a
single lever in a below-ground recess. Also, immense hydraulic rams applied
the operating forces to the dock gates. It's likely that all this buried
machinery would simply be left in place when the docks were cleared and
infilled. And easy to image how it could give rise to local spots of high
magnetic deviation.

Degaussing systems, great copper coils that were used to demagnetise vessels
in World War 2, to reduce their susceptibility to magnetic mines, may come
into use in this case. But for the magnetic field to penetrate to the depth
below ground that this machinery may live, such coils would need to be very
large in radius, and to carry a high current, slowly alternating, as they
are dragged around the area to be treated. I have my doubts whether it would
work. And the problem may not be caused by any permanent magnetisation of
these metal-masses, anyway, but simply due to their "soft-iron" magnetic
susceptibility, drawing the Earth's field-lines toward them below the
ground. In which case, nothing but exhumation would do the trick.

But is it really true that commercial aircraft are still using magnetic
heading sensors? Paul refers to a gyro-stabilised magnetic compass. I was
aware that commercial aircraft were still required to carry a magnetic
compass in the cockpit, for use it all else fails, but it's quite a surprise
(to me) to discover that magnetic sensors  remain in use, even if runways
are still numbered according to their magnetic bearing.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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