On Sat, 17 Apr 2010, Richard J. wrote:

Tom Anderson wrote on 17 April 2010 14:03:46 ...
On Fri, 16 Apr 2010, Basil Jet wrote:

Changing the subject slightly, http://en.wikipedia.org/wiki/Runway says
"Runway designations change over time because the magnetic poles slowly
drift on the Earth's surface and the magnetic bearing will change. When
runway designations do change, especially at major airports, it is often
changed overnight as taxiway signs need to be changed and the huge
numbers at each end of the runway need to be repainted to the new runway
designators. In July 2009 for example, London Stansted Airport in the
United Kingdom changed its runway designations from 05/23 to 04/22
overnight."

I'm surprised that the bearings are magnetic rather than true (which would
never change, up to continental drift). Is the idea that the poor pilots
shouldn't have to deal with correcting their compasses in flight?

Poor pilots can only afford a magnetic compass.

What do they do now they use (laser) gyrocompasses? Apply a magnetic
decorrection so they can work out where the runway points?

According to http://en.wikipedia.org/wiki/Heading_indicator the
gyrocompass (heading indiactor) has to be reset several times an hour so
that it matches the magnetic compass, otherwise it will drift for
various reasons. In other words, it's the heading given by the magnetic
compass in straight and level flight that is the reference.

That's for light aircraft. Commercial aircraft use laser gyros and so on.

Still, i suppose the point is that the system has to be set up so that
pilots with even the most spartan equipment can navigate safely, and that
means using magnetic bearings everywhere - apparently directional radio
beacons also use magnetic bearings.

Still, i'm surprised even light aircraft pilots have to correct their
indicators manually. 125 USD buys you this:

http://www.sparkfun.com/commerce/pro...oducts_id=9623

Which is a hobbyist component, but should cost no more for a real
manufacturer, which includes solid-state accelerometry, turn-rate sensing,
and magnetometry in all three dimensions, and a processor. That could do
magnetically-corrected heading indication without too much trouble. I
suppose doing it robustly and then getting it certified would cost quite a
bit more.

tom

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