Originally Posted by
Webby737
Certain steel parts of an aircraft can become magnetised such as the flap tracks or parts of the landing gears.
Very often this is cased by lightening strikes.
Dependant upon the location of the part/component it can sometimes have an effect on the magnetic (standby) compass, in other cases there is little or no effect.
I remember one case in a B737 where the standby compass would move by several degrees every time the nose gear was raised.
Dependant upon the size and location of the part that's magnetised you can either use a hand held magnetic yoke (also used for magnetic particle inspection) or in some cases a larger power pack with a large coil of cable.
Often, after demagnetisation a compass swing is required.
In the case of Boeing aircraft there is a demagnetisation procedure in the AMM.
I hope this helps.
Many, many moons ago I was involved in degaussing a whole herd of chipmunks and was surprised at the amount of residual magnetism that some of them had.
We naturally carried out a compass swing on them all after the degaussing process and some required a lot of twiddling on the compensating magnets to get correct compass readings.
One thing to bear in mind is that although the B737 nose gear example above is a good one, if the residual magnetism is in a static component then you will not see it as an obvious movement of the compass and the "deviation error" will be consistent.
So it is likely that the last compass swing and any adjustments carried out would have "dialled out" this deviation.
So simply degaussing alone will actually create an error in the compass readings.