Magnetic Compass Errors

A favorite of check ride examiners is to ask about one of the simplest
instruments in the aircraft, the magnetic compass.

“Say what instrument is that?” The poor sap you think, he’s trying to catch me
out by asking about the simplest instrument in the aircraft. thank god he
didn’t ask how the Artificial horizon works.

“Oh, that’s just the magnetic compass, it works with magnetism”. Then he moves
in for the kill.

“Yes, that’s sort of right. Can you tell me from what kind of errors does it
suffer?”

“Ehhm”.

We all remember that the magnetic compass is not one of our often looked at
instruments and it’s always wobbling and bobbling. Oh yes that’s one of the
errors:

1.    Hard to read because the floating compass is constantly moving up and
down and from side to side. This is why to set the DI from the magnetic compass
you need to be flying straight and level for a while to let it settle down.
2.    Deviation is the one where the reading on the compass has to be
corrected for the influences around the aircraft. The deviation card will have
these corrections written on it. If you had an anvil in the back which you were
carrying for a friend that would influence your magnetic compass.
3.    ENOS. Early North Overshoot South. From a Easterly or Westerly heading,
if you turn North you stop turning before your chosen heading because the
magnetic compass will be lively and be ahead of your true magnetic heading.
Overshoot South, keep going past your heading the magnetic compass is lagging
and will catch up. Accelaration errors on an Easterly or Westerly heading,
which are difficult to achieve in a GA aeroplane.
4.    Variation finally you have to have compensated for before you get in
the cockpit and converts your magnetic heading into a true heading to get you
where you want to be.