Magnetic Compass Errors

The magnetic compass is the simplest instrument on the panel, but it is subject to several errors that you must take into account. First come the two corrections you calculate: variation and deviation. Then come turning errors and acceleration errors.

Two corrections: variation and deviation

Variation

The difference between true and magnetic direction is called variation. Lines of equal variation are called isogonic lines. Variation does not change with your aircraft’s heading.

The FAA handbook works through two cases. At 10° west you add the variation to the true course: 180° becomes 190°. At 14° east you subtract it: 180° becomes 166°.

Deviation

Magnetic fields inside the aircraft pull the compass off. This error is called deviation. Causes include flowing electrical current and magnetised parts. Deviation depends on the aircraft’s heading, not on its position.

A maintenance technician reduces deviation by swinging the compass. They record the remainder on a compass correction card that is kept in a holder near the compass. You fly using the values on this card.

From true course to compass course

You work in two steps. True course plus or minus variation gives the magnetic course. Magnetic course plus or minus deviation gives the compass course. You read the deviation, with its sign, from the correction card.

Errors caused by dip

The Earth’s magnetic field runs parallel to the surface only at the magnetic equator. The closer you get to the magnetic poles, the more steeply the field lines tilt against the surface. This angle is called the dip angle.

The compass is built to rotate only in the horizontal plane: its centre of gravity lies below the pivot point. If the card tilts out of the horizontal, the vertical component of the Earth’s field acts on it. This leads to the turning errors. Close to the magnetic poles the horizontal component is too small: the compass is unusable for navigation there.

Turning errors

When you fly a turn by the compass, the FAA handbook says:

Turn towards	What you do
North	Stop the turn before the desired heading.
South	Let the compass reading pass the desired heading, then stop the turn.

The handbook gives a rule of thumb: 15° plus half the latitude. At about 40° latitude that is 15° + 20° = 35°: you stop a turn towards north 35° before the heading. At about 30° latitude that is 15° + 15° = 30°: you stop a turn towards south 30° after the heading. Both errors grow the closer you get to the magnetic poles.

Acceleration error

On easterly and westerly headings, dip and inertia falsify the compass when you accelerate or decelerate. When you accelerate it shows a turn towards north, when you decelerate a turn towards south. The handbook gives the mnemonic ANDS: Acceleration North, Deceleration South.

Oscillation and steep turns

Oscillation is a combination of all the errors above. It makes the compass card fluctuate against the actual heading. When you set the heading indicator, you use the average indication between the swings. Up to about 18° of bank the card can tilt freely. At steeper bank angles the indications are unpredictable.

Aligning the heading indicator with the compass

The heading indicator is designed to make the magnetic compass easier to use. The forces that make the compass hard to read do not act on it. According to the FAA handbook, you set it to the compass in straight and level flight at constant speed, so that compass errors do not corrupt the result.

What the EASA syllabus covers

The EASA syllabus for the PPL(A) theory lists “Turning and acceleration errors” for the magnetic compass. It also names “Compass deviation” and the relationship between true and magnetic direction. Flight training includes, after the first solo flight (Exercise 14), a revision of “turns using magnetic compass, compass errors”.