The magnetic compass: errors and the swing

Navigation · Chapter 17 · DGCA CPL and ATPL

How the aircraft compass works, why it lies when you speed up or turn, and how its errors are measured and reduced.

Remember

Easier to steer south than north in the northern hemisphere (lively, not sluggish).
The compass magnet hangs below its pivot (pendulous) and sits in liquid that damps and buoys it.
Acceleration error: E/W headings only; speeding up shows a turn towards the nearer pole.
Turning error: through N/S only; nearer pole sluggish, further pole lively; at the magnetic equator only liquid swirl.
Deviation: the aircraft's own magnetism. A: same on all headings; B: nose magnet, biggest E/W; C: wing magnet, biggest N/S. B = (E − W) ÷ 2, C = (N − S) ÷ 2.
A swing finds, removes and records deviation; it is needed after shocks, lightning, electrical changes, big latitude changes.

What does a magnetic compass do?

What the compass does

A magnet that points north • Inside the compass is a small magnet • Like any compass needle it swings to point at magnetic north • It does that whatever the aircraft does
Read it at the line • From the cockpit you look through a window at the edge of a card • The number under the white lubber line is your magnetic heading: here 030°
Turn, and the card stays • When the aircraft turns, the compass case turns with it • The card does not: it keeps pointing north • So the numbers slide past the line: watch the heading change

What is inside a direct-reading compass?

Inside the compass

Take the glass away • Here is the compass with its front glass removed • One part at a time now
1. The magnets • Two short, strong bar magnets (red end north) • Short and strong so they are sensitive: they feel even a weak pull
2. The card • The magnets are fixed under a card • The card has the headings printed round its edge • Card and magnets turn together, as one
3. The pivot • Card and magnets sit on a jewel on top of a post • A jewel is very smooth: little friction, so the card turns freely
4. The liquid • The bowl is full of liquid • Knock the compass: the liquid slows the swing and the card settles fast (aperiodic) • It also floats the card, taking weight off the pivot
Without the liquid • The same knock with no liquid: the card swings on and on • That is why the bowl is filled
5. The bellows • Warm liquid expands • A small bellows at the back gives it room, so the glass does not crack and no bubble forms
6. The lubber line • A white line on the glass, in line with the aircraft's nose • You read the heading against it
All the parts • Pulled apart: glass, card, magnets, pivot, bellows
Three design goals • Horizontal: the magnets must stay level to feel only the level pull (next lesson) • Sensitive: strong short magnets, a jewel pivot, a floated card • aperiodic: settles fast, thanks to the liquid
Kinds and limits • Vertical-card compasses read like a dial; grid-ring compasses are set with a rotating ring • Accuracy limit after correction: ±10° (European rule)