Magnetic compass construction
Compass points north because earth is a giant magnet with invisible lines of magnetic flux from magnetic north pole to magnetic south pole. Any magnet free to rotate lines up with them (PHAK ch.8).
Parts:
| Part | Detail |
|---|---|
| Magnets | 2 small, attached to a metal float |
| Compass fluid | Clear, kerosene-like. Float sealed in bowl of it, fills case. Damps float and card oscillation |
| Card | Graduated, wrapped around float. Read through glass window with lubber line |
| Pivot | Float/card assembly rides on it |
- Float assembly centre of gravity sits below the pivot.
- Result: magnetic dip's vertical pull too weak to tip card off horizontal in normal flight (PHAK ch.8).
Reading it, backward:
- Card stays fixed to magnets. Housing and pilot rotate around it.
- North heading: "33" (330°, west of north) sits on right. Graduations run reverse of a normal compass rose.
- Markings: cardinal letters N/E/S/W, plus a number every 30° with final zero dropped:
| Mark | Heading |
|---|---|
| 3 | 030° |
| 6 | 060° |
| 33 | 330° |
Deviation sources: aircraft electrical equipment, radios, magnetic cargo.
Serviceable compass: housing entirely full of compass fluid (PHAK ch.8).
Trap: card stays still, aircraft turns; "33" on right on north heading is correct.
References
- PHAK ch.7 (FAA-H-8083-25)
- PHAK ch.8 (FAA-H-8083-25)