The magnetic compass is an invention-free, power-independent backup instrument — required in every certified aircraft per CS-23.1303(b)(2) and functions even on total electrical failure.
Operating principle
A pivot-mounted magnet aligns with the horizontal component of the Earth's magnetic field. Display typically via a compass card with a lubber line, immersed in a fluid-filled capsule for damping.
Geomagnetic field — components
The Earth's magnetic field has two components:
- Horizontal component (H) — aligns the compass.
- Vertical component (Z) — causes magnetic dip: the compass magnet is pulled down toward the nearest magnetic pole.
Magnetic dip
| Latitude | Dip | Effect |
|---|---|---|
| Magnetic equator | 0° | Pure horizontal force, no dip-related issues |
| Mid-latitudes (e.g. Germany ~67°) | moderate | Acceleration and turning errors present |
| Polar regions | nearly 90° | Horizontal component near zero — compass unusable |
To compensate for dip, manufacturers use pendulous mounting and a counterweight — the compass floats in the fluid such that in straight-and-level flight the horizontal component is read correctly. During acceleration or turns, the characteristic errors appear (see §8.2).
Construction
| Component | Function |
|---|---|
| Compass card | 0–360° scale on the magnet bar |
| Magnet bar(s) | Aligns the card with the field |
| Lubber line | Reference mark on the case against which the card is read |
| Compensation screws | N/S and E/W for deviation correction |
| Fluid | Damps oscillation and reduces friction |