Magnetism — Foundations for Navigation
This lesson covers geomagnetic basics as relevant for navigation. Equipment details (magnetic compass types, gyrocompass, etc.) are covered in Subject 020 Aircraft General Knowledge. Only the essentials here.
Earth as a magnet
The Earth behaves like an oblique bar magnet inside. The magnetic axis is tilted about 11° against the rotation axis and shifts slowly.
- Magnetic North Pole: in 2024 about 86°N, 137°E (per IGRF-13/NOAA WMM 2020), moving from the Canadian Arctic toward Siberia at about 55 km/year.
- Magnetic South Pole: about 64°S, 137°E (south of Australia).
The position changes continuously — hence the World Magnetic Model (WMM) by NOAA with a five-year update cycle (currently WMM 2020, next WMM 2025).
Geomagnetic field
At each point on Earth, the field is characterised by three quantities:
| Quantity | Symbol | Meaning |
|---|---|---|
| Total field strength | F | magnitude of the magnetic field vector (in nT, nanotesla) |
| Inclination / magnetic dip | I | tilt angle of the field vector against the horizontal |
| Declination / variation | D | angle between geographic and magnetic north (in the horizontal plane) |
Magnetic dip (inclination)
- 0° at the magnetic equator — field vector horizontal.
- +90° at the magnetic North Pole (vector points vertically into the Earth).
- −90° at the magnetic South Pole (vector points vertically out of the Earth).
- In Central Europe (≈ 50°N): inclination about 65°–68° toward the Earth's interior.
Consequence for the magnetic compass: the strong inclination makes the horizontal component of the field small at high latitudes → the magnetic compass becomes inaccurate (caution above 60° latitude, hardly usable above 70°).
Variation (see previous lesson)
- Local angle between TN and MN.
- Shown on ICAO charts as isogonals (dashed lines).
- Annual change noted, due to slow drift.
Deviation
- Distortion of the magnetic field inside the aircraft caused by:
- Iron-bearing structures (engine, engine cowling, steel members).
- Electric currents (alternator, avionics wiring).
- Permanently magnetised parts.
- Calibrated on a compass rose (compass swing) and documented on a Compass Correction Card.
- Changes over the aircraft's life (e.g. new avionics installed → new correction card needed).
Magnetic compass errors in flight
The magnetic compass is correct in straight, level cruise flight. In acceleration and turning phases, systematic errors occur:
1. Acceleration error
- On east and west headings (NH): on acceleration the compass appears to indicate more north heading, on deceleration appears more south.
- Mnemonic: ANDS — Accelerate North, Decelerate South.
2. Turning error (Northerly/Southerly)
- On north heading (NH): when rolling into a turn the compass lags, the pilot must roll out early to reach the desired heading.
- On south heading (NH): when rolling into a turn the compass leads, the pilot must roll out late.
- Mnemonic: UNOS — Undershoot North, Overshoot South.
- On east/west headings: no turning error.
Source: FAA-H-8083-25B Chapter 8 Flight Instruments.
3. Oscillation and pendulum errors
- In turbulence the compass rose oscillates; read the mean.