Magnetic Compass: Construction, Turning and Acceleration Errors
Construction and operation
The direct-reading magnetic compass consists of magnetised needles attached to a float and compass card, pivoted in a bowl filled with damping fluid. It aligns with the horizontal component of the Earth's magnetic field. It is the only instrument that indicates magnetic heading without a power supply, but it is accurate only in steady, level, unaccelerated flight.
Errors
- Variation: angle between true and magnetic north; taken from the chart.
- Deviation: aircraft-induced field error; taken from the compass swing card.
- Turning error (northerly/southerly): caused by the vertical component of the Earth's field and the compass's dip. In the Northern Hemisphere, when turning through north the compass under-reads (lags) and when turning through south it over-reads (leads). The magnitude is greatest at high magnetic latitude and is approximately equal to the latitude in degrees. The mnemonic is UNOS (Undershoot North, Overshoot South).
- Acceleration error: on acceleration in the Northern Hemisphere on an easterly or westerly heading, the compass indicates a turn toward north; on deceleration it indicates a turn toward south. Mnemonic ANDS (Accelerate North, Decelerate South).
- Oscillation and fluid issues: rough air causes oscillation; bubbles reduce damping.
Worked example
At magnetic latitude 40°N, rolling out of a turn onto 360° (north), the compass should be read when it indicates approximately 040° (lag of about 40°). Rolling out onto 180° (south), the compass should be read when it indicates approximately 140° (lead of about 40°). On east or west headings, acceleration causes a brief northern indication.
Question traps
Turning error applies only near north and south; acceleration error applies only near east and west. Questions often reverse the hemisphere - remember the errors reverse in the Southern Hemisphere.