Aircraft Magnetic Field Effects: Deviation, Compensation, and Compass Swing
The aircraft itself generates a magnetic field due to its metal structure, electrical circuits, and engines. This disruptive field affects the compass accuracy.
Compass Deviation
Deviation is the angular difference between Magnetic North and the Compass Heading. It is the error caused by the aircraft's own magnetism.
- Deviation West (-): Compass points West of Magnetic North ("Compass Best").
- Deviation East (+): Compass points East of Magnetic North ("Compass Least").
Types of Magnetism
- Hard Iron Magnetism: Permanent magnetism acquired by the aircraft's steel components during manufacture or due to lightning strikes/maintenance. It is relatively constant.
- Soft Iron Magnetism: Temporary magnetism induced in the aircraft's structure by the Earth's field. It changes as the aircraft changes heading or latitude.
Compass Swing
A Compass Swing is a calibration procedure performed to measure and correct deviation.
- Process: The aircraft is positioned on a known heading (using a calibrated "Compass Rose" on the ground). Engines and electrical systems should be running/on to simulate flight conditions.
- Correction: Adjustable magnets in the compass housing are used to cancel out most of the deviation (Hard Iron).
- Residual Deviation: The error that cannot be removed is recorded on a Deviation Card placed near the compass.
When is a Compass Swing required?
- After long-term storage (aircraft stationary > 1 year).
- After a lightning strike.
- After significant changes to electrical systems/avionics.
- After maintenance involving magnetic materials.
- When errors are suspected.
Conversion Formula (Cadet)
To convert between headings, use the mnemonic C-D-M-V-T: Compass $\pm$ Deviation = Magnetic $\pm$ Variation = True. (East is Plus, West is Minus when going Left $\rightarrow$ Right).