Compass Errors

Types of Compass Errors

• Deviation: This is the error caused by local magnetic fields within the aircraft, such as those from electrical systems, metal structures, or magnetic cargo. Deviation varies with heading and must be measured and recorded on a compass correction card after performing a compass swing.

• Variation: The angle between true north and magnetic north, which is not a compass error per se, but must be considered when converting between true, magnetic, and compass headings.

• Turning Error: When turning, especially near the poles, the compass may lag or lead due to the inertia of the compass card and the dip of the Earth's magnetic field. This is most noticeable when turning from north or south headings.

• Acceleration Error: When accelerating or decelerating on east or west headings, the compass may momentarily indicate a turn due to the dip of the magnetic field and the inertia of the compass card. In the northern hemisphere, acceleration causes a turn indication to the north, while deceleration shows a turn to the south (ANDS: Accelerate North, Decelerate South).

Causes and Symptoms

• Aircraft modifications, electrical changes, or the presence of magnetic materials can introduce or worsen deviation.
• Erratic, sluggish, or reversed compass indications during taxi or flight may signal a malfunction or severe error.
• Compass errors are checked during preflight by comparing compass readings with known references and through regular compass swings.

Managing Compass Errors

• Always refer to the compass correction card for deviation values on specific headings.
• Be aware of compass turning and acceleration errors, especially during manoeuvres on northerly and southerly headings.
• Regular maintenance, including compass swings, is essential after significant changes to the aircraft or after events like lightning strikes.