Magnetic Compass Errors

Types of Magnetic Compass Errors

Turning Error

Turning error occurs mainly when the aircraft turns through north or south headings. This is caused by the vertical (dip) component of Earth's magnetic field interacting with the pendulous design of the compass. The error is greatest on north and south headings and zero on east and west. In the northern hemisphere, the compass will lag (undershoot) when turning through north and lead (overshoot) when turning through south.

Acceleration Error

Acceleration error is most pronounced on east or west headings. When the aircraft accelerates or decelerates, the offset centre of gravity of the compass card causes it to tilt and rotate, leading to a false heading indication. In the northern hemisphere, acceleration causes the compass to indicate a turn toward north, and deceleration toward south.

Deviation

Deviation is caused by magnetic fields within the aircraft, such as those from electrical equipment or structural components. This error varies with heading and is corrected as much as possible during a compass swing. Residual deviation values are recorded on a deviation card, which pilots must use to convert between compass and magnetic headings.

Compass Swing, Compensation, and Calibration

A compass swing is a ground procedure performed by engineers to measure and correct deviation. The aircraft is aligned with known headings, and the compass is adjusted using built-in magnets. After correction, any remaining deviation is documented for pilot reference. Compass swings are required after compass installation, significant maintenance, lightning strikes, or changes in aircraft magnetism.

Serviceability Checks

Before flight, pilots should check the compass for physical integrity, correct fluid level, and compare its indication with known references (such as runway heading or another compass).

Limitations Near Magnetic Poles

Near the magnetic poles, the horizontal component of Earth's field is too weak for reliable compass operation, making the instrument unusable in these regions.

Timed Turns

To avoid turning error, pilots can use timed turns (rate-1 turns) to change heading accurately, calculating the time needed for a specific heading change at a standard rate.