Compass Swing and Deviation

What Is Compass Swing?

A compass swing is a ground-based calibration procedure performed by maintenance engineers to assess and minimise compass deviation. The aircraft is aligned with surveyed magnetic headings (typically North, South, East, and West) on a compass swing site, with all normal electrical and avionics systems operating to simulate flight conditions. The compass reading is compared to the known heading, and the difference is the deviation for that heading.

Correcting Compass Deviation

Deviation is caused by magnetic fields from aircraft components, electrical systems, and structural materials. During a compass swing, engineers adjust compensating magnets (E/W and N/S screws) near the compass to reduce deviation as much as possible. This process is known as compass compensation. After adjustments, residual deviation values are recorded for each heading.

The Compass Card

The residual deviation for each heading is documented on a compass correction card (compass card) mounted near the instrument. Pilots use this card to convert between compass heading and magnetic heading, applying the listed corrections for accurate navigation.

When Is a Compass Swing Needed?

Compass swings are required after compass installation, significant maintenance, structural modifications, loading magnetic cargo, lightning strikes, or after operating in different magnetic latitudes. Regulatory standards set limits for acceptable residual deviation (e.g., ±5° for main compasses).

Deviation Behaviour and Limitations

Deviation is not constant; it varies with heading, aircraft configuration, and latitude. Pilots must understand that the direct-reading compass is only reliable in straight, level, unaccelerated flight. Errors can occur during turns, acceleration, or pitch changes, so the compass card remains vital for accurate heading information.