How often must the compass be swung for correct functioning?

The compass must be swung at least annually to ensure correct functioning and accuracy. This process involves calibrating the compass in relation to the aircraft's magnetic field and magnetic variation in the area where it operates. Over time, the Earth's magnetic field can change due to shifts in the magnetic poles, as well as local magnetic disturbances caused by other equipment or structures near the aircraft. Thus, an annual calibration helps to account for these changes, ensuring that pilots can rely on the compass for navigational purposes.

Regular compass swinging is crucial for flight safety, as an inaccurate compass can lead to navigational errors. While some aircraft might require more frequent checks based on specific operating conditions, the general requirement is set at one year in most regulations, which is why that choice stands out as correct.