How do magnetic compasses work, how do you adjust them, what are their tolerances?

How do magnetic compasses work, how do you adjust them, what are their tolerances?

• A. They align with Earth's magnetic field; adjust with compass correction cards and swinging; tolerances commonly within ±5 degrees
• B. They rely on GPS signals; adjusted with magnetic tape; tolerance within ±1 degree
• C. They use accelerometers; adjust with autopilot; tolerance within ±10 degrees
• D. They are gyroscopes; adjust with reference star; tolerance within ±15 degrees

Answer: (A)

Magnetic compasses read heading by aligning a magnetized element with Earth’s magnetic field, giving a magnetic heading. But the instrument is never perfect because the aircraft itself creates magnetic disturbances and because Earth’s field varies from place to place. Those disturbances are called deviation (caused by the airplane’s own ferrous equipment) and variation (the difference between true north and magnetic north at a location). To navigate accurately, pilots apply corrections: the variation shown on charts and a deviation correction, usually captured on a compass correction card. The compass is calibrated by swinging the aircraft or using a calibration process to map how the readings off the instrument deviate across headings; this information is used to adjust readings so the indicated heading matches the true or magnetic heading as needed. The typical tolerance for an aviation magnetic compass is about plus or minus five degrees. This means, after correction and calibration, the heading shown should be within five degrees of the actual magnetic heading under normal operating conditions.

Magnetic compasses read heading by aligning a magnetized element with Earth’s magnetic field, giving a magnetic heading. But the instrument is never perfect because the aircraft itself creates magnetic disturbances and because Earth’s field varies from place to place. Those disturbances are called deviation (caused by the airplane’s own ferrous equipment) and variation (the difference between true north and magnetic north at a location). To navigate accurately, pilots apply corrections: the variation shown on charts and a deviation correction, usually captured on a compass correction card. The compass is calibrated by swinging the aircraft or using a calibration process to map how the readings off the instrument deviate across headings; this information is used to adjust readings so the indicated heading matches the true or magnetic heading as needed.

The typical tolerance for an aviation magnetic compass is about plus or minus five degrees. This means, after correction and calibration, the heading shown should be within five degrees of the actual magnetic heading under normal operating conditions.