What causes heading indicator drift during acceleration and deceleration, and how is it mitigated?

• A. A failure of the magnetometer.
• B. Air speed changes.
• C. Gyro precession during acceleration/deceleration; mitigated by aligning with the magnetic compass regularly.
• D. Captain's correction.

Answer: (C)

The heading indicator drifts during changes in speed because the directional gyro inside it experiences precession when the aircraft accelerates or decelerates. As you speed up or slow down, torques on the gyro’s gimbals cause its spin axis to move away from the true heading, so the instrument reading slowly drifts from the actual direction. To keep readings accurate, you regularly align the heading indicator with the magnetic compass, which provides a known reference heading and resets the gyro so it matches the real magnetic heading again. This is why the best answer emphasizes gyroscopic precession due to acceleration/deceleration and periodic realignment with the magnetic compass. The other options don’t explain the drift mechanism: a magnetometer failure would affect the compass itself, not the gyro’s drift pattern; airspeed changes alone don’t cause the directional gyro to drift in this way; and a captain’s correction isn’t a mechanical mitigation for the instrument.