FAA Instrument Rating Airplane (IRA)Flight Instruments and SystemsMedium
A pilot flying in smooth air notices that after 20 minutes, the heading indicator (directional gyro) has drifted 8° from the magnetic compass reading. No abnormal maneuvering occurred. What is the most likely cause of this discrepancy?
- ACompass turning error from magnetic dip
- BInherent gyroscopic precession caused by internal friction and the earth's rotation
- CCompass deviation error from electrical equipment
- DA leak in the pitot-static system
Show answer & explanationAnswer & explanation
Correct answer: B. Inherent gyroscopic precession caused by internal friction and the earth's rotation
Heading indicators are subject to real precession caused by friction within the gimbals and bearings, as well as the earth's rotation, which is why pilots must periodically realign the heading indicator with the magnetic compass, typically every 15 minutes.
Why the other options are wrong
- A. Turning error affects the magnetic compass during maneuvers, not the mechanical heading indicator in level flight.
- C. Compass deviation is a magnetic compass error, not something that would cause the heading indicator to drift relative to the compass.
- D. A pitot-static leak would affect airspeed/altitude instruments, not the heading indicator.
Heading Indicator Precession
Real precession in a directional gyro, caused by internal friction and the earth's rotation, causes gradual drift requiring realignment with the magnetic compass approximately every 15 minutes.
- Not the same as gyroscopic (apparent) precession from applied force
- Realignment should be done during straight, level, unaccelerated flight
- Excessive drift rate may indicate instrument malfunction
Memory trick: Every 15 minutes, reset the DG before it 'wanders off.'