A CFII observes that after 20 minutes of straight-and-level flight, the vacuum-driven heading indicator has drifted 6° from the magnetic compass reading, even though the aircraft flew a constant heading the entire time. What is the most accurate explanation and correct corrective action?
- AThis indicates the compass, not the heading indicator, is in error and requires compass swing
- BThis indicates a failing vacuum pump; the flight should be discontinued immediately
- CThis is normal gyroscopic precession from internal friction; the pilot should reset the heading indicator to the compass in straight-and-level, unaccelerated flight
- DThis is caused by the Coriolis effect and can only be corrected by changing latitude
Show answer & explanationAnswer & explanation
Correct answer: C. This is normal gyroscopic precession from internal friction; the pilot should reset the heading indicator to the compass in straight-and-level, unaccelerated flight
Vacuum-driven heading indicators normally precess a few degrees over time due to internal bearing friction and are designed to be periodically reset to the magnetic compass during stabilized, unaccelerated flight. A drift of this magnitude over 20 minutes is within the expected range for gyro precession error, not a system failure requiring termination of flight.
Why the other options are wrong
- A. The compass is the reference for resetting; it is not the source of this type of error.
- B. A single instance of normal precession drift does not indicate pump failure by itself.
- D. Coriolis effect is unrelated to heading indicator drift; that term applies to weather systems.
Heading Indicator Precession
The gradual drift of a vacuum or electric heading indicator caused by internal gyro friction, requiring periodic realignment with the magnetic compass.
- Normal drift is a few degrees per 15-20 minutes
- Reset only during straight-and-level, unaccelerated flight
- Excessive or rapid drift may indicate instrument or vacuum system malfunction
Memory trick: Friction makes the gyro drift—reset it like resetting a clock