A CFI notices during a cross-country flight that the heading indicator has drifted 12 degrees from the magnetic compass heading after 45 minutes of flight, despite no turns being made. What is the most likely explanation, and what should the pilot do?
- AThe heading indicator has failed completely and must be replaced before further flight
- BThis is normal mechanical precession error inherent to the vacuum-driven gyro, and the pilot should periodically realign the heading indicator with the magnetic compass
- CA strong crosswind has physically pushed the gyro off its axis, requiring a return to level flight to correct it
- DThe magnetic compass is experiencing turning error and should be disregarded in favor of the heading indicator
Show answer & explanationAnswer & explanation
Correct answer: B. This is normal mechanical precession error inherent to the vacuum-driven gyro, and the pilot should periodically realign the heading indicator with the magnetic compass
Heading indicators are subject to small amounts of mechanical precession due to friction and other factors, typically causing drift of a few degrees per 15 minutes even in normal operation. Pilots are trained to realign the heading indicator with the magnetic compass approximately every 15 minutes during straight-and-level, unaccelerated flight to correct for this normal drift.
Why the other options are wrong
- A. Some drift is normal and expected; it doesn't necessarily indicate failure unless drift is excessive or erratic.
- C. Wind does not physically affect a gyro's orientation; drift is due to internal friction, not aerodynamic forces.
- D. The compass is reliable in steady, unaccelerated, wings-level flight, unlike during turns or acceleration.
Heading Indicator Precession
Small, normal drift in a vacuum-driven heading indicator caused by internal friction, requiring periodic realignment with the magnetic compass.
- Typically realigned every 15 minutes in level, unaccelerated flight
- Excessive or erratic drift may indicate a failing gyro or vacuum system
- Different from acceleration/turning errors of the magnetic compass itself
Memory trick: 'Every 15 minutes, sync the spinning compass to true north'