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Heading indicator (DG / HSI) precession means
Choices
the gyro speeds up over time.
Gyros don't speed up; they slow down with friction.
✓ the gyro slowly drifts off the correct heading and must be periodically re-aligned with the magnetic compass.correct
Precession means the gyro slowly drifts off the correct heading and must be realigned periodically with the magnetic compass (typically every 15 minutes during straight-and-level flight). Mechanical (vacuum-driven) heading indicators precess due to gyro friction and bearing wear, drifting ~3° per 15 minutes. Pilot must periodically reset the DG to the magnetic compass during straight-and-level flight (compass is only accurate when not turning/accelerating). Modern electronic ADAHRS units don't precess but have other failure modes.
the heading indicator only works above 5,000 ft.
DG works at all altitudes.
the indicator reverses readings.
DG doesn't reverse.
Why
Precession means the gyro slowly drifts off the correct heading and must be realigned periodically with the magnetic compass (typically every 15 minutes during straight-and-level flight). Mechanical (vacuum-driven) heading indicators precess due to gyro friction and bearing wear, drifting ~3° per 15 minutes. Pilot must periodically reset the DG to the magnetic compass during straight-and-level flight (compass is only accurate when not turning/accelerating). Modern electronic ADAHRS units don't precess but have other failure modes.
FAA source: FAA-H-8083-25C, Ch. 8, Flight Instruments - Heading indicator / gyroscopic precessionbrowse the reference library →
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Original study question written for this course — representative of FAA knowledge-test topics, not an actual current FAA exam question. Reviewed by a panel of three CFIs + AI review.