A pilot relies on a vacuum-driven heading indicator that has zero internal friction and a mechanically perfect gyro. Even so, the pilot must reset the heading indicator to the magnetic compass roughly every 15 minutes. What causes this necessary correction?
- AMagnetic dip near the vertical component of the Earth's field pulls the gyro card toward west
- BThe gyro's spin axis stays fixed in inertial space while the Earth rotates beneath it, creating an apparent drift of up to about 15° per hour
- CThe gyro exhibits real precession caused by internal bearing friction that must be corrected
- DStatic pressure changes with altitude cause the heading card to shift position
Show answer & explanationAnswer & explanation
Correct answer: B. The gyro's spin axis stays fixed in inertial space while the Earth rotates beneath it, creating an apparent drift of up to about 15° per hour
Because a gyroscope maintains rigidity relative to inertial space, and the Earth rotates approximately 15° per hour, the heading indicator will appear to drift even without any mechanical defect. This apparent wander is greatest at the poles and zero at the equator, requiring periodic realignment to the magnetic compass.
Why the other options are wrong
- A. Magnetic dip affects the magnetic compass, not a gyroscopic heading indicator.
- C. This describes real mechanical precession, not the earth-rotation effect described here.
- D. Static pressure has no effect on a vacuum gyro instrument.
Apparent Gyroscopic Wander (Earth Rotation)
Even a friction-free gyro appears to drift because its spin axis is fixed in inertial space while the Earth rotates beneath it, at a rate up to 15°/hour depending on latitude.
- Maximum apparent drift at the poles (15°/hr)
- Zero apparent drift at the equator
- Requires periodic realignment with the magnetic compass, separate from mechanical/friction precession
Memory trick: The Earth spins under a gyro that doesn't care — that's 'apparent' drift, not real error.