A flight instructor explains that magnetic compass turning error and acceleration/deceleration error share a common underlying cause. What is that cause?
- AThe rotation of the Earth beneath a fixed compass card
- BElectrical interference from onboard avionics during turns and acceleration
- CStatic pressure fluctuations acting on the compass card during maneuvering flight
- DThe pendulous mounting of the compass card, combined with magnetic dip, causes the card to swing out of alignment during turns and speed changes
Show answer & explanationAnswer & explanation
Correct answer: D. The pendulous mounting of the compass card, combined with magnetic dip, causes the card to swing out of alignment during turns and speed changes
The magnetic compass card is mounted on a pendulous assembly so it can align with the horizontal component of the Earth's magnetic field. Because of magnetic dip (the vertical component of the field pulling on the card), banking during turns and changes in speed cause the card to swing and produce turning and acceleration/deceleration errors.
Why the other options are wrong
- A. Earth's rotation causes apparent gyro wander, not magnetic compass errors.
- B. Avionics interference can cause deviation, not the systematic turning/acceleration errors described.
- C. The compass is a magnetic instrument, unaffected by static pressure.
Magnetic Dip and Pendulous Compass Card
The compass card's pendulous mounting allows it to react to magnetic dip (the vertical field component), which is the root cause of both turning error and acceleration/deceleration error.
- Dip is greatest near the magnetic poles, negligible near the equator
- Pendulous mounting lets the card swing during banked turns
- Same root cause explains both turning and ANDS errors
Memory trick: Dip makes the card dip and swing — that's the root of all compass trouble.