FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsHard
A directional gyro (DG) is correctly aligned with the magnetic compass at the start of a flight at 30° North latitude. Apparent precession caused by the earth's rotation occurs at a rate equal to 15° multiplied by the sine of the latitude, per hour. If the DG is not realigned during a 2-hour flight, how many degrees of drift due to apparent precession should the technician expect?
- A7.5°
- B15°
- C30°
- D22.5°
Show answer & explanationAnswer & explanation
Correct answer: B. 15°
The apparent precession rate is 15° × sin(30°) = 15° × 0.5 = 7.5° per hour. Over 2 hours, total drift = 7.5° × 2 = 15°, requiring periodic realignment of the DG with the magnetic compass.
Why the other options are wrong
- A. This is only the hourly rate, not the total drift over 2 hours.
- C. This value results from omitting the sine factor and using the full 15°/hr rate for 2 hours.
- D. This value results from using 3 hours instead of 2 in the calculation.
Apparent (Real) Precession of a Directional Gyro
Drift of a directional gyro caused by the earth's rotation, occurring at a rate of 15° × sin(latitude) per hour, requiring periodic realignment with the magnetic compass.
- Drift rate depends on latitude: 15° × sin(latitude) per hour
- Zero drift rate at the equator, maximum at the poles
- Pilots must periodically reset the DG to the magnetic compass heading
Memory trick: Sine of latitude scales how fast the earth 'steals' your gyro's heading.