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The indication of the directional gyro is valid only for a limited period of time. The causes of this inaccuracy are:

1 - rotation of the earth
2 - longitudinal accelerations
3 - aircraft's moving over the surface of the earth
4 - vertical components of the earth's magnetic field

The combination that regroups all of the correct statements is:

  • A
    1, 2, 3.
  • B
    1, 2, 4.
  • C
    1, 3.
  • D
    1, 3, 4.
The DGI needs to be reset at regular intervals by reference to the magnetic compass. The instrument is suffering from the following errors:
  • Gimballing error – produced due to the shape of the gimbal system.
  • Random wander – change of direction in space of the rotor’s spin axis due to manufacturing imperfections.
  • Apparent wander due to Earth’s rotation – the gyro is initially aligned with a given meridian, but over time this meridian has moved due to Earth’s rotation, and the gyro is not correctly aligned anymore.
  • Rotor speed variations – which will be sensed and corrected by streaming more air to the bucket vanes of the rotor.
  • Transport wander – apparent wander due to the change of aircraft position.

Apparent wander due to Earth’s rotation can be calculated using the following formula:

Apparent wander (°/h) = 15 x sin (latitude)

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