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Compass deviation is defined as the angle between..
  • A
    True North and Magnetic North.
  • B
    True North and Compass North.
  • C
    Magnetic North and Compass North.
  • D
    the horizontal and the total intensity of the Earth’s magnetic field.

True Direction, the North Pole is taken as a geographic reference and is assigned an angular value of 000º T or 360º T.

In most light aircraft True directions aren’t the most precise because the airplanes aren’t equipped with instruments that will align itself with the True North Pole.

The primary source for directions inside a cockpit is the magnetic compass and he will align itself with the Magnetic North Pole and not with the True North Pole, those are not co-located.

Magnetic Direction, have as reference the Magnetic North Pole and is assigned an angular value of 000º M or 360º M.
Variation is the angle between the True and Magnetic North and is assigned in degrees East or West of the True North.

  • If the Magnetic North is west of the True North, westerly variation, the magnetic direction will have a higher angular value than the true direction.
  • If the Magnetic North is east of the True North, easterly variation, the magnetic direction will have a lower angular value than the true direction.

Variation (-) West – Magnetic Best
Variation (+) East – Magnetic Least

The utility of a magnetic compass on the aircraft will depend on the accuracy of its response to the magnetic field of the earth.

The metallic structure of the airplane, its engine and equipment can affect the accuracy of the compass, an effect called the compass deviation.

Compass deviation is defined by the number of degrees by which unwanted magnetic fields deflect the compass from magnetic north indication (desired indication).

Deviation (-) West – Compass Best
Deviation (+) East – Compass Least

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