An aircraft's compass heading is 004°. The deviation card indicates 1°W on northerly headings and the variation is 5°W. What is the aircraft's true heading?
Refer to figure.
MAGNETIC VARIATION
The earth behaves as though a huge permanent magnet were situated near the centre producing a magnetic field over the surface.
The poles of this hypothetical earth-magnet do not lie on the earth’s spin axis, this lack of symmetry giving rise to magnetic variation - the angular difference between the True North (TN) and the Magnetic North (MN).
Variation is designated west or east depending on whether the magnetic pole lies to the west or to the east of 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
COMPASS DEVIATION
Magnetic material and electrical circuits surround the flight deck that influence the Earth’s magnetic field and cause the compass needle to deflect away from the local magnetic meridian.
This is known as compass deviation and can result in deviation either east or west of magnetic north by an amount, which depends on the aeroplane’s heading and latitude.
Deviation (-) West – Compass Best
Deviation (+) East – Compass Least
Any deviation can be analysed and resolved into components acting along the aeroplane’s major axes. The causes of the deviation are analysed and corrected for by carrying out a compass swing, although some residual deviation remains, and recording them on a deviation card.
To obtain a True Heading from the Compass Heading:
Use the acronym C-D-M-V-T, and replace the values you are given in the question:
From left to right, magnetic deviation W is subtracted from Compass Heading to get Magnetic Heading and magnetic variation E is added to Compass Heading to get Magnetic Heading.
From left to right, magnetic variation W is subtracted from Magnetic Heading to get True Heading and magnetic variation E is added to Magnetic Heading to get True Heading:
Compass Heading (C) |
Deviation (D) |
Magnetic Heading (M) | Variation (V) |
True Heading (T) |
004º | 001ºW | 003º | 5ºW | 358º |
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