Direction (061.01.03)

Direction in navigation is measured as an angle from a datum. On Earth the datum for direction is the geographic North Pole. This is the point where the Earth’s axis of rotation meets its surface. Directions are measured in degrees in a clockwise direction from the north datum, the so called “True North (TN)“. True directions are shown with the letter T after the degrees sign, like: 035°T.

It is possible to read true directions straight of an aeronautical chart, by aligning the protractor with the True North reference, the meridians.

In flight, compasses are used to find direction, and compasses indicate the direction with reference to the Earth’s magnetic pole, not the geographic pole. The magnetic pole is the point through which the magnetic field lines re-enter the planet.

The magnetic pole is not at the same location as the geographic pole. Actually, this magnetic pole is not even fixed and changes position over time due to fluctuations in the Earth’s magnetic field. (Currently, Magnetic North is located in the Arctic region, near Canada, and is moving towards Siberia.)

While the True North is a fixed point, Magnetic North (MN) varies, as stated, over time and varies based on the geographical location as the Earth’s magnetic field is not uniform.The difference is referred to as Magnetic Variation.

Magnetic Direction

Variation is the difference between True and Magnetic heading.

Variation is measured in degrees, and is named east or west according to whether the Magnetic North lies to the east or west of the True North at a specific point.

So magnetic heading, plus or minus variation, equals true heading. Whether to “add” or to “subtract” is depending on whether the MN is west or east of the TN at a certain location.

• East variation: MN is east of TN, in this case you should subtract the variation from the True North.
• West variation: MN is west of TN, in this case you should add the variation to the True North.

Mnemonic:
Variation east, magnetic least (-)
Variation west, magnetic best (+)

Important: Variation can be given like, 4°W or 16°E. But also like: -4° or +16°, which is exactly the same. So in those cases read + as east, and – as west.

Mnemonic (rewritten):
Variation east (+), magnetic least (-)
Variation west (-), magnetic best (+)

Compass Direction

The readings of the Earth’s magnetic field by a compass installed in an airplane is distorted by the surrounding metal and electrical equipment. The needle slightly deviates from magnetic north and indicates a direction known as the Compass North (C).

Deviation is the difference between Compass North and Magnetic North.

Deviation is also measured in degrees and is one again described as east (+), west (-).

Mnemonic :
Deviation east (+), compass least (-)
Deviation west (-), compass best (+)

Calculations

In order to convert an aircraft’s compass heading to a true heading and vice versa, deviation and variation has to be applied in the correct sequence. Using this table might help:

C (compass)	D (deviation)	M (magnetic)	V (variation)	T (true)

Example 1: Given, true heading is 005°, variation 32°„, deviation 4°E. What is the compass heading?

Example 2: Given, compass heading is 233°, true heading 246°„, deviation -3°. What is the variation?

Richting in navigatie wordt gemeten als een hoek ten opzichte van een referentiepunt.

Heading, Track and Drift

The heading of an aircraft is the direction the airplane’s nose is pointing, measured as an angle from the north (T, M or C).

The track is the aircraft’s path over the ground, of which we distinguish two:

• Required track, the track we wish to follow;
• Track made good, the actual track;

Track Error (TKE), is the angle between the required track and the track made good.

Drift, is the angle between the aircraft heading and the track made good (actual).

Drift is expressed as to the left (port) or to the right (starboard). Where, important to remember, drift goes from heading to track (air to actual).

Drift goes from heading to track (air to actual).

---

Wind Correction Angle

If there is wind hitting the airplane, let’s say, from the left, and we don’t compensate for it, then it will push (drift) the plane to the right. So with the same heading, the actual track will be drifted to the right of the required track.

In order to maintain the required track, we have to compensate for the wind, by pointing the noise of the plane slightly into the wind. Actually, by the same angle as the drift would have been, but in opposite direction. This what is called the Wind Correction Angle (WCA).

So the WCA applied to the required track, gives the required heading to be flown. For example: