We are used to travelling on the surface of the earth. To go from one place to another, we follow a highway that is built between the two locations. In the air, we do not have any highways and thus for basic navigation, we need to follow directions to go from place A to B.
Background: Traditionally, the direction from which the sun rises is called the East; the direction in which it sets is called West. When facing East, the direction on your left is called North, and direction to the right is called South. These four directions are called the cardinal directions. Directions in between the four cardinal directions, i.e. NE, SE, SW and SE, are called the quadrantal directions.
Definition of Direction: Direction is the angle measured clockwise from datum North. The direction is called True if the datum is the True North or the Geographical North; it is Magnetic if the datum is the magnetic North; and is Compass if the datum is the compass North. Direction is always expressed in a three figure group, with suffix of (T), (M), and (C) defining the datum, True, Magnetic or Compass respectively. An angle measured 1 degree from the datum Magnetic North would thus be annotated as 001* (M).
True North: True North is the direction of the North pole, and is indicated on maps by the meridians. Thus to measure True directions on a map the protractor should be aligned with the true meridian. Guess what is the direction of the geographic North pole?
Magnetic North: Magnetic North is the direction in which a freely suspended magnetic needle, which is only subject to the earth’s magnetic field, would point. The direction of this freely suspended magnetic needle is also known as the magnetic meridian and is the datum for measuring magnetic directions.
Compass North: A freely suspended magnetic needle when positioned in an aircraft is influenced by the electro-magnetic fields in the aircraft, in addition to the earth's magnetic field, and would thus point in a direction that is different from the magnetic North. This direction towards which it would point in a given aircraft is called the Compass North.
Practical Aspects: In navigation, we need to travel from one place to another on the surface of the earth. The direction to be flown between two places on the surface of the earth is measured from the map or chart. This direction is the true direction. However, in basic navigation we do not have the means to fly the true direction and thus need to use a magnetic compass. Since magnetic and compass direction may not be the same as true direction, we can only use the magnetic compass to steer the true direction if we know the angular difference between the magnetic North and true North (variation) and also the difference in the compass North and magnetic North (deviation).
Variation: The angular difference between the true and magnetic North at any given place is called Variation. It is measured in degrees East or West, depending on whether the magnetic North is to the East or West of the True North. Variation at any given place on the map/ chart can be found by looking for the lines joining places of equal variation – isogonic lines or isogonals. Lines joining places of zero variation are known as agonic lines.
Deviation: The angular difference between the magnetic and compass North on any given aircraft is called deviation. Deviation is measured in degrees E or W of the magnetic North depending on whether the compass North is E or W of the magnetic North. Since deviation is dependent on the electro-magnetic fields of the aircraft, it is logical that deviation would be different on different aircraft, and also on different headings of the aircraft. Deviation for any particular aircraft can be found on a deviation card installed next to the magnetic compass on the aircraft.
Finding Magnetic direction from True direction: Variation E is designated (+) and W is (-). To find true direction from magnetic direction, one needs to algebraically add the variation to the magnetic direction. It is much easier to remember the thumb rule
· Variation East Magnetic least
· Variation West Magnetic best.
Finding Magnetic direction from Compass direction: Deviation E (+) and W (-) are algebraically added to the compass direction to get magnetic direction. It is once again easier to remember the thumb rule
- Deviation East Compass least
- Deviation West Compass best.
Compass Error: Compass error is the algebraic sum of the variation and deviation.
Practically while preparing the flight plan on the ground, we start with measuring the true direction on the map or chart; look for the variation at that place on the same map or chart and apply that to get the magnetic direction to be flown. All this is calculated and put down in the flight plan form before we proceed to the aircraft. Once we reach the aircraft, we check the deviation card and apply the relevant deviation to the magnetic direction calculated as per the flight plan and we can now fly the true directions on the magnetic compass in that aircraft.