The Magnetic Compass of an Aircraft

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The magnetic compass of an aircraft (top right). Photo Credit: Nicholas Volodimer

The magnetic compass is perhaps the simplest instrument employed to indicate direction, and has been used widely by sailors and aviators alike. In airplanes, the magnetic compass is primarily used for navigational purposes. In spite of the fact that it is quite simple to use, and requires no input from the pilot to indicate direction, the magnetic compass is often the most poorly understood aircraft instrument in the airplane cockpit. Nonetheless, its wide use in all airplanes, ranging from single-engine aircraft such as the Cessna – 172 to large commercial jets such as the airbus A380, demonstrates the need for pilots to thoroughly understand the instrument.

Understanding the Earth’s Magnetic Field

Similar to the encapsulation of our planet by the ozone or the atmosphere, a flux or lines of magnetic force surround the Earth.

The magnetic field of the Earth is quite similar to that of this bar magnet. Photo Credit: Windell Osk

This pattern of magnetic flux surrounding the Earth behaves similarly to the pattern of a magnetic field around a large but weak magnet. Thus, the Instrument Flying Handbook, by the Federal Aviation Administration (FAA) refers to our planet as “a huge magnet, spinning in space”.

Magnetic lines of force have a unique property that gives them the power to influence magnets on the surface of the Earth. Any magnetic compass, be it in an aircraft, a ship or in a land vehicle, will align itself with these lines of force.

The Aircraft Compass – Construction

The magnetic compass of an aircraft consists of a float, fixed to a bar magnet. The float is placed on a pivot which is joined to the outer casing of the airplane compass. A compass card graduated in 360 degrees is attached to this float in such a manner that when the bar magnet moves, the motion of the compass card can be viewed from the face of the compass. The float assembly is filled with a liquid to ease the movement of the bar magnet suspended on the pivot.

The face of the aircraft compass has a permanent vertical line on the outer side. This line is referred to as the lubber line. All readings of magnetic heading are read with reference to the lubber line.

How does an Aircraft Compass Work in Flight?

The working of an airplane compass relies on the Earth’s magnetic field, so an aircraft in flight, maintaining a straight and level attitude, uses the airplane compass to relay magnetic heading as follows:

  1. The bar magnet is aligned with the respective magnetic line, generated by the Earth’s field.
  2. As an aircraft changes its direction, the compass card appears to move.
  3. This motion is due to the aircraft’s change in direction and should not be interpreted as movement in the magnetic compass. The bar magnet of a free airplane compass remains aligned at all times with the Earth’s magnetic field.
  4. The aircraft rotates around this stable, pivoted bar magnet giving the illusion of an apparent rotation of the bar magnet itself.
  5. This movement is read on the compass card, with reference to the lubber line.
  6. Magnetic heading is attained and used for navigational purposes.

An aircraft magnetic compass currently showing a magnetic heading of 020. Photo Credit: Maurizio Pucci

Compass Movement and Errors

The compass swings freely inside its casing, cushioned by the liquid, while the outer casing (fixed to the plane) moves in cohesion with the aircraft. These inertial properties of the magnetic compass create certain errors and lags in the instrument which make it difficult to read the compass. The readings of a magnetic compass are unreliable when the aircraft is turning, accelerating, decelerating, climbing or descending.

These errors are, of course, manageable, with common corrective measures, but an gyroscopic instrument referred to as the heading indicator has been incorporated into most aircraft to minimize reliance on the compass.

Resources:

Aeroplane General Knowledge and Aerodynamics, Aviation Theory Centre, 2004.

Instrument Flying Handbook, Federal Aviation Administration, 2007.

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