Aircraft Acceleration Errors: The Magnetic Compass

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The float-type assembly of the magnetic compass in an aircraft makes it vulnerable to acceleration errors.. Photo Credit: Maurizio Pucci

The magnetic compass of an aircraft is one of the most primitive and sensitive instruments available in the cockpit of an airplane.  It works by translating magnetic flux, created by the magnetic lines of force, into the direction in which the aircraft is currently flying.

The compass assembly comprises of a bar-magnet, suspended in a liquid with a float attached to it. The float is held in place by a single pivot joined to the aircraft through its casing. However, the float is not balanced at its center of gravity which is a major contributing factor to acceleration errors of the magnetic compass found in modern day aircraft.

This arrangement of the magnetic compass is responsible for somewhat predictable, yet unwanted movements of the bar magnet in response to aircraft motion through the air.

Magnetic Compass and Accelerating Aircraft

Here, the hand of the performer on the chair acts as a pivot through which her weight acts on the chair. Photo Credit: Brent Moore

The magnetic compass of today is modified to decrease an effect categorized as “magnetic dip.” This modification incorporates a pivot assembly that is not connected to the float of the bar magnet at its center of gravity, but is in fact connected to the float at a point well-displaced of the center of gravity. Anything balanced on a point other than its center of gravity would inherently be unstable.

This setup is inline when an aircraft takes flight in northerly or southerly headings; the reason why no acceleration errors of the magnetic compass are experienced at these headings.

However, as an airplane accelerates on either a westerly or an easterly heading, the center of gravity (free of the pivot) swings accordingly. This causes acceleration errors in an aircraft’s magnetic compass.

How Aircraft Acceleration Errors Happen

Consider an aircraft flying straight and level, maintaining its desired airspeed. Since the airplane is neither trying to accelerate nor trying to climb, the forces of lift, weight, thrust, and drag cancel one another out, thus enabling the plane to maintain a state of equilibrium. As the aircraft accelerates from this state, to increase its airspeed, the following happens:

  1. A pivot of the magnetic compass, along with the compass casing, to accelerate with the aircraft.
  2. The float, being free of any direct bonds with the aircraft, retains its inertia.
  3. The lubber line, marked on the compass face, moves with the aircraft.
  4. The compass card, attached to the float, resists this acceleration.
  5. Inertia of the float, along with the bar magnet, forces the rotating float to lag behind in its attempt to catch-up with the speeding airplane.
  6. Balanced at a point other than its center of gravity, the float swings in a direction opposite to the force of acceleration, to indicate the current direction incorrectly.

Similar would be the effect on the magnetic compass if an aircraft decelerates in flight. However, since these acceleration errors are entirely due to the design of the magnetic compass, it is possible to correctly predict these swings.

As an Emirates Airbus A330 accelerates for take-off, the magnetic compass swings to indicate incorrectly. Photo Credit: Maximilian Wolf

For an aircraft in flight, in the northern hemisphere:

  1. Aircraft acceleration in easterly or westerly headings would cause the magnetic compass to indicate an apparent turn to north.
  2. Aircraft deceleration in easterly or westerly headings would cause the magnetic compass to indicate an apparent turn to south.
The direction of apparent turns reverse for accelerating and decelerating aircraft in the southern hemisphere. For instance, a magnetic compass accelerating on easterly or westerly headings would indicate an apparent turn to the south, when flying in the southern hemisphere.

Tom, an instructor from the Caesar Creek Soaring Club, explains acceleration errors in a magnetic compass on his glider:


Trainee pilots unaware of this error in the magnetic compass are more susceptible to deviation from their planned course, whenever accelerating or decelerating in flight.

In order to read the magnetic compass correctly, one must first establish and maintain the desired aircraft speed, wait for the accelerating characteristics to die out, and then read the magnetic heading from the compass.

Resources:

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

Instrument Flying Handbook, Federal Aviation Administration, 2007.

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