by Jonathan M. Stern
Errors Inherent in a Magnetic Compass
There are several types of errors associated with the use of a magnetic compass. The first, variation, was discussed previously. The most significant of the other errors are deviation, Northerly turning error, and acceleration error. None of these other errors, however, is re-created on Flight Simulator.
Compass deviation is caused by magnetic disturbances within the aircraft. These can be caused by electrical fields within the aircraft's systems and components or by metal structures within the aircraft.
Deviation is dealt with by periodic adjustments to the magnets in the compass and a compass correction card. The compass correction card indicates for the pilot the compass headings to fly to achieve desired magnetic headings. Although illegible on Flight Simulator, the center post on the Lear 35 instrument panel, which is depicted in Figure 2.31 contains a compass correction card.
Figure 2.31. The compass correction card addresses magnetic deviation.
The Northerly turning error causes compass readings to be incorrect at the beginning of a turn from a Northerly or Southerly heading. The error is smaller from initial headings that are more Easterly or Westerly, and there is no error from turns initiated from East or West headings. When initiating a turn from a Northerly heading, the compass actually turns opposite the direction of turn for a moment and then reverses course and turns in the correct direction.
When a turn from a Southerly heading is initiated, the compass turns in the proper direction but at a rate faster than the actual turn (see Figure 2.32).
Figure 2.32. Northerly turning error is one of the errors inherent in the magnetic compass.
The other significant error of the magnetic compass is acceleration error. Unlike Northerly turning error, acceleration error is most prevalent on East or West headings and disappears when the aircraft is headed due North or South. On an Easterly or Westerly heading, the compass indicates a heading North of the actual heading during acceleration and South of the actual heading during deceleration. This can be remembered by the acronym ANDS, which stands for Accelerate North, Decelerate South.
Note: For readers in the Southern Hemisphere, the Northerly turning error and acceleration error are just the opposite. The acceleration error yields a Northerly indication during deceleration and a Southerly indication during acceleration. The Northerly turning error is also opposite of that described.