Magnetic compass

The magnetic compass

Magnetic compass: Basic theory

  • The compass is mounted usually in acid-free kerosene that dampens movement.
  • Compass Card is marked with direction.
  • Rotates freely and can tilt up to 18°.
  • There are three primary errors associated with a compass:
    • Compass deviation.
    • Magnetic dip.
    • Acceleration Error.
  • The magnetic compass is accurate only while the aircraft is flying wings-level in steady-state, non-accelerated flight.

Variation

  • The angular distance between the true and magnetic pole.
  • When the variation is westerly it is added to True heading to find Magnetic heading.
    • 030 ‎° True + 10 ‎° west variation = 040 ‎° Magnetic heading.
    • « West is best »
  • When the variation is easterly it is substracted from True heading to find Magnetic heading.
    • 030 ‎° True – 15 ‎° west variation = 015 ‎° Magnetic heading.
    • « East is least »

Compass deviation

  • Compass deviation is deviation errors of the compass caused by the aircraft’s magnetic fields.
  • It is checked regularly as part of aircraft maintenance. 
  • Effects of deviation are minimized by corrector magnets attached to the compass housing, while the remaining deviation errors are recorded and displayed in the cockpit on the compass deviation card.

Magnetic dip errors

  • The Northerly Turning Error occurs during shallow turns through southerly and northerly headings.
  • The compass “leads” when turning from southerly headings.
  • The compasslags” when turning from northerly headings.
  • There is approximately no turning error when turning east and west headings.

Acceleration Error

  • Acceleration/Deceleration Error occurs on easterly and westerly headings.
  • Acceleration causes the compass to show a northerly error.
  • Deceleration causes a southerly error.
  • Remember this by “ANDS.”
  • There is approximately no such error on northerly and southerly headings.

Review questions

  • What is the cause of Deviation error of the magnetic compass?
  • What is magnetic variation?
  • What errors are magnetic compasses subject to?
  • What is northerly and southerly turning error?
  • What is acceleration error?
  • During flight, when are the indications of a magnetic compass accurate?