Earth's Magnetic Poles

Magnetic Poles

• The magnetic pole in the northern hemisphere is a magnetic south pole, and vice versa.
• The Earth's magnetic south pole is located at about 74°N 101°W, which is 2000 km from the geographic North Pole.

Magnetic Variation or Declination

• The grid of an aeronautical navigation chart is aligned to the geographic north pole (True North, TN).
• A magnetic compass aligns to the Earth's magnetic flux, which may be oriented either side of true north (Magnetic North, MN).
• The difference between true north and magnetic north at a geographic location is termed magnetic variation, variation, or declination.
• Variation is measured in degrees of error east or west of true north.
• Variation changes with geographic location, but it is not affected by aircraft heading.

Deviation

• Deviation is an error in the indication of an installed magnetic compass caused by local magnetic fields in the aircraft.
• Deviation error changes as aircraft heading changes.
• Deviation error is measured during a compass calibration.
• If the measured deviation is out-of-limits, it can be minimised by adjusting the compass (compass compensation or compass swinging).

Compass Correction Card

• The compass correction card is used to correct for deviation errors by the pilot.
• The card is mounted close to the compass and indicates the difference between the compass reading and the actual magnetic heading.
• The recorded deviation for each Magnetic Heading (MH) is used to calculate the corresponding Compass Heading (CH).

Direct Reading Compass Compensation

• Any components fitted to an aircraft that are magnetisable (i.e., contain iron) have the potential to distort the Earth's magnetic field.
• The effect of acceleration on the pendulous card can be classified into two distinct errors: Turning error and Acceleration error.

Turning Error

• Turning error occurs when an aircraft is in a coordinated turn, causing the pendulous card to remain vertical to the plane of the aircraft.
• The extent and direction of turning error are dependent on the aircraft heading, the hemisphere in which the aircraft is flying, the magnetic angle of dip, and the angle of tilt of the magnetic card.

Acceleration Error

• Acceleration error is caused by the force of centrifugal acceleration produced by the turn.

Remote-Reading Compass

• A remote-reading compass is mounted away from the instrument panel to eliminate interference from magnetic fields.
• The system consists of a magnetic field detector and a heading indicator, which may incorporate a directional gyro for increased stability.

Compass Terminology

• Calibration is the measurement of the deviation of a compass installed in an aircraft, any necessary compensation of this deviation, and the recording of the residual deviation.
• Compensation is the adjustment of a compass to minimise deviation errors caused by the magnetic properties of an aircraft.
• Compass correction card is a card mounted near a compass that indicates the difference between the compass reading and the actual magnetic heading.
• Deviation is an error in the indication of a magnetic compass caused by local magnetic fields in the aircraft.
• Direct Reading Compass is a compass that has the magnetic sensing element and heading indication located in the one instrument.