Skip to content
PilotSchool Knowledge Base

Magnetic Compass- Principles and Corrections

Purpose and Use of a Compass Correction Card

Purpose of the Compass Correction Card

The compass correction card is an essential tool in aviation for correcting errors in the magnetic compass readings due to magnetic interference from the aircraft’s instruments and structure. It records the necessary corrections (deviation) that must be applied to the readings of the magnetic compass to obtain the true magnetic heading.

  • Magnetic Deviation: The card provides a set of corrections for each compass heading to equate it with the actual magnetic heading. These deviations are typically caused by magnetic fields within the aircraft.
  • Compass Calibration: After compensating for liability errors in the aircraft system, the compass correction card accurately records adjustments necessary for precise navigation.

Use in Determining Magnetic Heading

The compass correction card aids pilots in translating compass headings into magnetic headings by applying the deviations indicated. For example, if the correction card indicates a 5° east deviation when the compass reads 090°, the true magnetic heading is 085°.

  • Example: A magnetic heading of 090° adjusted to a compass heading of 085° due to a 5° east deviation.

Procedures for Preparing Compass Correction Card

The preparation involves aligning the aircraft along cardinal compass directions and noting residual errors after compensation adjustments. This is documented at 30-degree intervals to ensure regulatory compliance.

  • Documentation and Compliance: Correcting and recording data in alignment with FAA standards ensures navigation reliability.

References:

Principles of Direct Reading Magnetic Compass Construction

Fluid in the Compass

The fluid used in magnetic compasses serves two main purposes: preventing bubbles and reducing oscillations and friction.

  • Bubble Prevention: Temperature-induced fluid contraction might cause bubbles, which impede accurate needle movement. Prevention involves ensuring complete fluid filling or applying heat.
  • Friction Reduction: The fluid also reduces friction, allowing the compass needle to move freely.

Pendulosity of Magnet Systems

Pendulosity, referring to a tilting effect caused by vertical magnetic components, can introduce errors due to changes in the aircraft’s pitch. A pendulous suspension can stabilize the compass to manage these pitch-induced errors.

Magnetic Needles Pointing to Magnetic North

The compass needle’s alignment with the Earth’s magnetic field ensures accurate orientation. The use of pendulous suspension and adjustment mechanisms help mitigate errors due to deviations and maintain the needle alignment toward magnetic north.

References:

Compass Errors in the Southern Hemisphere

Acceleration Errors

In the southern hemisphere, when facing east or west, acceleration causes the north end (considered the ‘heavy’ end) of the compass to lag, giving a false reading of a southward turn. Conversely, deceleration gives a false northward indication.

Turning Errors

Turning errors are most significant when turning through the southern direction in the southern hemisphere due to magnetic dip. The dip causes the south-seeking end of the compass needle to dip, leading to erroneous readings, particularly during turns through North and South.

To counter these errors, pilots often use gyros (Directional Gyros) aligned during stable flight to reduce dependence on error-prone compass readings. Pilots need to make necessary corrections and be vigilant, particularly at headings through south.

References: