Magnetic Compass in Aviation

Importance of the Magnetic Compass

• Basic but often overlooked equipment on airplanes
• Critical backup when modern avionics fail
• Used for heading information when AHRs or magnetometer fails

Errors in Magnetic Compass

Deviation and Variation

• Basic errors from private pilot training

Turning and Acceleration Errors

• Due to magnetic dip caused by the Earth's magnetic field
• Magnetic field lines dip near the poles
• Turning and acceleration errors are noticeable primarily at higher latitudes

Turning Errors

• Acronym: UNOS (Undershoot North Overshoot South)
• In the Northern Hemisphere:
  • Turn towards north: Compass lags behind (undershoot)
  • Turn towards south: Compass overshoots (overshoot)
• In the Southern Hemisphere: Errors are the opposite
• Rule of thumb for degrees to undershoot/overshoot: (Latitude/2) + 15 degrees
  • Example: Miami (26 degrees latitude) -> 28 degrees
  • Example: New York (40 degrees latitude) -> 35 degrees

Practical Example

• New York (40 degrees latitude)
  • Left turn from 090 to 360: Undershoot by 35 degrees (035)
  • Right turn from 090 to 180: Overshoot by 35 degrees (215)

Acceleration Errors

• Acronym: ANDS (Accelerate North Decelerate South)
• Only occurs on East-West headings
• Accelerate: Compass pulls north
• Decelerate: Compass pulls south

Techniques for Navigation with Magnetic Compass

Utilizing Timed Turns

• Essential when primary instruments fail
• Standard rate turn = 3 degrees of heading change per second
• Calculate turn duration: Desired heading change (degrees) ÷ 3 = Turn time (seconds)
  • Example: 90-degree turn = 30 seconds of turn time

Practical Example

• Heading 270 to 360:
  • Change of 90 degrees
  • Turn time = 90 ÷ 3 = 30 seconds
  • Use timer for accurate turn

Maintaining Standard Rate Turns

• 15-18 degrees of bank at 100-120 knots
• Smaller turns (e.g., 10-degree heading change from ATC): Count 3 seconds in head
• Eliminates confusion and reduces pilot workload

Summary

• Magnetic compass is a pivotal tool in emergencies
• Understanding and counteracting errors ensure accurate navigation
• Timed turns provide a reliable method for heading changes