A magnetic compass is one of the simplest and most important navigation instruments in an aircraft. It points toward magnetic north, giving pilots a basic heading reference even when electrical systems fail. Because Earth’s magnetic field is not lined up exactly with true north, compass readings must be corrected before they can guide accurate navigation.
Pilots learn these corrections because a small heading error can grow into a large position error over distance.
The compass is affected by variation, deviation, and motion related errors during turns and speed changes. Variation comes from the difference between true north and magnetic north at a location, while deviation comes from magnetic fields created by the aircraft itself. In flight, the compass also tilts and lags because the magnet assembly floats in fluid, causing turning errors and acceleration errors.
Safe compass use requires applying corrections and interpreting the instrument carefully, especially during maneuvering.
Key Facts
- Magnetic heading is measured relative to magnetic north, not true north.
- Variation is the angle between true north and magnetic north at a location.
- Deviation is compass error caused by magnetic fields in the aircraft.
- True course plus or minus variation = magnetic course.
- Magnetic heading plus or minus deviation = compass heading.
- UNOS rule: Undershoot north headings and overshoot south headings when rolling out of turns in the Northern Hemisphere.
Vocabulary
- Magnetic compass
- An instrument that uses a magnetized element to align with Earth’s magnetic field and show aircraft heading.
- Variation
- The angular difference between true north and magnetic north at a specific location.
- Deviation
- The compass error caused by magnetic fields from the aircraft structure, electrical equipment, or metal objects.
- Compass heading
- The heading indicated by the compass after accounting for magnetic effects and instrument error.
- Acceleration error
- A compass error caused by acceleration or deceleration, most noticeable on east or west headings.
Common Mistakes to Avoid
- Using compass heading as true heading without corrections is wrong because variation and deviation can shift the indicated direction by several degrees.
- Adding east variation when the navigation method requires subtracting it is wrong because sign conventions determine whether the final heading increases or decreases.
- Reading the compass during a turn as if it were stable is wrong because turning error makes the compass lag, lead, or swing depending on heading and hemisphere.
- Ignoring metal objects near the compass is wrong because phones, headsets, tools, and cockpit electronics can create deviation and change the indicated heading.
Practice Questions
- 1 An aircraft has a true course of 090 degrees and the local variation is 8 degrees west. What is the magnetic course if west variation is added?
- 2 A pilot plans a magnetic heading of 215 degrees. The compass deviation card shows 4 degrees east on that heading. What compass heading should the pilot steer if east deviation is subtracted?
- 3 A pilot in the Northern Hemisphere is turning from east to north and wants to roll out on 360 degrees. Explain whether the pilot should roll out early or late, and why.