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.
Understanding Aviation: The Magnetic Compass
Inside a traditional wet compass, one or more small magnets are attached to a lightweight card marked with directions and numbers. The assembly sits on a pivot inside a bowl filled with liquid. The liquid damps sudden movement, so the card does not swing wildly after turbulence or a turn.
This useful damping creates a delay. The compass card needs time to settle, so its reading is most trustworthy during straight, level, unaccelerated flight. Pilots check the instrument before flight for a clear fluid bowl, readable markings, no air bubbles large enough to interfere with movement, and a card that turns freely.
The Earth’s field does more than pull the compass toward north. In most of the Northern Hemisphere, magnetic field lines slope downward into the ground. This magnetic dip pulls the north-seeking end of the compass downward.
The pivot and the shape of the magnet system reduce the effect, but they cannot remove it completely. During a turn, the tilted magnetic system responds differently depending on the aircraft heading. A turn starting near north can appear slower than it really is.
A turn starting near south can appear faster. The familiar rollout rule is a practical response to this behavior, but it applies only under the conditions for which it was taught. A pilot must still use the heading indicator, outside references, and the actual compass movement.
Acceleration error comes from the same dip effect. When an aircraft speeds up or slows down on an east or west heading in the Northern Hemisphere, the compass can briefly show a false turn. On an east or west heading, acceleration makes the compass indicate a turn toward north.
Deceleration makes it indicate a turn toward south. Students often remember this as accelerate north, decelerate south. It is not a command to turn the aircraft.
It is a warning that the compass is temporarily misleading. This error becomes less important near the magnetic equator because the field lines there are more nearly horizontal.
Accurate navigation uses a sequence of references rather than one compass reading. A chart gives a direction based on true north. Local magnetic information converts that planned direction to a magnetic reference.
The aircraft compass correction card then accounts for errors caused by radios, wiring, metal structure, and other equipment. This card is made during a compass swing, when the aircraft is placed on known headings and the difference is recorded. Deviation can change after equipment is installed, removed, or moved.
A headset, tablet mount, loose tool, or magnetic phone case near the compass can matter. Good practice is to keep magnetic objects away, read the compass only when conditions are stable, and cross-check it against other instruments. The magnetic compass is simple, but using it well requires patience and careful observation.
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.