A magnetic compass shows the heading of the aircraft relative to magnetic north, but steel, wiring and the running engine bend the local field, so the reading must be corrected by a card made from a compass swing, and it misreads in banked turns and under acceleration. Magnetic north differs from true north by an amount that depends on where you are, and wind pushes the aircraft sideways from the heading, so the course made good differs from the heading. A 20 km/h crosswind on an aircraft flying 80 km/h across it means a drift of about 14.5°.
Part of Aerodynamics and Flight
A heading reference for an aircraft: mounting it, correcting it, and allowing for wind and declination.
The Compass Itself Is Not New
The field navigation is covered in the Phase 0 orientation section; the corpus’s real navigation articles are Navigation Without Technology, Making a Magnetic Compass, True vs Magnetic North, Dead Reckoning and the rest of that section. This article covers only what changes in an aircraft. The carrier calls for a liquid-damped compass mounted away from engine and electrical interference, with a correction card in the cockpit.
Why The Aircraft Changes The Reading
- Deviation: iron and steel in the engine, mounts and tubing, and any current-carrying wire, deflect the needle. The error changes with the aircraft’s heading and with the engine running.
- Declination: the angle between magnetic north and true north depends on location (True vs Magnetic North).
- Turning and acceleration errors: a compass is a pendulum and tilts in a bank, so it reads wrongly during and after a turn; read it in steady, level flight.
- Vibration: needle swing and a stuck card.
Method: Swing The Compass
Step 1: Place the aircraft on a level site away from steel and vehicles, with the engine running at a normal setting.
Step 2: Point the aircraft on known magnetic headings (north, east, south, west and between), marked on the ground with a trustworthy reference such as a surveyed line, and read the compass at each.
Step 3: Record the difference between the true magnetic heading and the reading at each. That is the deviation.
Step 4: Write the correction card and fix it where the pilot can see it. Repeat after any change to the engine, wiring or steel parts.
Worked Example: Wind Drift
Heading is not the path over the ground. For an aircraft flying at airspeed V across a wind w at right angles, the drift angle is arcsin(w ÷ V). With V = 80 km/h and w = 20 km/h, arcsin(0.25) = 14.5°. The inputs are illustrative. A slow aircraft is pushed far off by quite a modest wind, and one with a 55-70 km/h stall speed is more affected than faster aircraft. Steering by the compass alone toward a distant field will miss it unless the drift is allowed for or the pilot navigates by landmarks (Terrain Reading).
For A First Aircraft
A pilot who stays within sight of the field, which is where first flights stay, does not need a compass. It is for later work, and it should always be backed by known landmarks and a plan for finding the way back.
Common Mistakes
- Mounting the compass near the engine, magnets or a steel tube.
- Using the uncorrected reading.
- Reading it in a turn.
- Forgetting the wind.
Safety
Getting lost in an aircraft with limited fuel
Fuel sets how long you can look for a landing place. Leave with enough fuel for the trip plus a reserve decided beforehand, and turn back when the plan says to, not when the engine coughs.
What’s Next
- Learn next: Inclinometer, the instrument that shows a skid
- Combine with: Wind Direction Reading, for finding the wind