Now, if errors are still too big or inconsistent after a standard swing, the problem's probably more than just calibration. Check for Fluid and Purity: Take a look at the compass fluid. Modern ones use a specific fluid like MIL-C-5020A aliphatic naphtha or Isopar L. Cloudiness, bubbles, or a low...

A little regular care prevents most major compass headaches. Know the Triggers for a Swing: According to FAA guidance in AC 43.13-1B , you need a compass swing after any big change involving ferrous metals (like installing a new radio), if you suspect the compass accuracy, or after a lightning...

How Do You Calibrate An Aircraft Compass?

Quick Fix Summary

The TL;DR: Calibrating an aircraft compass, known as a "compass swing," is a two-part process. First, you physically adjust small magnets inside the compass housing to counteract the aircraft's own magnetic field. Second, you create a correction card that pilots use to apply any remaining, uncorrectable errors to their indicated heading.

What's Happening

An aircraft's magnetic compass aligns itself with the Earth's magnetic field. But the metal, electronics, and engine create their own magnetic fields that interfere, which causes some pretty significant heading errors. Honestly, this isn't just a minor inconvenience—it's a direct threat to navigation safety. The calibration process, which the FAA requires, systematically finds and minimizes these errors to make sure the compass gives a reliable heading. That's actually a legal requirement for VFR operation under FAR 91.205.

Step-by-Step Solution: Performing a Compass Swing

Safety First: You'll want the aircraft in its "normal flight" configuration: on level ground, with all systems (like avionics and fuel pumps) set up as they would be in cruise. And don't forget to take all the ferrous metals out of your pockets—tools, keys, that sort of thing.

Establish a Datum: Using a precise surveyor's transit or a certified compass rose painted on the ramp, align the aircraft's longitudinal axis with a known magnetic heading (say, 0° North). Then chock the wheels securely.
Record Errors: With the aircraft pointed at each cardinal heading (0°, 90°, 180°, 270°) and several points in between, write down the heading the aircraft's compass shows. The difference between that known magnetic heading and the compass reading is your error.
Adjust Compensators: Find the two small adjustment screws on the compass face. One's usually marked "N-S" and the other "E-W." Using a non-magnetic screwdriver, adjust these internal magnets following the compass maker's instructions to get those recorded errors as small as you can.
Create the Correction Card: Even after adjustment, a little error typically sticks around. For every 30° of aircraft heading, record the compass reading and the correction needed (something like "Steer 005° to fly 000°"). Then legibly put this data onto the official correction card mounted next to the compass.
Final Verification: Do one last check by aligning the aircraft with all the cardinal headings again. Make sure the errors are now within the limits in the aircraft's maintenance manual and that the correction card matches them accurately.

If This Didn't Work

Now, if errors are still too big or inconsistent after a standard swing, the problem's probably more than just calibration.

Check for Fluid and Purity: Take a look at the compass fluid. Modern ones use a specific fluid like MIL-C-5020A aliphatic naphtha or Isopar L. Cloudiness, bubbles, or a low fluid level can make the card stick or dampen poorly. Refilling it needs special care to avoid introducing air, by the way.
Inspect for Local Magnetic Fields: A major source of error is a new magnetic source you haven't accounted for. Systematically check for recently installed gear (headsets, tablets with magnets, new radios) or forgotten tools stuck in the instrument panel. Honestly, even a steel clipboard left on the glareshield can throw everything off.
Consider Component Failure: The compass itself might be failing. If the card oscillates wildly, sticks, or the fluid is discolored, the unit may need an overhaul or replacement. For remote-indicating "master compass" systems, the issue could also be in the transmitter or the repeater indicators.

Prevention Tips

A little regular care prevents most major compass headaches.

Know the Triggers for a Swing: According to FAA guidance in AC 43.13-1B, you need a compass swing after any big change involving ferrous metals (like installing a new radio), if you suspect the compass accuracy, or after a lightning strike.
Mind the Cockpit Environment: Set a strict rule for pilots and mechanics: no magnetic materials (certain tablet cases or speakers are common culprits) should be placed near the compass or instrument panel. This is easily the most common source of error after a calibration.
Schedule Proactive Checks: Don't wait for the compass to feel "wrong." Just include a quick verification of the compass correction card during annual or 100-hour inspections. Comparing current errors to the card helps you catch any drift early on.