The TL;DR: A compass swing removes magnetic interference from the aircraft and creates a correction card for residual errors. You’ll need a known magnetic heading, non-magnetic tools, and the aircraft in normal flight configuration. The whole job takes about 30–45 minutes if you’re prepared.
What’s Happening Inside an Aircraft Compass
A direct-reading magnetic compass aligns with Earth’s magnetic field to give heading, but the airplane itself generates magnetic “noise.” Engines, avionics, fuel, and even the pilot’s headset can pull the compass off by several degrees. These errors pile up fast and can mislead a pilot flying under VFR—especially when close to terrain or in poor visibility. The FAA requires a compass swing every time the aircraft’s magnetic environment changes, as spelled out in FAR 91.205.
Step-by-Step Solution: Performing a Compass Swing
Safety first: Do the swing on a calm day, aircraft chocked and brakes set. Remove all ferrous tools and metal objects from your person and the cockpit. Follow the aircraft maintenance manual, which in most cases references AC 43.13-1B.
- Choose a Reference Point: Use a certified compass rose painted on the ramp or a surveyor’s transit aligned to true north. Rotate the aircraft’s longitudinal axis to 0° (North).
- Power-Up and Stabilize: Start the engine and run radios, lights, and pumps exactly as in normal cruise. Let avionics warm up—cold screens can throw off flux-gate sensors in electronic compasses.
- Log the Errors: At each cardinal heading (0°, 90°, 180°, 270°) and the four intercardinal points (45°, 135°, 225°, 315°), record the compass reading versus the known magnetic heading. The difference is your deviation.
- Adjust the Compensators: Behind the compass face you’ll find two small slots marked “N-S” and “E-W.” Insert a non-magnetic (brass or plastic) screwdriver and turn the screws per the compass manufacturer’s table until each error is ≤ 10°.
- Build the Correction Card: For every 30° heading, note the compass reading and the correction needed. Example: “005°” next to “000°.” Print the card on non-ferrous stock and mount it beside the compass.
- Verify and Sign Off: Re-run the 8-point check. Errors must fall within the limits in the aircraft’s type certificate data sheet (TCDS). If they do, sign the maintenance log and attach the new correction card.
If This Didn’t Work
When deviation is still large or erratic, dig deeper.
- Check the Fluid: Modern compasses use a clear, inert fluid (MIL-C-5020A aliphatic naphtha or Isopar L). Cloudiness, air bubbles, or a low level can cause the card to stick or oscillate. Refill slowly with a syringe to avoid trapping air.
- Hunt for Hidden Magnets: Walk the cabin with a small handheld compass. Anything that deflects its needle—new tablet, headset magnet, even a steel clipboard—must be moved at least 12 inches from the compass or removed entirely.
- Inspect the Compass Assembly: If the card wobbles, sticks, or the fluid is brown, the unit may be internally damaged. For remote-indicating systems, swap the master compass or check the flux-valve transmitter for corrosion.
Prevention Tips: Keep the Compass Honest
- Know When to Swing: The FAA’s AC 43.13-1B lists triggers: installation of any ferrous item, lightning strike, or if the compass card shows >10° error during pre-flight.
- Enforce a Magnetic-Free Cockpit: Prohibit magnetic phone cases, speakers, and some kneeboard clips. Make it part of the pre-flight checklist.
- Add a Quick Verification: During each 100-hour or annual inspection, do a 2-minute check: align the aircraft to a known heading, note compass vs. GPS, and compare to the correction card. Any drift >3° triggers a full swing.
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—sometimes by several degrees. Honestly, this isn’t just a minor annoyance; it’s a real safety risk when you’re flying VFR. The FAA makes calibration mandatory for good reason. A proper compass swing systematically finds and minimizes these errors so the compass gives a reliable heading. That’s not just best practice—it’s the law under FAR 91.205.
If This Didn’t Work
Now, if errors are still too big or inconsistent after a standard swing, the problem’s probably deeper than 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 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—or if you suspect the compass accuracy. Lightning strikes count too.
- 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.
