Calibrating the magnetic compass

A required, independent instrument

The magnetic compass remains a required, independent standby navigation instrument on virtually all certificated aircraft, and under 14 CFR Part 91 an aircraft may not take off for VFR or IFR flight with it inoperative. However, because it relies on the Earth's magnetic field, its accuracy is affected by magnetic interference generated by the airframe itself.

Variation and deviation

• Magnetic variation: the angular difference between true north and magnetic north. Variation depends on geographic location and is shown on aeronautical charts.
• Magnetic deviation: the error caused by magnetic fields within the aircraft, from ferrous steel components, engines, fasteners and direct-current electrical wiring.
• Deviation is not fixed: it changes with the aircraft's heading, equipment configuration and electrical load.

The purpose of calibration

Compass calibration, or "swinging" the compass, has two steps. First, the compass's internal compensating magnets are adjusted to minimize deviation. Second, the remaining residual error is recorded on a compass correction card mounted on or next to the instrument, so the pilot can correct for it.

When a compass swing is required

AC 43-215, Standardized Procedures for Performing Aircraft Magnetic Compass Calibration, recommends swinging the compass:

• At periodic intervals recommended by the manufacturer or the operator's maintenance program.
• After a major component change.
• After major servicing.
• When the aircraft is relocated to a significantly different latitude.

Other common triggers

Manufacturers' manuals and maintenance programs typically add events that can change the aircraft's magnetic field or the compass itself:

• Installation, replacement or relocation of the compass, or of avionics, instruments or electrical wiring near it.
• Engine changes, major modifications or structural repairs near the compass.
• A lightning strike or a severe electrical fault that may have magnetized structure.
• Whenever the compass is suspected of being inaccurate.

Who may do it

Compass calibration and adjustment of the compensators are maintenance. Certificated airframe mechanics and appropriately rated repair stations may perform a compass swing. Repairs or alterations to the compass itself may only be made by a repair station with an appropriate instrument rating.

Five accepted methods (AC 43-215)

AC 43-215 describes five acceptable ways to establish the aircraft's true magnetic heading for compensation and card preparation. Whichever is used, the reference equipment must be properly calibrated and certified for use.

• 1. Surveyed compass rose: the aircraft is positioned on a calibration pad surveyed to magnetic north, away from magnetic disturbances. This is the most familiar method.
• 2. Master site compass: a self-contained, calibrated sighting compass set up away from the aircraft. The mechanic stands facing the aircraft and sights along it to determine its precise magnetic heading. No painted compass rose is needed.
• 3. Simulated rotation system: a test apparatus neutralizes the Earth's field at the compass and creates an adjustable simulated field, so the aircraft stays stationary. Its initial alignment still needs a reference to magnetic north from a compass rose or master site compass.
• 4. Portable magnetic standard: a highly accurate digital compass with magneto-resistive sensors, aligned to the aircraft with an internal laser and read on a remote display as the aircraft is turned.
• 5. Air swing: the aircraft is flown in smooth air on eight headings (N, NE, E, SE, S, SW, W, NW) and the compass is compared with a calibrated heading reference or an inertial navigation system. GPS is not acceptable, because it shows ground track, not heading. Air swings are flown in day VFR conditions with a certificated mechanic on board to make the adjustments.

Step 1: Site and aircraft preparation

Position the aircraft as far as possible from magnetic disturbances such as steel-reinforced concrete, underground power lines and metal hangars. Place it in its normal flight configuration: engines running, doors and canopy closed, controls centered and at normal flying attitude. Tailwheel aircraft may need the tail raised.

Step 2: Compensate on north and east

Align the aircraft to magnetic north (000°) and adjust the N-S compensating magnet until the compass reads north. Then align the aircraft to magnetic east (090°) and adjust the E-W compensating magnet until the compass reads east.

Step 3: Split the error on south and west

Align the aircraft to magnetic south (180°) and use the N-S compensator to remove half of the observed error. Then align to magnetic west (270°) and use the E-W compensator to remove half of the observed error.

Make adjustments with a non-magnetic screwdriver, lightly tap the compass after each adjustment, and let it settle before taking a reading.

Step 4: Record 12 headings

Turn the aircraft through the full circle in 30° increments, 12 headings in all, and record both the aircraft compass reading and the reference heading at each one.

If the aircraft has an electrical system, perform two complete checks: one with minimum electrical equipment operating, and one with all electrical accessories on, such as radios, navigation, radar and lights.

Step 5: Make the correction card

For each heading, the difference between the compass reading and the reference heading is the residual deviation. The correction card shows, for each desired magnetic heading, the compass heading to steer. Mount it on or next to the compass, and record the calibration in the aircraft's maintenance records, because a compass swing is a maintenance action that requires a logbook entry.

Electrical loads and two cards

If the compass readings with equipment on and equipment off are not identical, AC 43-215 says to make two separate correction cards: one with all the equipment on, and one with the equipment off. For Part 23 aircraft, the placard must also state which electrical loads would cause a deviation of more than 10° when turned on.

Reading an example card

A typical card lists the desired headings in 30° steps with the heading to steer beneath each one, for example:

• For N (000) steer 001, for 030 steer 032, for 060 steer 061.
• For E (090) steer 090, for 120 steer 118, for 150 steer 149.
• For S (180) steer 179, for 210 steer 208, for 240 steer 238.
• For W (270) steer 271, for 300 steer 301, for 330 steer 331.
• The card also shows the date of the swing and whether radios were on or off.

Key takeaways

• Deviation comes from the aircraft: internal metal parts and electrical currents cause heading-dependent errors that must be compensated.
• Compensate first, card the rest: adjust the N-S and E-W magnets to remove most of the error, then record the residual error on the correction card.
• Check with equipment on and off: if the readings are not identical, two correction cards are required.
• Follow AC 43-215: use one of its five methods, such as a surveyed compass rose or a master site compass, with calibrated reference equipment, and record the work in the maintenance records.