Determine whether a compass swing is required. The Federal Aviation Administration (FAA) mandates that a compass swing be performed when the accuracy of the compass is suspect, after a cockpit modification involving ferrous metal or the electrical system has been made, whenever a compass has been subject to shock, after a lightning strike or after flying through an electrical storm. Regardless, it is good practice to re-swing a compass periodically.
Line the aircraft up with magnetic north. This can be accomplished in two ways -- either with a calibrated master compass or, more commonly, using a painted compass rose that can be found on the ground at most airports. With the engine running and all electrical and radio systems on, use a non-metallic screwdriver (usually a slot-type screwdriver) and adjust the N-S compensator on the compass until it reads zero. Turn the aircraft to an east heading and repeat the process. Each time, the aircraft must have its engine running and be properly configured. This time, adjust the compass to indicate the east -- or 090 -- heading using the E-W compensator screw.
Turn the aircraft to a south heading and repeat the process, except this time only take out half the difference between the magnetic south and the indicated compass heading. For example, if the compass reads 184 degrees, adjust the screw to 182 degrees (half the correction). Turn the aircraft to a west heading and perform these identical steps. Keep in mind that the airplane must be properly configured during each step.
Create a compass deviation card. This is done by again turning the aircraft around the compass rose, starting from north, and then by 30 degree increments clockwise. This time, note the actual heading the compass is indicating. The difference between this and the actual magnetic heading is your compass error. While it is normal to have some error on the compass (due to the nature of the magnetic field and aircraft interference), errors should not exceed 10 degrees. The errors can be used to create a compass deviation card.
Install the completed compass deviation card with all the errors of each 30 degree increment onto your compass. The compass can now be used and is properly calibrated with an accurate deviation card.
