Because the autopilot contains a built-in magnetometer for a backup source of heading in the event of GPS loss, it is important to try to locate the programmer away from known sources of magnetic disturbance. The calibration procedure can account for a moderate amount of fixed disturbance (for example, nearby iron objects) but it cannot adjust for changing magnetic fields such as would be generated by aircraft compasses or certain electrical devices. One such source of such problems is the “Flag” mechanism in some older DG or HSI devices. These units use a solenoid to hold the flag out of sight, and the magnetic field will then change when the flags come and go. If at all possible, place the autopilot so as to be as far as possible from such devices. A hand-held compass can be used to assist in finding fixed or variable disturbances prior to the installation of the autopilot. Even a few inches can make an appreciable difference in the magnetic disturbance level. It should be noted also that strobe light controls, landing lights, navigation lighting, and pitot heat, generate very strong currents in their wiring, thus they will create a periodically pulsating magnetic field disturbance. Shielding has no effect on this problem; the only solution is to keep the wiring as far away as possible from any electronics which can be affected by pulsating magnetic fields.
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