Magnetic Headings

The Inertial Reference System (IRS) itself doesn’t bother with the magnetic information. It carries on navigating by reference to True North. The magnetic variation information is used to provide a calculated magnetic heading based on “true” headings and is used in areas where magnetic references are prevalent as with runway headings and navigation aids such as VOR. The continued use of the increasingly unstable magnetic reference is intended to keep the compass calibration industry gainfully employed for the next millennium.

The magnetic compass is the simplest of all flight deck instruments yet it can be the most susceptible when it comes to outside forces. A common error that affects the compass’ accuracy is the mounting using steel hardware rather than nonferrous. Any magnetized device in the general area of the compass will produce errors along with improperly routed electrical wiring. In some cases the magnetic effects of direct electrical current in wiring can be mitigated by utilizing a twisted wire pair.

Compass rose

When it comes to adjusting or “compensating” a magnetic compass, the first step in most maintenance instructions is to move the aircraft to a “Compass Rose.” Back in the good old days this was readily available at most airports and was located in a remote area free of stray electromagnetic fields and metal buildings. And, the ramp was marked with precise locations to achieve the exact heading including the cardinal and ordinal settings.

In years past, this was a primary means of ensuring navigation accuracy so the alignment of the compass rose was validated yearly. Once on the rose, the technicians would first position the aircraft relative to the North heading. Having a tug or tow tractor in close proximity creates a significant challenge to achieving proper adjustment.

In lieu of the compass rose, a master sighting compass provided a reliable means of ensuring accuracy during alignment. This testing device does require yearly calibration when used to ensure the airworthiness or return to service of aircraft directional systems.

A compass swing is a must in the event of any of the following:

•