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:

• When the accuracy of the compass is suspected

• After flight deck modifications or changes that alter the ferrous metal content

• Any time the compass has been subject to a significant mechanical shock

• After an aircraft encounters a severe electrical storm or a lightning strike

• When a change is made to the aircraft electrical system

• In the event cargo is carried that may affect the compass

• When the primary global operating region of the aircraft changes

• If the aircraft is parked at a continuous heading for at least a year

• Anytime a compass system component is replaced

If a compass cannot be adjusted to proper manufacturer’s tolerance even though it appears to function properly on the bench, and after a full assessment of the surroundings, a compass key may be the solution. The intent is to introduce a calibrated field to counter the effects of the compass location.

If the aircraft has an electrical system, two complete compass checks should be performed. One with minimum electrical equipment operating and the second with all electrical accessories such as radios functioning. If the compass readings are not identical, two separate compass correction cards should be created and displayed in view of the pilot.

An alternative to the use of a compass rose or master compass may be provided by airframe manufacturers and involves utilizing the inertial reference systems (IRS). Although normally oriented to True North, selections are available to display magnetic heading. Orienting the aircraft to the IRS heading and then noting the compass display can provide a significant time savings. If this method is published as a maintenance procedure and the airworthiness authority considers the maintenance publications to be “acceptable data” then these procedures may supply a viable certification means.

Having worked on aircraft utilizing a star gazing window for the navigator and listening to recent press about astrological anomalies and the changing positions of celestial bodies coupled with the variation, deviations, and reversals in the Earth’s magnetic field, the train is a sure bet as it still utilizes the tried and true “Iron Compass” (fixed steel rails joining destinations).

Jim Sparks has been in aviation for 30 years and is a licensed A&P. He is the manager of aviation maintenance for a private company. He can be reached at sparks-jim@sbcglobal.net.