Magnetic Headings

The earliest adventurers became savvy navigators employing the positions of celestial bodies relative to the seasons and even time of day. At some point it was discovered directional enhancements could be made using a chunk of loadstone or even a piece of iron ore floating in a bowl of water. The first documented use is believed to have occurred in China around 1000 BC and although not initially used for navigation it did assist in alignment of buildings and monuments.

Initial use of a geomagnetic compass is believed to have occurred in the 11th century.

The earth is a rather significant permanent magnet spinning in space and is surrounded by a magnetic field. A sensitive layer of the atmosphere called the Magnetosphere is a region susceptible to solar winds and other ionizing phenomena which can have a pronounced impact on magnetically sensitive equipment. A key feature of this physical property is the ability to enable other magnets to align with the surrounding lines of flux.

The magnetic poles are considered points on the Earth’s surface where the magnetic field is entirely vertical. That is, the inclination of the Earth’s field is +90 degrees at the North Magnetic Pole and -90 degrees at the South Magnetic Pole. A compass held in a horizontal plane by Santa Claus while at home points randomly and at his off-season retreat in Ecuador it points to the poles.

Deviations in accuracy exist based on latitude and may require compensation in more sophisticated directional systems. Both of Earth’s poles wander independently and are usually not directly opposite. If the Earth’s magnetic field were perfectly dipolar, the geomagnetic poles would coincide. However, significant occurrences within our planet’s molten core and even severe solar flares will result in shift. The position of the two poles will vary and can migrate rapidly with movements of up to 22 nautical miles per year on record.

Geomagnetic reversals

The Earth is a form of dipole whose orientation is related to the rotational axis. There are two positions where the axis best fits and are referred to as the North and South geomagnetic poles. The geomagnetic field should be placed about 270 nautical miles off the center of the Earth causing the inner reaches of the Magnetosphere to skim lower in the vicinity of the southern Atlantic ocean creating what is called the South Atlantic Anomaly.

It has been suggested that the Earth’s magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 300,000 years. The last such event, called the Brunhes-Matuyama Reversal, is believed to have occurred some 780,000 years ago.

There is no clear theory as to how the geomagnetic reversals might have occurred. Some scientists have produced models for the core of the Earth where the magnetic field is unstable and the poles can abruptly shift from one orientation to the other over the course of a few hundred to a few thousand years.

Other scientists propose that the geomagnetic dynamo turns itself off, either spontaneously or through some external action like a comet impact, and then restarts itself with the magnetic “North” pole pointing either North or South. External events are not likely to be routine factors in this reversal as there is a lack of data to correlate the age of impact craters and the timing of reversals. Regardless of the cause, the magnetic poles do periodically flip from one hemisphere to the other. Temporary variations also occur where the dipole axis crosses the equator and then returns to the original polarity and are known as excursions.

“Mag-Var” (Magnetic Variation) tables are included in the flight management system (FMS) software and are stored in memory. These charts include predicted changes in the magnetic variations between Latitudes 82 North and 82 South and are periodically revised to ensure a match with the latest available data. By the nature of the magnetic changes, these charts are never completely up to date.

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.

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