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The magnetic compass: the simplest instrument in the panel also has simple requirements, but it can be the most difficult to use.

It's rather quaint these days that aircraft still carry a magnetic compass. The cellphone in the pilot's pocket probably has a more stable and less error-prone electronic sensor to know which way it's pointed. When turned or accelerated, that solid-state device remains accurate in situations where a conventional "whiskey" compass will twist and turn like a bobble-head toy on a student pilot's first solo. With all that technology going on, the magnetic compass remains a staple of most cockpits around the world.

Technological advances have trickled down to the compass over the years--including innovative mounting and damping, plus error minimization and trick lighting--but its basic foibles will never be fixed. We also can have the same basic cellphone technology in our instrument panels and tied to our autopilots, reducing the compass to something we crane our necks to see past. But the magnetic compass is a simple device, needing little maintenance while its limitations are well known.


The basic design of an aircraft's magnetic compass like the top one in the sidebar on the opposite page places two small magnets on a metal float inside a bowl of clear fluid similar to kerosene. The float and its magnets are wrapped with the compass card, a scale indicating a magnetic heading. The card is viewed through a glass window which forms part of the fluid container and features a vertical line, called a lubber line, against which is compared the compass card.

The float/magnets/compass card assembly rides on a metal pivot that helps keep it from tilting in any direction beyond a certain angle. The compass fluids damps but does not eliminate this motion. Thanks to the magnets' interaction with Earth's magnetic field, the assembly wants to point to the north, and the compass card is attached 180 degrees opposite that indication. The pilot effectively reads the compass from its backside. Compensating magnets also are installed in the compass.

Many airports have a compass rose painted on the pavement for aligning compasses. Using adjusting screws and the compass rose for alignment, a technician can adjust the compass and its compensating magnets to minimize the deviation error (see sidebar below), which is caused by competing magnetic fields in the aircraft to which it's mounted.

For this reason, the FAA cautions, "A common error that affects the compass' accuracy is the mounting of a compass or instruments on or in the instrument panel using steel machine screws/nuts rather than brass hardware, magnetized control yoke, structural tubing, and improperly routed electrical wiring, which can cause unreasonable compass error."

Any error that can't be corrected is noted on a compass deviation card mounted on or near the compass. A typical compass/aircraft installation can be adjusted, so eliminate all but a couple of degrees. The maximum acceptable deviation in level flight is 10 degrees on any heading. The technician notes which actual compass heading to fly for every 30 degrees, and straight-line interpolation between them is used for headings other than on the card.

The sidebar above compares the basic fluid-filled magnetic compass with the vertical-card version. While the more modern vertical card compass can be easier to read, it still suffers from the standard compass errors. It is, however, much better damped, which generally makes it easier to read in turbulence.


The fluid-filled magnetic compass is relatively bulletproof, but it does have some special needs. For one, it should be part of every personal airplane's annual or condition inspection. For example, the inspecting technician should examine it to ensure there's a compass deviation card present (and perhaps review logbook entries to see if the compass has been swung since any avionics or electrical work). In fact, the FAA has a list of 10 events dictating when a compass swing must be performed for large aircraft.

During a preflight inspection, the compass should indicate an approximately correct heading, with the magnet/float assembly free to float in its case. Any fluid leakage should be corrected. The vertical card compass has similar requirements (except for possible fluid leaks). Both should be securely mounted and include operable internal lighting, if equipped.

When checking a compass for accuracy, the location must be free of steel structures, including underground pipes or cables. Equipment producing a magnetic field also is a no-no. No magnetic or ferrous material should be nearby, and only nonmagnetic tools (e.g., a brass screwdriver) should be used when adjusting the compass.

We all do it, but it's a bad idea to place a headset on the instrument panel's glare shield next to a compass. The headset contains all kinds of electronics and can generate electromagnetic interference, which can play havoc with a compass. For that matter, take care to not place anything metallic near a compass, especially in flight.


The typical magnetic compass in the typical airplane isn't used that much in day-to-day operations. It's there to serve as a standard against which to set and compare the heading indicator/directional gyro in a steam-gauge airplane and as a last-resort backup after an electrical system failure when flying a glass panel. But that doesn't mean pilots can ignore their responsibility to be familiar with how to fly with it when the chips are down. That ancient technology just may get you home, or at least out of trouble.


Of Dips And Deviations

As we all should remember from ground school or instrument flight training, the basic liquid-filled, magnetic compass has predictable characteristics often making a direct observation impossible. To be reliable, this type of compass needs to be in steady, unaccelerated flight and even then, it's susceptible to error. The different types of errors include:

* Variation, which is the difference between true and magnetic directions, usually corrected for in instrument flight during the charting process.

* Deviation results from the influence of local magnetic fields in an aircraft caused by electrical current flow or magnetized aircraft structure. A compass correction card should be placed next to the instrument to allow pilots to compensate for this error.

* Dip errors are produced whenever the aircraft's attitude changes, altering the presented heading.



In addition to the inherent errors of a floating magnetic compass, it also can lead to confusion because its card appears backward. An inexperienced or distracted pilot easily can begin a turn in the wrong direction when relying on it as a primary heading indicator. Enter the vertical card magnetic compass, an example of which is pictured at bottom right.

The vertical card compass eliminates that confusion and is better damped while providing the pilot "with a true relationship of the aircraft to azimuths as opposed to the reverse reading requirements of the liquid filled compass," according to the FAA's AC 43.13B, Acceptable Methods, Techniques, and Practices--Aircraft Alterations. The vertical card compass is dry internally, eliminating the possibility of leaks, another advantage over its fluid-filled counterpart.

But because it's a magnetic compass, it's still susceptible to the types of errors described in the sidebar on the opposite page.
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Publication:Aviation Safety
Date:Jun 1, 2017
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