The Mystifying Compass: Why this age-old device still exists

Why does this age-old device still exist? By Jim Sparks Just to put everything in perspective , Albert Einstein commented that when he was still a child his father gave him a magnetic compass. He spent hours pondering how the small pointer...


The flux valve does exactly as the name implies: it opens and closes allowing the magnetic Earth's flux field flow through its frame at 800 times per second. This frame consists of two pieces of metal formed together and joined by a center post surrounded with an excitation coil. The metal of the frame is formed in three spokes going outward from the center and each spoke is wound with a sensing coil. In addition, the frame is mounted using gimbals or universal joints to allow it to remain somewhat centered during climb, decent, or bank conditions and like the whiskey compass the flux valve contains a dampening fluid usually alcohol (not a drinkable variety).

When in operation and the exciter coil is energized it effectively closes the gate and prevents the flow of the Earth's magnetic field through any of the three legs. As the voltage of the exciter coil begins to drop, the gate opens and the Earth's flux lines begin to flow. The relationship of the flux valve to the Earth's North and South Pole will determine which of the three legs has the highest saturation. Then as the voltage of the excitation coil builds once again the gate is again closed and the sensor coil voltages drop. By sensing the voltage and polarity of the three sensing coils a very accurate determination can be made as to the aircraft position relative to the magnetic poles. The fact that the excitation current cycles at 400 cycles per second effectively means the flux valve is opened and closed at a rate of 800 times per second.

Once the magnetic position is determined the information is fed to a remote compass indicator. This device is most often run from a directional gyro. Flux valve information is only used to update gyro position. This is the reason for a stable heading indication during various flight maneuvers whereas on the magnetic compass the indication may be erroneous.

Flux valve housings generally have slotted receptacles for the attaching hardware along with a way of dictating which end of the device should point forward. The slots are for making adjustments for compass error. Similar policies and procedures should be considered around flux valves as with the magnetic compass. Use only nonmagnetic hardware for the attachment and always consult airframe manufacturer's documentation for appropriate part numbers and specific locations when installing hardware in the area surrounding the flux sensor.

Reliability of flux valves, as a rule, is quite good. Unfortunately there will be adverse effects resulting from improper hardware installation or bonding problems. With wingtip mounted flux sensors in some cases navigation or strobe light operation may impact compass operation. In other cases the source of the 400 Hz-excitation power may also become corrupt. In some cases an out of phase condition on multiple inverter-equipped aircraft may result in a bleed over into the signal coil resulting in an abnormal compass drift. Aircraft indirect cabin fluorescent lighting systems have been known to cause directional indicating problems as well.

So with all the new equipment available such as GPS, who needs a magnetic compass?

Well according to the FAA in Federal Air Regulation 91.33, all aircraft require a magnetic direction indicator. Aircraft electrical systems do experience failures from time to time and a device that can provide a means of navigating to a safe destination without the benefit of aircraft power will be most appreciated. Who knows maybe the term whiskey compass has something to do with what comes after a flight where a power failure did occur and the magnetic compass was the sole means of navigation.

I'll drink to that!

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