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 always oriented itself to a specific direction. It was this allure of the compass that first attracted Einstein to the wonders of the natural world.
Most of today's most sophisticated commercial aircraft still include in the otherwise all electronic flight deck, a small permanent magnetic (or whiskey) compass. Every now and then a maintenance requirement surfaces where this device must be tested and if needed an adjustment made. Why does this age-old device still exist in our current world of technological advancement? More importantly why do we have to adjust it and what can introduce errors requiring these adjustments?
Magnetism and troubleshooting
The early explorers knew of the lodestone and ancient sea captains considered this chunk of rock to be a prized possession. They observed that the orientation of a suspended lodestone was always relative to a specific direction when it came to a stop. They also realized that these directional properties could be transferred to various metal pointers mounted on bearings. This was accomplished by mechanical contact or simply having the loadstone in close proximity of the pointer.
One of the key elements in troubleshooting compass systems is a basic understanding of magnetism. There are only three other elements that can become magnetized at other than extreme ambient temperatures: iron, cobalt, and nickel. It is the unique property of their atoms and more specifically their electron arrangement that allows polarization. Unfortunately, in all cases these materials will lose their magnetic properties shortly after the source of magnetism is removed.
There are, however, several alloys that will retain polarity and can be classified permanent magnets. Alnico is one of these alloys and is widely used in the construction of permanent magnetic compasses. This material is a result of joining iron with aluminum nickel and cobalt, and magnets manufactured from it are 50 times stronger than its predecessors.
Polarity and magnetic strength
Each magnet has both a North Pole and a South Pole. Like poles tend to repel while opposite poles will attract. An invisible force exists between the North and South Pole of any magnet and this flux field intensity is a direct result of the strength of the magnet.
Magnetic field strength is based on a mathematical definition that takes into account the amount of force the field has on an electrical current flow. The unit of measure of magnetic force is the Tesla (T), which is calculated by multiplying the current flow through a wire by the wire length in a magnetic field providing the field is perpendicular to the conductor. This is then multiplied by the field strength determined in Newtons per ampere.
A Newton is a measurable force calculated in a fashion similar to foot pounds or inch pounds. In fact one Newton is equal to 100,000 Dynes and one Dyne is the force required to move one gram one centimeter.
The strongest magnetic field ever created is around 40 Tesla. This means a Tesla is a very large unit of measure. In fact the Earth's magnetic field is only rated at .06 millitesla. Unfortunately this means aircraft magnetic sensors will react to the strongest magnetic field available and this is not always the Earth.
The Earth is a magnet
Yes, the Earth is a permanent magnet. Many physicists theorize that the core of the Earth has a solid mass of iron and nickel surrounded by hot molten metal. This causes a reaction, which produces electrical current flows that in turn cause polarity. It is also theorized that the Earth's magnetic field will reverse itself every several hundred thousand years. You may have heard the terms Magnetic North and True North. True North applies to the Earth's rotational axis where as Magnetic North actually lies along the East Coast of Greenland at about 70 degrees north latitude.
Remember how the formula for determining the strength of a magnetic field called for having the conductor perpendicular to the field. This is also true when it comes to using a magnetic compass. The perpendicular factor becomes known as Angle of Dip. The perpendicular or greatest field strength will occur at the magnetic equator while the weakest point will be at the two magnetic poles. Angle of Dip in an aircraft is not always a given. At high angles of attack or during dives it has been observed that magnetically driven compasses will actually spin and usually in the direction opposite of an aircraft's turn. This phenomenon is explained by observing that when an aircraft is not in level flight the perpendicular reference to the Earth's magnetic field is distorted.
While the FAA has no rules governing the adjustment or certification of the whiskey compass, many airframe manufacturers will make it part of their normal maintenance and inspection programs. One requirement that does exist is that there must be a compass correction card installed. Maintenance on these devices is somewhat limited. Other than lamp replacement and basic adjustment most repairs become cost prohibitive. It is often more prudent to purchase a replacement unit.
A typical magnetic compass consists of a compass card with an overhead suspension point. Secured to, but just below the card is a pair of magnets. This mechanism moves about in a fluid bath for dampening purposes. Originally whale oil was the fluid of choice but alcohol is the prevalent fluid today.
When the magnets are level as when the aircraft is flying straight and level the predominant factor should be the Earth's magnetic field and subsequently the compass will provide a reliable indication of magnetic heading. Adjustable needle magnets are also installed to provide some limited means of correction for surrounding magnetic interference.
The adjustment procedure will vary between airframe manufacturers according to the primary function of the compass. In most modern commercial aircraft the whiskey compass is considered a backup device to be used in the event of electrical failure. Therefore these devices are typically adjusted without aircraft generator functioning and only essential aircraft systems operating. When this device is considered a primary means of navigation, appropriate airframe systems need power applied and should be operational. The adjustment of the needle magnets should always be accomplished using a nonmagnetic screwdriver. Be aware a watch or other jewelry may have a negative impact on maintenance.
Always refer to the manufacturer's parts documents when replacing hardware in and around the flight deck. If the compass is installed using steel hardware directional anomalies will be present. Many airframe manufacturers require only nonmagnetic screws be used in and around the instrument panel. Also the bolts used to install the windshield or flight deck structures may be specific in nature to prevent compass problems.
The source of the problem . . .
Many of the problems that are observed with magnetic compass error might in fact not be the fault of the compass. When considering the relatively weak magnetic field generated by the planet, a field produced by the airframe may influence the compass display. Sometimes taking the unit to an open area free of metal and power lines along with a calibrated sighting compass may be a quick way of determining the problem.
Heated windshields, electrical blower fans, and any inductive component even remotely close to a magnetic compass may influence the display, not to mention electrical bonding problems.
Top: Flux valve installed. Bottom: Placard addressing nonmagnetic
hardware by flux valve.
Flux sensors or valves
Remote compass sensors were created in an attempt to minimize the effects of aircraft generated fields impacting directional data. These devices are most often located in areas such as a wing tip or horizontal stabilizer where aircraft system interference would be at a minimum. Often these components are referred to as "flux valves or flux gates;" operationally they all accomplish the same thing.
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!