Put yourself in this situation, for a moment. You are cranking along on a cross-country, an IFR cross-country. For the moment you are VFR on top with a thick solid layer below. You are flying single pilot IFR with no VFR alternates within range.
Things are going along fairly smoothly, then trouble starts. The first indication on your ammeter/loadmeter goes unnoticed. You make a radio call to ATC they report as fading badly. The VOR pegs to one side of the instrument. The alternator warning light comes on. Electrical failure. That unplanned close-by VFR airfield would look pretty good about now.
ATC does notice that your transponder reply has vanished, but they cant call you. You have always wondered if a battery-operated standby radio wouldnt be a good idea, even if they are expensive. They dont seem so expensive right now.
There are several points in your favor, however. You are on top of the stratus, its daylight, the stratus below is warm with no threat of icing, and you still have the gyro-operated instruments. You pull out the handheld GPS, but are dismayed to find that the nicad batteries are dead because you havent plugged it in lately.
The problem now is whether you will be heading in the right direction to get to your destination or alternate. Although your heading indicator is working, you feel an occasional twinge as you recall the way the vacuum gauge swung during your runup – a typical sign of a vacuum pump about to go.
The only thing between you and almost certain disaster is a small strip of magnetized metal inconspicuously mounted near the center of your windshield. Despite all of the advances in navigation, the final arbiter of your success is one of the first flight instruments installed in airplanes.
In aircraft equipped only for VFR flight, the magnetic compass is still sometimes the only instrument that indicates the direction of flight. Even with the explosion of development in aircraft instrumentation, the faithful magnetic compass is still standard equipment.
It doesnt need any external power or vacuum source to operate. That makes it an extremely useful emergency or standby navigation instrument. While its simple to operate, a pilot using a magnetic compass does need to understand the various errors you can run into navigating with it.
The magnetic compass points to a north that is a fair distance from the mapmakers north. The magnetic north pole is actually on Prince of Wales Island, which is 900 miles from the geographic north pole. Most of the time when you navigate cross-country you need to use the angular relationship to magnetic north and the true course that you measure. This is known as magnetic variation.
If you flew a cross-country from Seattle to Boston there will be a wide range of magnetic variations. Zero variation occurs roughly in a line from Oshkosh, Wis., to the Florida Panhandle. The variation will range from 20 degrees easterly in the Pacific Northwest to 15 degrees westerly in the Boston area.
Why does it matter? Some directions are given in terms of true north and others are given in terms of magnetic north. The magnetic compass and the direction finding instruments in the aircraft give heading information in terms of magnetic north, as are courses, headings and runways. Maps are based on true north, as is upper air wind direction. Surface winds are generally reported with reference to magnetic north.
If you want to convert a direction that has been given to you in true north to magnetic north, variation must be applied. Easterly variation should be subtracted from a true heading to get magnetic, and westerly variation is added.
Dips and Other Deviant Behavior
As the compass tries to align with the earths magnetic force, a number of compass errors are caused. At the equator, the lines of force in the earths magnetic field are parallel to the earths surface. Closer to the poles they begin to curve downward as they approach the magnetic poles.
Even though the magnets may be lined up horizontally with the earths magnetic field, the compass bars would be pulled downward if the effect was not countered in the instrument.
To counterbalance the vertical component of the earths magnetic field, compensating magnets are located inside the compass housing. The vertical force is neutralized by the magnets when the aircraft is level, but banking to turn is a different matter. If you are headed north or south and bank the aircraft, the compass card will tilt and rotate.
Whenever the aircraft is banked on these north/south headings the largest turning errors will result. The compass bars have farther to dip when the heading is north or south and the card is tilted, because they are aligned with the nose-to-tail axis of the airplane. The compass card will swing back to its former (and correct) reading, once the wings are leveled. On headings of east and west, there is no turning error.
The magnetic compass is located away from electrical equipment (and radios), to dampen out the effects caused by electromagnetic fields created when the units are on. Even so, steel in the structure of the airplane will cause distortions in the way the compass reads. Either way, there will be errors in the accuracy of the magnetic compass. These errors are lumped under the single name – deviation.
Deviation is measured by pointing the airplane in a known direction and measuring how far off the indication is. The compass rose painted on the ramp at some airports pays tribute to the almost forgotten art of solving this problem.
The differences between the actual heading and the reading on the instrument are noted on a compass correction card, which then becomes part of the airplanes required documents. The correction card is usually located close to the compass itself. What the pilot wants to know is what number should show up under the lubber line, when the airplane is pointed in a direction that he wants to go. Lets say the desired direction is east, or 90 degrees, the correction card may call for a +6 degree reading on that heading. The heading to fly must be adjusted to 96 degrees.
That may seem like a chunk of misdirection, but unless you are flying in very smooth air it wont mean that much in practice. The average flight contains a fair amount of bumps and pilot oscillations that will make it hard to read the compass accurately. This is usually called oscillation error. When flying manually in turbulence while trying to hold a heading on the magnetic compass, your best bet is to average out the swings.
Of course, there are many limitations with the magnetic compass. There is the relative instability and sluggishness near the magnetic poles. There are turning and acceleration/deceleration errors. Magnetic disturbances from other electrical equipment subject the magnetic compass to errors.
If its errors and characteristics are thoroughly understood, the magnetic compass gives the pilot a very reliable means of determining the heading of his aircraft. The airplane must be flown level, smoothly and at a constant airspeed, however, to present an accurate compass reading to the pilot.
Even though the gyroscopic instruments have pushed the magnetic compass back to being a standby instrument, there have been times that it has saved pilots when the sophisticated gear has let them down. Old-school navigators still maintain that they can go anywhere armed only with a magnetic compass, a clock, a speed indicator and an accurate chart.
A good (and challenging) IFR training exercise is to take along a safety pilot and a hood and see what you can do with a round-robin course, over three or four check points, about 50 or 60 miles apart. Your aim is to make your headings hold to the nearest degree, and your turns to the nearest second. Youll need to correct your heading and ground speed for winds.
After takeoff, the safety pilot will watch for traffic and other problems. You will put on the hood, and keep it on until the trip is finished. The objective is to see if you are able to navigate to your selected check points, and return to your departure airport on your ETA. You begin timing at the start or the takeoff roll. Using only a chart, the magnetic compass, the airspeed and a clock, you may be surprised at the accuracy that you can get.
But before betting your life on the accuracy of a magnetic compass, make sure that the compass has been swung recently and that it has an accurate compass correction card.
Remember that the best time to read the compass is when the aircraft is at a constant speed and in straight and level flight. Dont use bank angles of more than 18 degrees when flying a magnetic compass turn.
From my experience, if you are descending through an overcast in an emergency, assuming the terrain below is flat, using the magnetic compass as your sole reference for heading, the best success can usually be obtained by using a stabilized southeasterly or southwesterly heading.
The magnetic compass has been around a long time, and it has its quirks and errors. Using one correctly is almost a lost art, but at least youll find a compass somewhere in your cockpit, ready to serve, when the chips are down.
-by Raymond Leis
Raymond Leis is a CFII and ATP with more than 23,000 hours.