Since the beginning of flight, pilots have been using the magnetic compass for navigation. It doesn't matter if you're flying a Piper Cub or a Boeing 747, you'll find a magnetic compass in the cockpits of almost any aircraft. That is, unless you're flying the latest, most technologically advanced glass cockpits, which we'll cover in a separate article. So here's what you need to know about an instrument that's been in aircraft cockpits for over 110 years, and the errors associated with it.
How They're Made
Magnets in a compass make it align with the magnetic North Pole. In airplanes, your compass is almost always set on top of the dash or hung from the top of the windshield frame, in order to keep it as far away from electrical gear as possible to reduce magnetic deviation (more on this later). Back in the days before kerosene filled the inside of a magnetic compass, alcohol was used as the primary lubricant and non-freezing liquid... Hence the nickname a "whiskey compass." Rumor has it that some pilots broke into their whiskey compasses on long layovers...but that's another story.
Instruments in your airplane cause interference that affects your compass, and that interference is called deviation. Inside the compass, there are compensating magnets to counteract these fields of interference. A compass card is normally attached, showing what error correction to add for different headings, although the changes are normally just a few degrees. Next time you're in the cockpit, try putting your headset on the dash near the compass and see if it changes the compass heading. The magnets in your headset speakers will almost certainly change the magnetic field enough to show a difference in heading.
Variation is the difference between true north and magnetic north. Isogonic lines are drawn on your sectional charts to show different lines of magnetic variation to help with planning your magnetic heading. To find your magnetic course (in no wind, the heading you see on your compass), you'll either subtract easterly variation or add westerly variation. A great memory tool for this is "East is least, West is best." In this example, the variation is 14 degrees East, so you'd subtract 14 degrees from your true course to get your magnetic course.
Magnetic dip creates the most substantial errors in a compass. As you get closer to the North or South Pole, magnetic flux lines point downwards towards the poles, and your compass magnets dip towards the low side of a turn. When magnetic dip is pronounced, it's difficult to get actual readings. This error isn't much of a problem near the equator, where your compass points more or less horizontally and magnetic dip isn't a concern.
Acceleration And Deceleration Error
Another major problem with magnetic compasses in airplanes is acceleration error. In the Northern Hemisphere, as you accelerate, your compass will show a turn to the North. And as you decelerate, your compass will show a turn to the South. When the speed stabilizes, the compass returns to normal. This error is greatest on East or West headings. So how are you supposed to remember this? Use the acronym ANDS: "Accelerate-North, Decelerate-South."
During a turn from a Northerly heading, the compass briefly indicates a turn in the opposite direction. As for aircraft turning from a Southerly heading, the compass indicates a turn in the correct direction, but at a faster rate than is actually being turned. How do you remember this one? Use the acronym NOSE: "North-Opposite, South-Exaggerated."
Not too bad, right? The magnetic compass is tried and true. If everything else fails in your cockpit, you always have your magnetic compass. And now you know how it works, as well as the problems it has, too.