When North is Not Truly North
Navigating over the surface of the earth has other complications. One of the most fundamental is that there’s no instrument on an airplane that can tell a pilot if he’s pointing directly at the North Pole. After all, the North Pole is simply an ordinary point on the earth’s surface with no special physical quality. It just happens to be the pivot location, one end of the spinning planet’s axle, if you will.
Fortunately, there are a couple of points on the earth’s surface that do possess a special physical quality. Near the North and South Poles, but still many hundreds of miles from the actual poles themselves, are points where the magnetic forces of the planet rise out of the earth’s core. These points are called the magnetic poles. For pilots, directions are measured from the magnetic north and south poles (though in the Northern Hemisphere the southern pole is mostly irrelevant, and vice versa).
Inside an airplane’s cockpit, the pilot has two instruments at his disposal to measure the direction to the magnetic north pole. The primary instrument is the simplest and most ancient—the magnetic compass. Compasses haven’t changed much since the Chinese first conceived them, and the aviation version is a lightweight card attached to a magnetic bar that points toward the magnetic north pole. The whole thing floats in a bath of white kerosene to stop it from jiggling too much in flight.
Even with the stabilizing help of kerosene, the compass can be somewhat jiggly and difficult to read in flight. So early aviators built the directional gyro, or DG. The DG
doesn’t point northward by itself, though, as the compass does. The pilot must set it before flight according to the magnetic compass. Periodically during flight, he must reset the directional gyro, which wobbles a bit due to friction in the stabilizing gyroscope. (We’ll talk about the array of flight instruments in more detail later in this chapter.)
Nowadays, magnetic compasses float in a pool of white kerosene, which is viscous enough to stabilize the compass and clear enough to see through. Legend has it that the white kerosene has come in handy for pilots who crash-landed their planes in cold weather and needed help starting a fire. But in the old days, pilots sometimes used a different fluid in their compasses—booze. The alcohol worked well in stabilizing the “whiskey compass," as they called it, and it came in handy after a crash landing, too—though old-timers didn’t waste it on starting a fire.
Pilots are responsible for their own navigation. Air-traffic controllers don’t provide much help, and pilots like it that way. Pilots use a number of navigation instruments and a network of ground-bound radio transmitters to help create electronic roadways in the sky; we’ll talk about both of these in Chapter 15, “From Takeoff to Landing,"