Sometimes, pilots must make the transfer from wings to wheels briskly.

Landing Gear

What Americans call "landing gear," the British call "undercarriage," which really is much more logical. That's because an airplane uses its wheels not just to land but also to take off, taxi and sit around in the hangar. Whatever you call it, the part of the aircraft that allows it to move around on the ground has several distinguishing characteristics.

One of the most obvious is whether or not the gear folds up, or retracts, into the fuselage in flight. This drag-reducing concept was devised in the 1930s. In fact, the airplanes used on some early record-setting flights were designed to jettison their wheels after takeoff and then belly land on their keels. Today, I know of no jets that don't tuck their wheels away while in flight. The drag of fixed gear, while acceptable for many slower airplanes, is prohibitively expensive at jet speeds.

Woe betide the pilot who forgets to extend the landing gear before returning to terra firma. (There's an old aviation joke about the hapless pilot who realizes he forgot to lower the landing gear only when he has to use full engine power to taxi to the ramp.)  Today's jets have warning systems and checklist procedures that make inadvertent belly landings by professional crews almost impossible-but they have happened. Scarcely a day passes, however, without an FAA report that an owner-flown light airplane somewhere landed "gear up." The good news is that these arrivals almost always end without injury-except to the pilot's ego and insurance rates. (Another old joke: There are two kinds of pilots-those who have landed gear up and those who almost have.)

Some subtle elements of landing-gear design include how they absorb the shock of transferring the airplane's weight from wings to wheels. On normal landings, the transfer is as smooth as a car exiting a freeway ramp. But aircraft designers have to account for the worst-case scenarios-stiff, gusty crosswinds on short runways. Under those circumstances, the pilot is obliged to make the transfer from wings to wheels briskly and without delay-not the time or place for one of those "are we down yet?" featherbed landings.

That's when well-designed tires, wheels and struts show their muscle. And unless you actually see sparks and shards of aluminum falling off the airplane, chances are the roughness of the landing was not a mistake, but rather the result of prudent airmanship. Don't worry, the wheels can take it. And when you step off the airstair, hold onto your hat because it's probably windy.

Less complicated landing-gear designs use a single strut with the spring and shock absorber incorporated inside-like the MacPherson struts on many automobiles' suspension systems. The advantage of this type is its simplicity and the little amount of room the gear takes up inside the wing when the wheels are retracted, space then afforded to fuel capacity. More complex systems involve what's called a "trailing link" mechanism that works with a scissors-like, up-and-down motion. This configuration has significantly better shock-absorbing action for smoother landings, but is heavier, more complicated to manufacture and takes up more room inside the wing. As with almost everything in airplane design, this decision is a compromise.

The height of the landing gear is another strategic question for the aircraft designer. Short landing-gear legs are simpler, lighter and take up less room, but make for stiffer landings. Longer gear legs have the opposite complement of pluses and minuses. When it comes to design decisions, influences occasionally come into play that are more related to our human frailties. For example, some maintain that the Beechcraft King Air series of twin turboprops (see photo) was designed, on purpose, to sit tall and regal on long-legged gear-the better to allow owners to look down on competing brands around the airport. But no one has ever admitted that, at least in public.

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