Landing a commercial airliner seems like one of technology's most improbable feats. A plane must descend from 35,000 feet (10,668 meters) to the ground and slow from 650 miles (1,046 kilometers) to 0 miles per hour. Oh, yeah, and it has to place its entire weight -- some 170 tons -- onto just a few wheels and struts that must be strong, yet completely retractable. Is it any wonder that landing gear takes the No. 1 spot on our list?
Up until the late 1980s, the majority of civilian and military aircraft used three basic landing-gear configurations: one wheel per strut, two wheels side by side on a strut or two side-by-side wheels next to two additional side-by-side wheels. As airplanes grew larger and heavier, landing gear systems became more complex, both to reduce stress on the wheel and strut assemblies, but also to decrease forces applied to runway pavement. The landing gear of an Airbus A380 superjumbo airliner, for example, has four undercarriage units -- two with four wheels each and two with six wheels each. Regardless of configuration, strength is far more important than weight, so you'll find steel and titanium, not aluminum, in the metal components of a landing gear.