"Flight attendants please be seated for landing." We've all heard this announcement on the plane as the plane flies low over the city, preparing to touch down at the airport. Around this time, you might here some hydraulic noises. If you're seated at the window, you might also see the wing start to reconfigure, the back of the wing will move out and down.
This is something that I've seen many people ask questions about. These rear edge extensions are called flaps. These flaps are variable in that the pilot can set how far out these extensions can be placed. Flaps can also be used for takeoff as well, though not set at the large angle that tend to be when landing.
So what do flaps do?
Airplanes are inherently built for higher speeds. Wings are swept back to make for a more aerodynamic aircraft. The more aerodynamic the aircraft, the faster it can travel and the more efficient it is at higher speeds. However, landing and takeoff speeds are slow by necessity. This is where the flaps come in.
When the flaps are extended from the wing, the shape of the wing changes to a more arc like shape. This shape is great for low speed flight, since it increases the lift of the wings at those crucial low speeds. This allows the plane to fly at lower speeds and still remain in the air with no problems. Activating flaps gives the pilots better control at low speeds and helps land the plane slower and make for a better and more comfortable landing for passengers. Slower speeds also means that the runways don't need to be as long as would be required if planes did not have this feature.
Next time you're in a window seat, take a look at the wing during landing and you'll see the flaps come into play.