Aircraft undercarriage

[Pootle]

Why do aircraft lower their undercarriage so early prior to landing? I live in London and often see planes heading west towards Heathrow with their undercarriage in position thousands of feet over what must be around Bermondsey

[Skids]

Undercarriage make good air brakes by introducing a lot of drag. And its better to be ready earlier rather than later.

[Ralph]

Because they're good air brakes like Skids said, they also upset the balance of the aircraft when they're lowered, causing it to pitch down a bit. It's best to recover from this early on and high up.

[Clanad]

Both skids and ralph are on the right track, so to speak. Early on, with the introduction of jet aircraft into commercial aviation, numerous radical changes occurred in airmanship as compared to the older propeller driven aircraft. One of the most pronounced was the recognition of airspeed control. A memory jogger emerged..."you can't go down and slow down..." meaning without the avialable breaking of an idled, windmilling propeller(s) slowing down during descents was difficult in jet aircraft. Lowering the landing gear (which has a rather high maximum allowable speed to do so) provides one avenue. Most folks aren't aware that aircraft must be slower than 250 knots below 10,000 feet above mean seal level (MSL) and lower than 220 knots within a certain distance of the airport (changes from country to country). You'll notice that a small amount of flaps and slats (devices on aft and leading edge of wing) are extended first and later the landing gear. Astute air travelers may have noticed the raising of rather large flaps in the mid sections of the top side of the wings as well. These are speed brakes and used in the descent portion of the flight. In addition to these, another panel immediately adjacent to the speed brakes is automatically raised, along with maximum speed brake extension when the landing gear touches down to destroy lift and make braking for slowing more effective...