Speed Brake

A speed brake, also called an air brake, is a flight control surface built to increase an aircraft's aerodynamic drag deliberately, typically lying flush with the surrounding surface when retracted so that it adds no drag of its own until the pilot extends it into the airstream. Jet powered aircraft need such a device because, unlike propeller aircraft, a jet engine at idle produces little of the natural braking drag a propeller and its disc provide, so pilots use the speed brake to control airspeed and the steepness of the descent during an approach to landing without having to reduce power so far that the engines respond too slowly if a go around becomes necessary. Speed brakes are distinguished from spoilers by their aim, since a spoiler is built to disrupt lift and increase drag together and can raise an aircraft's stall speed, while a speed brake is built to add drag with as little effect on lift as possible, though on many modern airliners the same movable surfaces are wired to perform both functions depending on how they are commanded.