Thrust reverser

Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine's thrust so that it is directed forward, rather than backwards. Reverse thrust acts against the forward travel of the aircraft, providing deceleration. Thrust reverser systems are featured on many jet aircraft to help slow down just after touch-down, reducing wear on the brakes and enabling shorter landing distances. Such devices affect the aircraft significantly and are considered important for safe operations by airlines. There have been accidents involving thrust reversal systems such as Lauda Air Flight 004.

Reverse thrust is also available on many propeller-driven aircraft through reversing the controllable-pitch propellers to a negative angle. The equivalent concept for a ship is called astern propulsion.

A landing roll consists of touchdown, bringing the aircraft to taxi speed, and eventually to a complete stop. However, most commercial jet engines continue to produce thrust in the forward direction, even when idle, acting against the deceleration of the aircraft. The brakes of the landing gear of most modern aircraft are sufficient in normal circumstances to stop the aircraft by themselves, but for safety purposes, and to reduce the stress on the brakes, another deceleration method is needed. In scenarios involving bad weather, where factors like snow or rain on the runway reduce the effectiveness of the brakes, and in emergencies like rejected takeoffs, this need is more pronounced.

A simple and effective method is to reverse the direction of the exhaust stream of the jet engine and use the power of the engine itself to decelerate. Ideally, the reversed exhaust stream would be directed straight forward. However, for aerodynamic reasons, this is not possible, and a 135° angle is taken, resulting in less effectiveness than would otherwise be possible. Thrust reversal can also be used in flight to reduce airspeed, though this is not common with modern aircraft. There are three common types of thrust reversing systems used on jet engines: the target, clam-shell, and cold stream systems. Some propeller-driven aircraft equipped with variable-pitch propellers can reverse thrust by changing the pitch of their propeller blades. Most commercial jetliners have such devices, and it also has applications in military aviation.

...
Wikipedia