Reverse thrust
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Image:Reverse.thrust.klm.fokker70.arp.jpg
Reverse thrust is the temporary diversion of a jet engine's output so that the exhaust gases are blown forwards rather than backwards. The resulting thrust force acts against the forward travel of the aircraft. Reverse thrust is used by many jet aircraft to help slow down just after touch-down, thus reducing wear on the brakes and enabling the aircraft to use shorter runways. It is also available on many propeller-driven aircraft through changing the pitch of the propeller blades to a negative angle, thereby directing air flow into the direction of travel.
Regulations dictate, however, that a plane must be able to land on a runway without the use of reverse thrust in order to be certified to land there.
The application of reverse thrust can be identified by a sudden increase in the volume and pitch of the engines' sound just after touch-down. The reversers are usually clearly visible at the rear of the engines during use.
Frequently landings will use "idle reverse thrust" in which the engines do not actually spool up, but the reversers are activated. This use of the reversers saves fuel, aids in noise abatement and improves engine life.
There are several different ways of reversing the thrust of a turbofan. The illustration shows a Target Reverser, where all of the efflux is reversed. Some high bypass ratio engines, use blocker doors in the bypass duct to redirect (just) the bypass air through a Cascade Reverser.
Thrust Reversers are not intended under any circumstances for use in-flight, and have directly contributed to the crashing of at least one airliner, specifically Lauda Air Flight NG004. The Boeing 767-300 aircraft suffered a premature deployment of the No.1 thrust reverser, which caused the airliner to stall, and crash.[1]