Thrust reversal
(Redirected from Thrust reverser)
Thrust reversal is the temporary diversion of a jet engine's output so that the exhaust gases are blown in a direction other than aft of the aircraft. The resulting lack of equilibrium acts against the forward travel of the aircraft, providing deceleration. Thrust reversers are 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 thrust reversers in order to be certified to land there.
The deployment of thrust reversers 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 illustrations show clamshell-type reversers, 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 typically activated via a set of auxiliary levers (one per engine) on the throttle assembly that are moved up after the main throttles have been pulled back into the idle position.
On most modern jet aircraft, thrust reversers are not intended under any circumstances for use in-flight. However, they 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]. On October 31 1996, a Brazilian Fokker 100 crashed shortly after take-off from Congonhas airport, striking an apartment building and several houses. All 90 passengers and 6 crewmember onboard died. Three people were killed on ground. The crash was attributed to a faulty thrust-reverser in the right engine deploying after take-off.
In a related incident, in the runway overrun of Southwest Airlines Flight 1248, one of the suggested reasons for the overrun was a delayed deployment of the thrust reversers.
However some aircraft are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. For example, the ATR 72 can reverse thrust in flight, should the appropriate control lock be withdrawn. The Hawker-Siddeley Trident, a 120-180 seat airliner, was capable of descending at up to 10,000 ft/min (3,048 m/min) by use of the thrust reversers, though this capability was rarely used. The US Air Force C-17A is one of the only modern aircraft that uses reverse thrust in flight. The Boeing manufactured aircraft is capable of deploying all four engines in reverse to facilitate steep, tactical descents up to 15,000 ft/min (4,600 m/min) into combat environments.
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