Thrust reversal
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Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine's output so that the thrust produced is directed forward, rather than aft. This 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, reducing wear on the brakes and enabling shorter landing distances. It is also available on many propeller aircraft through reversing the controllable pitch propellers to a negative angle.
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[edit] Operation
Reverse thrust is typically applied immediately after touchdown, often along with spoilers, to improve deceleration early in the landing roll when residual aerodynamic lift and high speed limit the effectiveness of the friction brakes located on the landing gear. In older aircraft, reverse thrust is always selected manually, using levers located near the throttles. In newer aircraft, reverse thrust can be armed while in flight for automatic deployment on touchdown. When thrust is reversed, passengers will hear a sudden increase in engine noise and can feel the sudden deceleration it causes.
The early deceleration reverse thrust provides can reduce landing roll by a third or more. 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 as part of scheduled airline service.
Once the aircraft's speed has slowed, thrust reverse is deselected to prevent the reversed airflow from raising debris in front of the engine intakes where it can be ingested, causing foreign object damage. Thrust reverse is effective at any speed, and, in unusual circumstances, can be used all the way to a stop, or even to provide thrust to push the airplane backward, though airplane tugs or towbars are more commonly used for that purpose.
If the full power of reverse thrust is not desirable, thrust reverse can be operated with the throttles set at less than full power, even down to idle power, which reduces stress and wear on engine components. Reverse thrust is sometimes selected on idling engines to eliminate residual thrust, particularly in icy or slippery conditions, or where the engines' blast could do damage.
[edit] In-flight operation
Some aircraft are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. In-flight use of reverse thrust has several advantages: It allows for rapid deceleration, enabling quick changes of speed; It also prevents the speed buildup normally associated with steep dives, allowing for rapid loss of altitude, which can be especially useful in hostile environments such as combat zones, and when making steep approaches to land.
For example, the ATR 72 turboprop can reverse thrust in flight, should the appropriate control lock be withdrawn. The Hawker-Siddeley HS121 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's C-17A is one of the only modern aircraft that uses reverse thrust in flight. The Boeing-manufactured aircraft is capable of in-flight deployment of reverse thrust on all four engines to facilitate steep tactical descents up to 15,000 ft/min (4,600 m/min) into combat environments.
[edit] Jet aircraft
On aircraft using turbojet engines, thrust reversal is accomplished by causing the jet blast to flow forward rather than aft. As the engine is not configured to run (and rotate) in reverse, the thrust reversers block the exhaust flow and redirect it forward. Two methods are commonly used: In the target type of thrust reverser, the reverser blades angle outward, giving the general appearance of flower petals. In the clamshell type (top right), two reverser buckets are hinged so that when they deploy, they intrude into the exhaust of the engine, capturing and reorienting the jet blast. This type of reverser is usually clearly visible at the rear of the engine during use. In addition to the two already mentioned, a third type of thrust reverser is found on some high-bypass turbofan engines. Doors in the bypass duct are used to redirect the air that has been accelerated by the engine's fan section but has not passed through the combustion chamber (called bypass air) so that it provides reverse thrust.
[edit] Propeller aircraft
Propeller aircraft generate reverse thrust by changing the angle of their controllable pitch propellers so that the propellers direct their thrust forward, instead of aft as normal. Reverse thrust has been available on propeller aircraft dating back to the 1930s. Reverse thrust became available due to the development of controllable-pitch propellers, which change the angle of the propeller blades to make efficient use of engine power over a wide range of conditions.
Small aircraft typically do not feature reverse thrust, except in specialized applications. Similarly, large aircraft (weighing more than 12,500 lbs) almost always have the ability to reverse thrust. Both reciprocating engine and turboprop aircraft can have reverse thrust, and almost all propeller aircraft with reverse thrust have the ability to set the propeller angle to flat pitch (called Beta range) which generates no forward or reverse thrust, but provides large amounts of drag. This is especially useful in aircraft with complex reciprocating or turbine engines, as it enables engine speed to be kept high as the airplane descends, avoiding doing damage to the engines by shock cooling them.
[edit] Multiengine
Early multiengine aircraft such as the Boeing 247 and Douglas DC-2 were among the first to feature reverse thrust. As piston aircraft became heavier and more complex, reverse thrust became more important to allow them to operate from airports originally configured to handle the smaller airplanes of previous years. Additionally, the higher performance and greater altitude attainable by post World War II piston airplanes like the Lockheed Constellation made the ability to use flat pitch, or, in extreme cases, reverse thrust, in order to descend and slow for landing without overcooling the engines or approaching the runway with excessive speed. Finally, the advent of turboprops like the Vickers Viscount and Lockheed Electra brought even higher speeds and cruising altitudes to the fleet, as well as increased horsepower that could be used both for improved performance and to provide reverse thrust.
One special application of reverse thrust comes in its use on seaplanes and flying boats. These aircraft, when landing on water, have no conventional braking method and must rely on slaloming and reverse thrust to slow or stop. Additionally, reverse thrust is necessary for maneuvering on the water, where it is used to make tight turns or even back the airplane, such as when leaving a dock or beach.
[edit] Single-engine
Single-engine aircraft tend to be of such limited size that the weight and complexity of reverse thrust is unwarranted. However, large single-engine aircraft like the Cessna Caravan do have reverse thrust available, and single-engine seaplanes and flying boats tend to have reverse thrust as well. In other respects, reverse thrust on single-engine aircraft works much like that on other propeller aircraft.
[edit] Thrust-reverse related crashes
On most modern jet aircraft, thrust reversers are not intended under any circumstances for deployment in-flight. However, in-flight deployment has directly contributed to the crashes of at least three airliners:
- On 9 February 1982, Japan Airlines Flight 350 crashed 1000 feet short of the runway at Tokyo Haneda Airport following the intentional deployment of reverse thrust on two of the DC-8's four engines in an apparent suicide attempt, resulting in 24 passenger deaths.
- On 26 May 1991, 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, TAM Linhas Aéreas Flight 402. The Fokker 100 crashed shortly after take-off from Congonhas International Airport, São Paulo, Brazil, striking an apartment building and several houses. All 90 passengers and 6 crewmembers onboard died. Three people were killed on the ground. The crash was attributed to the uncommanded deployment of a faulty thrust-reverser in the right engine shortly after takeoff.
- On 8 December 2005, Southwest Airlines Flight 1248 overran the runway at Chicago Midway Airport. One of the suggested reasons for the overrun was a delayed deployment of the thrust reversers.
[edit] References
- ^ 26 May 1991 - Lauda 004. Tailstrike.com: Cockpit Voice Recorder Database (2004-09-23). Retrieved on December 14, 2006.
