Thrust reversal

Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine's exhaust or changing of propeller pitch 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.

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. Reverse thrust is always selected manually, either using levers attached to the thrust levers, or by moving the thrust levers into a reverse thrust 'gate'. When thrust is reversed, passengers will hear a sudden increase in engine noise, particularly those seated just forward of the engines.The early deceleration provided by reverse thrust 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 shut down 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 aircraft speed, and, if circumstances require, can be used all the way to a stop, or even to provide thrust to push the aircraft backward, though aircraft 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' jet blast could do damage.

In-flight operation

Some aircraft are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. Many commercial aircraft cannot use reverse thrust in flight. In exception, Russian and Soviet aircraft enable the use of reverse thrust in flight (mostly before touchdown). 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 Trident, a 120-180 seat airliner, was capable of descending at up to 10,000 ft/min (3,050 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 few 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 (this means that the aircraft's descent rate is just over 170mph, or 274kph). The Saab 37 Viggen (retired in November 2005) also had the ability to use reverse thrust before landing, enabling the use of many roads constructed in Sweden to double as wartime runways.

The Shuttle Training Aircraft, a highly modified Grumman Gulfstream II, uses reverse thrust in flight to help simulate the Space Shuttle aerodynamics so astronauts can practice landings.

Types of aircraft

Small aircraft typically do not feature reverse thrust, except in specialized applications. Conversely, large aircraft (weighing more than 12,500 lb) 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 aircraft descends, avoiding doing damage to the engines by shock cooling them.

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.

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 planes of previous years. Additionally, the higher performance and greater altitude attainable by post World War II piston aircraft 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 power that could be used both for improved performance and to provide reverse thrust.

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.

Seaplanes and flying boats

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/or reverse thrust, as well as the drag of the water in order to slow or stop. Additionally, reverse thrust is often necessary for maneuvering on the water, where it is used to make tight turns or even back the aircraft, such as when leaving a dock or beach.

Jet aircraft

On aircraft using jet engines, thrust reversal is accomplished by causing the jet blast to flow forward rather than aft. The engine does not run or rotate in reverse; Instead, thrust reversers are used to block the blast and redirect it forward. Two methods are commonly used: In the target-type thrust reverser, the reverser blades angle outward, giving the general appearance of flower petals, and forcing engine thrust to flow forward. In the clamshell type, 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.

Turbofan

In addition to the two types used on turbojet and low-bypass turbofan engines, 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.

The Boeing C-17 has a rare form of the above type in which even the exhaust from the core is redirected along with the main fan's air. This gives the C-17 unrivaled stopping ability among large jet powered aircraft.

Thrust-reverse related accidents

In-flight deployment of thrust reversers on aircraft not having that function as a design feature has directly contributed to the crashes of several transport-type aircraft:

*On 9 February, 1982, Japan Airlines Flight 350 crashed convert|1000|ft|m|-2 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 August 29, 1990, a Lockheed C-5A crashed shortly after take-off from Ramstein Air Base in Germany. As the aircraft started to climb off the runway, one of the thrust reversers suddenly deployed. This resulted in loss of control of the aircraft and the subsequent crash. Of the 17 people on board, only 4 survived the crash.
*On 26 May, 1991, Lauda Air Flight NG004. The Boeing 767-300 aircraft suffered an uncommanded deployment of the No. 1 thrust reverser, which caused the airliner to stall and crash. cite web
title = 26 May 1991 - Lauda 004
publisher = Tailstrike.com: Cockpit Voice Recorder Database
url = http://www.tailstrike.com/260591.htm
date = 2004-09-23
accessdate = 2006-12-14
] All 213 passengers and 10 crew were killed.
*On October 31, 1996, TAM Linhas Aéreas Flight 402. The Fokker 100 crashed shortly after take-off from Congonhas/São Paulo International Airport, São Paulo, Brazil, striking an apartment building and several houses. All 90 passengers and 6 crew members on board died. Three people were killed on the ground. The crash was attributed to the uncommanded deployment of a faulty thrust-reverser on the right engine shortly after take-off.
*On July 9, 2006, S7 Airlines Flight 778 crashed after landing, killing 124 people. One thrust reverser did deploy, while the other didn't, thus creating asymmetrical thrust. The aircraft failed to stop, veered off the runway and crashed into garages.

At least two accidents have been blamed in part on late deployment of reverse thrust:
*On 8 December, 2005, Southwest Airlines Flight 1248 overran the runway at Chicago Midway Airport, killing a 6-year old boy in a car. It was confirmed in the official crash report that delayed reverse thrust played a role.

*On 2 August, 2005, Air France Flight 358 overran the runway at Pearson International Airport, Toronto, Canada. The plane came to a skidding halt 200 metres past the end of the runway, with the nose pointing down a ravine. This crash was partly attributed to the late deployment of the thrust reversal systems. All passengers and crew were able to exit the plane and survive.

Gallery


Air Canada Boeing 777 with thrust reversers deployed, visible as a "split" in the engine nacelle
Rolls-Royce RB199 jet engine
Controllable pitch propeller on the singe-turboprop of a FedEx Cessna 208B Grand Caravan
KLM Fokker 100 landing with reverse-thrust buckets deployed.
Antonov An-74 moving with reverse-thrust

References

External links

* [http://www.aerospaceweb.org/question/propulsion/q0181.shtml Reducing Landing Distance]


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