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Thrust reversal, also called reverse thrust, is the temporary diversion of an
aircraft engine An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years ma ...
's
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
for it to act against the forward travel of the aircraft, providing deceleration. Thrust reverser systems are featured on many
jet aircraft A jet aircraft (or simply jet) is an aircraft (nearly always a fixed-wing aircraft) propelled by jet engines. Whereas the engines in propeller-powered aircraft generally achieve their maximum efficiency at much lower speeds and altitudes, jet ...
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
airline An airline is a company that provides air transport services for traveling passengers and freight. Airlines use aircraft to supply these services and may form partnerships or alliances with other airlines for codeshare agreements, in wh ...
s. There have been accidents involving thrust reversal systems, including fatal ones. 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.


Principle and uses

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 Landing gear is the undercarriage of an aircraft or spacecraft that is used for takeoff or landing. For aircraft it is generally needed for both. It was also formerly called ''alighting gear'' by some manufacturers, such as the Glenn L. Mart ...
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.


Types of thrust reversal systems

Small aircraft typically do not have thrust reversal systems, except in specialized applications. On the other hand, large aircraft (those weighing more than 12,500 lb) almost always have the ability to reverse thrust.
Reciprocating engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fe ...
,
turboprop A turboprop is a turbine engine that drives an aircraft propeller. A turboprop consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle. Air enters the intake and is compressed by the compressor. ...
and jet aircraft can all be designed to include thrust reversal systems.


Propeller-driven aircraft

Propeller-driven aircraft generate reverse thrust by changing the angle of their controllable-pitch propellers so that the propellers direct their thrust forward. This reverse thrust feature became available with 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. Reverse thrust is created when the propeller pitch angle is reduced from fine to negative. This is called the beta position. Piston-engine aircraft tend not to have reverse thrust, however
turboprop A turboprop is a turbine engine that drives an aircraft propeller. A turboprop consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle. Air enters the intake and is compressed by the compressor. ...
aircraft generally do. Examples include the
PAC P-750 XSTOL The PAC P-750 XSTOL, (formerly known as the PAC 750XL) is a utility aircraft of conventional all-metal low-wing monoplane design, with fixed tricycle undercarriage. Combining the engine and wings of the PAC Cresco with a new large fuselage ...
,
Cessna 208 Caravan The Cessna 208 Caravan is a utility aircraft produced by Cessna. The project was commenced on November 20, 1981, and the prototype first flew on December 9, 1982. The production model was certified by the FAA in October 1984 and its Cargo ...
, and Pilatus PC-6 Porter. One special application of reverse thrust comes in its use on multi-engine
seaplane A seaplane is a powered fixed-wing aircraft capable of taking off and landing (alighting) on water.Gunston, "The Cambridge Aerospace Dictionary", 2009. Seaplanes are usually divided into two categories based on their technological characteri ...
s and
flying boat A flying boat is a type of fixed-winged seaplane with a hull, allowing it to land on water. It differs from a floatplane in that a flying boat's fuselage is purpose-designed for floatation and contains a hull, while floatplanes rely on fuselag ...
s. 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. In addition, reverse thrust is often necessary for maneuvering on the water, where it is used to make tight turns or even propel the aircraft in reverse, maneuvers which may prove necessary for leaving a dock or beach.


Jet aircraft

On aircraft using jet engines, thrust reversal is accomplished by causing the jet blast to flow forward. The engine does not run or rotate in reverse; instead, thrust reversing devices are used to block the blast and redirect it forward. High bypass ratio engines usually reverse thrust by changing the direction of only the fan airflow, since the majority of thrust is generated by this section, as opposed to the core. There are three jet engine thrust reversal systems in common use:


External types

The target thrust reverser uses a pair of hydraulically operated ''bucket'' or ''clamshell'' type doors to reverse the hot gas stream. For forward thrust, these doors form the propelling nozzle of the engine. In the original implementation of this system on the
Boeing 707 The Boeing 707 is an American, long-range, narrow-body airliner, the first jetliner developed and produced by Boeing Commercial Airplanes. Developed from the Boeing 367-80 prototype first flown in 1954, the initial first flew on December 2 ...
, and still common today, two reverser buckets were hinged so when deployed they block the rearward flow of the exhaust and redirect it with a forward component. This type of reverser is visible at the rear of the engine during deployment.


Internal types

Internal thrust reversers use deflector doors inside the engine shroud to redirect airflow through openings in the side of the nacelle. In
turbojet The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, a ...
and mixed-flow bypass turbofan engines, one type uses
pneumatic Pneumatics (from Greek ‘wind, breath’) is a branch of engineering that makes use of gas or pressurized air. Pneumatic systems used in industry are commonly powered by compressed air or compressed inert gases. A centrally located and ...
ally operated ''clamshell'' deflectors to redirect engine exhaust. The reverser ducts may be fitted with cascade vanes to further redirect the airflow forward. In contrast to the two types used on turbojet and low-bypass turbofan engines, many high-bypass turbofan engines use a ''cold-stream'' reverser. This design places the deflector doors in the bypass duct to redirect only the portion of the airflow from the engine's fan section that bypasses the
combustion chamber A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process. Intern ...
. Engines such as the CFM56 direct the airflow forward with a ''pivoting-door'' reverser similar to the internal clamshell used in some turbojets. ''Cascade'' reversers use a vane cascade that is uncovered by a sleeve around the perimeter of the engine
nacelle A nacelle ( ) is a "streamlined body, sized according to what it contains", such as an engine, fuel, or equipment on an aircraft. When attached by a pylon entirely outside the airframe, it is sometimes called a pod, in which case it is attache ...
that slides aft by means of an air motor. During normal operation, the reverse thrust vanes are blocked. On selection, the system folds the doors to block off the cold stream final nozzle and redirect this airflow to the cascade vanes. In cold-stream reversers, the exhaust from the combustion chamber continues to generate forward thrust, making this design less effective. It can also redirect core exhaust flow if equipped with a hot stream spoiler. The cold stream cascade system is known for structural integrity, reliability and versatility, but can be heavy and difficult to integrate into nacelles housing large engines.


Operation

In most cockpit setups, reverse thrust is set when the thrust levers are on idle by pulling them farther back. Reverse thrust is typically applied immediately after touchdown, often along with
spoiler Spoiler is a security vulnerability on modern computer central processing units that use speculative execution. It exploits side-effects of speculative execution to improve the efficiency of Rowhammer and other related memory and cache attacks. ...
s, to improve deceleration early in the landing roll when residual aerodynamic lift and high speed limit the effectiveness of the brakes located on the landing gear. Reverse thrust is always selected manually, either using levers attached to the thrust levers or moving the thrust levers into a reverse thrust 'gate'. The early deceleration provided by reverse thrust can reduce landing roll by a quarter or more. Regulations dictate, however, that an aircraft must be able to land on a runway without the use of thrust reversal in order to be certified to land there as part of scheduled airline service. Once the aircraft's speed has slowed, reverse thrust is shut down to prevent the reversed airflow from throwing debris in front of the engine intakes where it can be ingested, causing foreign object damage. If circumstances require it, reverse thrust 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. When reverse thrust is used to push an aircraft back from the gate, the maneuver is called a powerback. Some manufacturers warn against the use of this procedure during icy conditions as using reverse thrust on snow- or slush-covered ground can cause slush, water, and runway deicers to become airborne and adhere to wing surfaces. If the full power of reverse thrust is not desirable, thrust reverse can be operated with the throttle 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, in particular in icy or slick conditions, or when the engines' jet blast could cause damage.


In-flight operation

Some aircraft, notably some Russian and
Soviet aircraft This list of the military aircraft of the Soviet Union and the Commonwealth of Independent States (CIS) includes experimental, prototypes, and operational types regardless of era. It also includes both native Soviet designs, Soviet-produced copies ...
, are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. Many commercial aircraft, however, cannot. In-flight use of reverse thrust has several advantages. It allows for rapid deceleration, enabling quick changes of speed. It also prevents the speed build-up normally associated with steep dives, allowing for rapid loss of
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
, which can be especially useful in hostile environments such as combat zones, and when making steep approaches to land. The Douglas DC-8 series of airliners has been certified for in-flight reverse thrust since service entry in 1959. Safe and effective for facilitating quick descents at acceptable speeds, it nonetheless produced significant aircraft buffeting, so actual use was less common on passenger flights and more common on cargo and ferry flights, where passenger comfort is not a concern. The Hawker Siddeley Trident, a 120- to 180-seat airliner, was capable of descending at up to 10,000 ft/min (3,050 m/min) by use of reverse thrust, though this capability was rarely used. The
Concorde The Aérospatiale/BAC Concorde () is a retired Franco-British supersonic airliner jointly developed and manufactured by Sud Aviation (later Aérospatiale) and the British Aircraft Corporation (BAC). Studies started in 1954, and France an ...
supersonic airliner could use reverse thrust in the air to increase the rate of descent. Only the inboard engines were used, and the engines were placed in reverse idle only in subsonic flight and when the aircraft was below in altitude. This would increase the rate of descent to around . The
Boeing C-17 Globemaster III The McDonnell Douglas/Boeing C-17 Globemaster III is a large military transport aircraft that was developed for the United States Air Force (USAF) from the 1980s to the early 1990s by McDonnell Douglas. The C-17 carries forward the name of tw ...
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 (a descent rate of just over 170 mph, or 274 km/h). The
Lockheed C-5 Galaxy The Lockheed C-5 Galaxy is a large military transport aircraft designed and built by Lockheed, and now maintained and upgraded by its successor, Lockheed Martin. It provides the United States Air Force (USAF) with a heavy intercontinental-rang ...
, introduced in 1969, also has in-flight reverse capability, although on the inboard engines only. The Saab 37 Viggen (retired in November 2005) also had the ability to use reverse thrust both before landing, to shorten the needed runway, and taxiing after landing, allowing many Swedish roads to double as wartime runways. The
Shuttle Training Aircraft The Shuttle Training Aircraft (STA) was a NASA training vehicle that duplicated the Space Shuttle's approach profile and handling qualities, allowing Space Shuttle pilots to simulate Shuttle landings under controlled conditions before attemptin ...
, a highly modified Grumman Gulfstream II, used reverse thrust in flight to help simulate
Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program n ...
aerodynamics so astronauts could practice landings. A similar technique was employed on a modified Tupolev Tu-154 which simulated the Russian Buran space shuttle.


Effectiveness

The amount of thrust and power generated are proportional to the speed of the aircraft, making reverse thrust more effective at high speeds. For maximum effectiveness, it should be applied quickly after touchdown. If activated at low speeds, foreign object damage is possible. There is some danger of an aircraft with thrust reversers applied momentarily leaving the ground again due to both the effect of the reverse thrust and the nose-up pitch effect from the spoilers. For aircraft susceptible to such an occurrence, pilots must take care to achieve a firm position on the ground before applying reverse thrust. If applied before the nose-wheel is in contact with the ground, there is a chance of asymmetric deployment causing an uncontrollable yaw towards the side of higher thrust, as steering the aircraft with the nose wheel is the only way to maintain control of the direction of travel in this situation. Reverse thrust mode is used only for a fraction of aircraft operating time but affects it greatly in terms of
design A design is a plan or specification for the construction of an object or system or for the implementation of an activity or process or the result of that plan or specification in the form of a prototype, product, or process. The verb ''to design' ...
, weight, maintenance, performance, and cost. Penalties are significant but necessary since it provides stopping force for added safety margins, directional control during landing rolls, and aids in rejected take-offs and ground operations on contaminated runways where normal braking effectiveness is diminished. Airlines consider thrust reverser systems a vital part of reaching a maximum level of aircraft operating safety.


Thrust reversal-related accidents and incidents

In-flight deployment of reverse thrust has directly contributed to the crashes of several transport-type aircraft: * On 4 July 1966 an Air New Zealand Douglas DC-8-52 with the registration ZK-NZB crashed on takeoff on a routine training flight from Auckland International Airport due to reverse thrust applied during a simulated failure of no.4 engine on takeoff . The crash killed 2 of the 5 crew on board. * On 11 February 1978, Pacific Western Airlines Flight 314, a Boeing 737-200, crashed while executing a rejected landing at
Cranbrook Airport Cranbrook/Canadian Rockies International Airport is an international airport located north of Cranbrook and south-east of Kimberley, British Columbia, in the Canadian Rockies. It is owned by the City of Cranbrook and operated by Elevate A ...
. The left thrust reverser had not properly stowed; it deployed during the climbout, causing the aircraft to roll to the left and strike the ground. Out of 5 crew members and 44 passengers, only 6 passengers and a flight attendant survived. *On 9 February 1982, Japan Airlines Flight 350 crashed short of the runway at Tokyo Haneda Airport following the intentional deployment of reverse thrust on two of the Douglas DC-8's four engines by the mentally unstable captain, resulting in 24 passenger deaths. *On 29 August 1990, a United States Air Force
Lockheed C-5 Galaxy The Lockheed C-5 Galaxy is a large military transport aircraft designed and built by Lockheed, and now maintained and upgraded by its successor, Lockheed Martin. It provides the United States Air Force (USAF) with a heavy intercontinental-rang ...
crashed shortly after take-off from Ramstein Air Base in
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. 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, 4 survived the crash. *On 26 May 1991, Lauda Air Flight 004, a
Boeing 767-300ER The Boeing 767 is an American wide-body aircraft developed and manufactured by Boeing Commercial Airplanes. The aircraft was launched as the 7X7 program on July 14, 1978, the prototype first flew on September 26, 1981, and it was certified o ...
, had an accidental deployment of the left engine's thrust reverser, which caused the airliner to go into a rapid dive and break up in mid-air. All 213 passengers and 10 crew were killed. *On 31 October 1996, TAM Linhas Aéreas Flight 402, a Fokker 100, crashed shortly after take-off from Congonhas-São Paulo International Airport,
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,
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, striking two apartment buildings and several houses. All 90 passengers and 6 crew members as well as 3 people on the ground died in the crash. The crash was attributed to the un-commanded deployment of a faulty thrust reverser on the right engine shortly after take-off. * On 10 February 2004, Kish Air Flight 7170, a
Fokker 50 The Fokker 50 is a turboprop-powered airliner, designed as an improved version of the successful Fokker F27 Friendship. The Fokker 60 is a stretched freighter version of the Fokker 50. Both aircraft were manufactured and supported by Dutch airc ...
, crashed while on approach to Sharjah International Airport. A total of 43 out of the 46 passengers and crew on board were killed. Investigators determined that the pilots had prematurely set the propellers to reverse thrust mode, causing them to lose control of the aircraft. * On 17 July 2007, TAM Linhas Aéreas Flight 3054, an
Airbus A320 The Airbus A320 family is a series of narrow-body airliners developed and produced by Airbus. The A320 was launched in March 1984, first flew on 22 February 1987, and was introduced in April 1988 by Air France. The first member of the fam ...
crashed after landing on Congonhas-São Paulo International Airport,
São Paulo São Paulo (, ; Portuguese for ' Saint Paul') is the most populous city in Brazil, and is the capital of the state of São Paulo, the most populous and wealthiest Brazilian state, located in the country's Southeast Region. Listed by the GaW ...
,
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
, hitting a Shell Gas station, cars, and finally the
TAM Express TAM may refer to: Biology * Thioacetamide, an organosulfur compound * Tumor-associated macrophage, a class of immune cells * Transparent Anatomical Manikin, an educational model Technology * Tanque Argentino Mediano, the main battle tank of Ar ...
building, killing a total of 199 people, 187 aboard the plane and 12 on the ground, leaving no survivors. An inoperative thrust reverser was one of the many causes for the accident.


See also

*
Afterburner An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and c ...
*
Thrust vectoring Thrust vectoring, also known as thrust vector control (TVC), is the ability of an aircraft, rocket, or other vehicle to manipulate the direction of the thrust from its engine(s) or motor(s) to control the attitude or angular velocity of the ve ...
* Vertical take-off and landing


References


External links


Reducing Landing Distance


– a 1945 ''Flight'' article on new engine developments showing a Power Jets reverse thrust device {{DEFAULTSORT:Thrust Reversal Aircraft controls Jet engine technology Aircraft propulsion components Articles containing video clips