What Are Thrust Reversers and Their Role?
As commercial aircraft have gotten larger and heavier, their landing speeds have gotten higher and higher as well. This has made bringing aircraft to a stop more difficult, as these aircraft require longer and longer landing strips. In many cases, brakes can no longer be solely relied upon to slow the aircraft. Therefore, many commercial aircraft now use thrust reversers. Thrust reversers, as their name implies, reverses the thrust generated by the engines to slow the aircraft. Thrust reversers come in two categories: mechanical blockage, and aerodynamic blockage.
Mechanical blockage is accomplished by placing a removable obstruction in the exhaust gas stream, somewhere to the rear of the nozzle. The engine exhaust gases are mechanically blocked and diverted to a suitable angle in the opposite direction by an inverted cone, half-sphere, or clam shell, all of which are placed in a position to reverse the flow of the exhaust gases. This type is typically used in ducted turbofan engines, where the fan and core flow mix in a common nozzle before exiting the engine. The clamshell-type or mechanical-blockage reverser operates to form a barrier in the path of the escaping gases, which nullifies and reverses the forward thrust of the engine. This system must be able to withstand high temperatures, be mechanically strong, relatively lightweight, and reliable. When not in use, the mechanical-blockage system must be able to retract and nestle neatly around the engine exhaust duct to provide for normal operation.
In an aerodynamic blockage type of thrust reverser (used primarily with non-ducted turbofan engines), only fan air is used to slow the aircraft. An aerodynamic thrust reverser system consists of a translating cowl, blocker doors, and cascade vanes that redirect the fan’s airflow to slow the aircraft. If thrust levers are at an idle position and the aircraft has weight on the wheels, moving the thrust levers aft activates the translating cowl to open, which in turn closes the blocker doors. This action stops airflow from going aft, and instead redirects it through the cascade vanes, where it goes forward to slow the aircraft.
Thrust reversers must have no adverse effect on engine operations, both when they are deployed and when they are stored. Thrust reversers are actuated by either hydraulic or pneumatic pressure and must be locked out from deploying while personnel are in the area of the reverser system during maintenance.
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