When an aircraft lands, it must decelerate quickly and safely to avoid overshooting the runway. While brakes and aerodynamic drag contribute to this, they may not always be sufficient on their own. This is where thrust reversers come into play. These temporarily redirect engine thrust forward, helping to slow the aircraft efficiently. This article explains their purpose, working mechanism, and the different types used in commercial and military aircraft.
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Why Are Thrust Reversers Necessary?
Jet engines produce enormous thrust to push an aircraft forward. After landing, this forward momentum must be curbed to bring the plane to a stop safely. Brakes and aerodynamic drag assist in this process, but they are not always enough. At the time of touchdown, the plane will be carrying decent speeds, making it harder for the wheel brakes and air brakes alone to take control. Thrust reversers provide quick deceleration by redirecting engine exhaust in the opposite direction. This reduces the plane’s speed more effectively and helps prevent excessive strain on the brakes.
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When Are They Used?
It isn’t common to find the extensive use to thrust reversers in most real-world landings. Many a time, the air brakes and other stopping mechanisms get the job done. However, landing in the rain and on slippery runways demands stronger deceleration and a stronger use of reverse thrust. It can also be employed in cases of rejected takeoffs, to prevent overshooting the runway.
You might now wonder… Do they use reverse thrust to reverse a plane from the gate? Well, not really. While technically it can be used for the purpose, doing so could suck objects (and even people around) into the engine if the turbine spins fast enough. Thus, a tug truck is often used for pushbacks.
How Do Thrust Reversers Work?
Reversers are integrated into the aircraft’s engines and activate only when needed. When the pilot engages them after landing, they redirect the engine’s airflow. Instead of pushing the plane forward, the air is deflected to slow it down.
The turbine inside the engine continues to rotate in the same direction. However, mechanical components alter the direction of the thrust being produced. This process reduces speed efficiently, decreases wear on the landing gear, and shortens the required runway length for stopping.
Different Types of Thrust Reversers
Aircraft use various types of thrust reversers depending on their engine designs and the way they work. While the core idea of deflecting the airflow forward remains the same, different models employ different techniques to achieve this. Below are the most common ones:
Target reverser: Found in some commercial jets, these use bucket doors to deflect the air in the opposite direction. Hydraulic actuators move these to redirect the airflow in the desired direction. Target reversers are seen on aircraft using low bypass ratio turbofan engines.
Cascade Reverser: These are found on planes with high bypass ratio engines. It is also called a clam-shell reverser and uses blocking doors that sit near the openings near the fan cowl’s aft edge. These obstruct the normal airflow and direct it out, at a sideward angle.
Cold stream Reversers: Some modern planes rely on cold stream reversers. This system redirects the bypass airflow using doors that sit in the bypass duct. These can be activated by an air motor, and when functional, will block the cold stream final nozzle. This will generate forward airflow out of cascade vanes, in other words, reverse thrust.
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Can Thrust Reversers Be Used in Flight?
The reverse thrust mechanism is designed to work only when the aircraft is on the ground. Modern systems include electronic and hydraulic safeguards to ensure that it doesn’t deploy mid-flight. This prevents accidents and ensure safe operation.
However, military aircraft sometimes use thrust reversers mid-air. In specific situations, such as high-risk landings or specialized maneuvers, military jets may engage these systems to decelerate more rapidly. Unlike commercial planes, these aircraft are built for such extreme operations.
Why Are They Important?
Thrust reversers play a critical role in ensuring safety in aviation. By assisting in deceleration, they help aircraft stop more efficiently, reducing the chances of runway overshoots. They also minimize wear on braking systems and the landing gear, and help in lowering maintenance costs and improving overall longevity. Without them, modern aircraft would require significantly longer runways and landings would place greater strain on their brakes.
