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Thrust reversal is a technique available for decelerating certain kinds of aircraft by reversing the direction of thrust, pushing forward instead of backward to slow the plane down. It is also called reverse thrust in some settings and is available on jet aircraft, along with planes using controllable pitch propellers. Generally, this technique is used in addition to braking and deploying spoilers to slow the plane down.
When planes land, passengers probably notice flaps on the wings rising to create more drag, acting to slow the plane's forward momentum. In addition, the brakes are applied to bring the plane to a stop. Thrust reversal is another method for slowing the plane, reducing wear and tear on the brakes and allowing planes to stop in a shorter distance. Planes cannot rely on thrust reversal alone to stop, and usually must land on airstrips long enough to allow the plane to come to a complete stop without this technology, unless there is an emergency situation and the plane needs to land at the closest airport for safety reasons.
With this technique in jet planes, the flow of gas from the engines is reversed, often with the use of a clamshell mechanism to block the normal outlet, redirecting the gas in the other direction and creating reverse thrust. People in the window seats near the wing will be able to see the mechanism in operation if they look out the windows while the plane is landing. For planes with controllable pitch propellers, the pitch is altered to push air in the other direction. In both cases, if the plane was standing still while reverse thrust was activated, it would move backward instead of forwards.
Planes with a thrust reversal system have indicator lights to let pilots know when it is active. Usually, failsafes prevent it from being activated in the air, with the exception of some military aircraft, where it may be used during landings to slow the plane at the last minute. Pilots can read the amount of thrust the engines are producing on displays in the cockpit, using this information as they prepare for takeoff and landing.
Many planes use thrust reversal when landing, if it is available, to reduce stress on the mechanical systems of the aircraft. Slowing the plane using this technique wears the brakes and tires less, allowing parts to last longer and cutting down on maintenance expenses needed to keep the plane safe and operational.