The kinetic energy of a jet airplane is extremely high due to the combination of the aircraft’s weight and speed. This energy is very difficult to eliminate because a jet aircraft has low drag when the nose wheel is on the ground, and the engines continue to produce forward thrust even when the power is idle. While brakes can normally suffice, there is need for a supplementary method of slowing. This is where thrust reversers come in. A thrust reverser is a device in the engine exhaust system that essentially reverses the path of exhaust gas flow. The flow is not able to reverse 180 degrees, but rather the final path of the gases is diverted 45 degrees from straight ahead. This, coupled with the losses in the reverse flow paths, results in an engine efficiency of about 50 percent, helping the brakes bring the aircraft to a stop.
A normal jet engine will have one of two types of thrust reverser: target or cascade. A target reverser features basic clamshell doors that open from the stowed position at the engine tailpipe. These doors swivel open, blocking and redirecting the outward flow of gases. Cascade reversers are markedly more complex. These are found on turbofan engines and are designed to reverse only the fan air. When activated, cascade reverser parts such as doors in the shroud open and block the airstream’s normal path, redirecting it forward to help slow the aircraft.
On most aircraft configurations, reverse thrust is applied with the thrust lever at idle by pulling up the reverse lever to a detent, which holds the lever in position. This leaves the engine at idle RPM during the slowing process. Reverse thrust is more effective at high speed than at low speed, for two reasons. The first is that the net amount of reverse thrust increases with speed, and the second is that the reverse power is greater at high speeds because of the engine’s increased work rate. In other terms, the aircraft’s kinetic energy is being displaced at higher rates when operating at higher power.
The proper time to apply reverse thrust often depends on the type or model of aircraft. Some aircraft tend to pitch nose up when reverse thrust is applied on landing, which can cause the aircraft to temporarily leave ground again. With aircraft of this type, the pilot must ensure that the aircraft is on the ground with the nose wheel firmly down before implementing reverse thrust. Specific procedures regarding operation of thrust reversers will be included in the FAA-approved AFM of an aircraft.
Thrust reversers on jet aircraft differ greatly from those on propeller aircraft. Idle reverse thrust on a propeller produces 60 percent of the reverse thrust available at full power, making it very effective when at this setting. Jet aircraft, on the other hand, produce very little reverse thrust at idle reverse. In a jet aircraft, the pilot must use full power reverse as soon as the plane is fully grounded and landing can be completed within the available runway distance. Accidental deployment of thrust reversers while airborne is a serious emergency, so reverser systems have multiple built-in lock systems. One system prevents the reversers from operating while airborne, and another to prevent operation when the thrust levers are improperly positioned.
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