Thrust reversal

Thrust reversal, often called reverse thrust, is a specialized operating mode for jet engines. It is used to direct engine thrust forward to help an aircraft slow down. This process is most common during the landing roll on a runway. While aircraft use wheel brakes to stop, thrust reversal provides a vital extra force. This helps the aircraft decelerate more effectively, especially on slippery surfaces.

Using this system also reduces the mechanical stress and wear on the landing gear brakes. It acts as a secondary braking device to ensure safety during high-speed landings.

The mechanism of thrust reversal does not involve running the engine in reverse. Instead, the engine continues to rotate normally, but the direction of the exhaust is changed. The device blocks the natural rearward path of the air and redirects it partially forward. This creates a force that pushes against the aircraft's forward motion.

To be most effective, pilots apply reverse thrust immediately after touchdown. This is when the aircraft is at its highest speed on the runway. As the aircraft slows, the pilot must cancel the reverse thrust. If the speed becomes too low, the forward-moving exhaust could be sucked back into the engine. This could cause foreign object damage by ingesting debris from the runway.

There are three primary types of thrust reversal systems used in jet engines. The first is the target-type system, which uses hydraulically operated doors called buckets or clamshells. These doors hinge at the rear of the engine to block the hot gas stream and redirect it forward.

The second type is the internal system. These systems use deflector doors inside the engine shroud to redirect airflow through openings in the side of the nacelle. Some internal versions use pneumatically operated clamshell deflectors to manage the exhaust.

A third common type is the cold-stream reverser, which is often found on high-bypass turbofan engines. These engines generate most of their thrust from the large fan section rather than the core. The cold-stream system places deflector doors in the bypass duct to redirect only this fan airflow.

This design uses a vane cascade to guide the air forward. Because the core exhaust continues to produce forward thrust, this method is slightly less effective than others. However, the cascade system is known for being reliable and having strong structural integrity.

Propeller-driven aircraft use a different method called operating in reverse pitch. These aircraft use variable-pitch propellers, which allow the angle of the blades to change. By reducing the pitch angle from a fine position to a negative position, known as the beta position, the propeller directs thrust forward.

This technology is common in turboprop aircraft like the Cessna 208 Caravan. Most large aircraft weighing more than 12,500 pounds are equipped with some form of thrust reversal capability.

History shows that thrust reversal has been used for many different flight needs. Some aircraft, like the Douglas DC-8, were certified to use reverse thrust while still in flight. This was used to help with steep descents, though it caused significant aircraft buffeting. The Concorde also used reverse thrust in the air to increase its rate of descent. In military aviation, the technology allows for much shorter landing distances. The Panavia Tornado can land on a 900-meter runway, and the Saab 37 Viggen can use 500-meter strips.

These short strips might even be straight sections of a road used as a wartime runway.

Thrust reversal is a critical component of modern aviation safety and efficiency. While the FAA does not require it for aircraft certification, airlines use it to provide extra stopping power. It is especially useful during rejected takeoffs or when rain and snow reduce brake effectiveness. In some cases, pilots use reverse thrust to move an aircraft backward from a gate. This specific maneuver is called a powerback.

Whether used for rapid deceleration or precise ground movement, it remains a vital tool for pilots.