Reverse Thrust: Slowing Down a Plane After Landing

In an earlier article, we explored various methods pilots use to decelerate an aircraft after landing. One such technique is the application of reverse thrust. If you’ve ever been on a plane, you may have heard a powerful roar from the engines just after touchdown as the aircraft begins to slow down. If you happen to be seated near the engines, you might also observe the engine cowling opening during this process. This is the moment when reverse thrust comes into play.

The Principle of Reverse Thrust

Thrust reversal, also known as reverse thrust, is the temporary diversion of an aircraft engine’s thrust so that it acts against the forward travel of the aircraft, providing deceleration. This technique is particularly important for large aircraft and is used to reduce wear on brakes and enable shorter landing distances. It is also employed in adverse weather conditions, when factors like snow or rain on the runway can reduce the effectiveness of the brakes.

A simple and effective method of achieving thrust reversal is to reverse the direction of the exhaust stream of the jet engine. Although directing the exhaust stream straight forward would be ideal, a 135° angle is used for aerodynamic reasons, resulting in slightly less effectiveness.

Types of Thrust Reversal Systems

There are three common types of thrust reversing systems used in aviation: propeller-driven aircraft, target, clam-shell, and cold stream systems.

1. Propeller-driven aircraft: These aircraft generate thrust reversal by changing the angle of their controllable-pitch propellers so that they direct their thrust forward. This reverse thrust feature became available with the development of controllable-pitch propellers. Reverse thrust is created when the propeller pitch angle is reduced to a negative position, known as the beta position. Examples of aircraft with this feature include the PAC P-750 XSTOL, Cessna 208 Caravan, and Pilatus PC-6 Porter.
2. Target system: The target thrust reverser uses a pair of hydraulically operated bucket or clamshell type doors to reverse the hot gas stream. For forward thrust, these doors form the propelling nozzle of the engine. When deployed, they block the rearward flow of the exhaust and redirect it with a forward component.
3. Clam-shell system: Internal thrust reversers use deflector doors inside the engine shroud to redirect airflow through openings in the side of the nacelle. In turbojet and mixed-flow bypass turbofan engines, one type uses pneumatically operated clamshell deflectors to redirect engine exhaust.
4. Cold stream system: Many high-bypass turbofan engines use a cold-stream reverser. This design places the deflector doors in the bypass duct to redirect only the portion of the airflow from the engine’s fan section that bypasses the combustion chamber. This type of system is less effective due to the exhaust from the combustion chamber continuing to generate forward thrust.

Applications of Reverse Thrust

Reverse thrust is particularly useful in multi-engine seaplanes and flying boats, which have no conventional braking method when landing on water. These aircraft rely on slaloming, reverse thrust, and water drag to slow or stop. Reverse thrust is also necessary for maneuvering on water, making tight turns, or even propelling the aircraft in reverse.

Jet aircraft, on the other hand, utilize thrust reversal by causing the jet blast to flow forward. The engine does not run or rotate in reverse; instead, thrust reversing devices are used to block the blast and redirect it forward.

How Reverse Thrust is Applied

In most cockpit setups, pilots activate thrust reversal by pulling the thrust levers farther back while they are set to idle. This mechanism works in tandem with spoilers to improve deceleration during the critical moments of touchdown, when high speed and residual aerodynamic lift limit the brakes’ effectiveness on the landing gear.

Reverse thrust is always manually selected, and its application can reduce landing roll by a quarter or more. However, aircraft must be able to land without reverse thrust to be certified for scheduled airline service on a runway. Once the aircraft slows down, reverse thrust is shut down to avoid debris ingestion by the engine.

Reverse Thrust in Flight

Although most commercial aircraft cannot safely use thrust reversal in flight, some aircraft, particularly Russian and Soviet models, are designed for this purpose. In-flight reverse thrust offers numerous advantages, including rapid deceleration for quick speed changes and preventing speed build-up during steep dives, allowing for rapid altitude loss. This can be especially useful in combat zones or when making steep approaches to land.

Several aircraft models, such as the Douglas DC-8, Hawker Siddeley Trident, Concorde, Boeing C-17 Globemaster III, and Lockheed C-5 Galaxy, are known for their in-flight reverse thrust capabilities. Military aircraft and space shuttle training aircraft have also utilized this technique for various purposes.

Effectiveness and Safety Considerations

Reverse thrust is most effective at high speeds, and for maximum impact, it should be applied quickly after touchdown. However, activating reverse thrust at low speeds can lead to foreign object damage. In certain aircraft, there’s a risk of momentarily leaving the ground due to the combined effects of reverse thrust and the nose-up pitch effect from spoilers. Pilots must ensure a firm position on the ground before applying reverse thrust.

Additionally, asymmetric deployment of reverse thrust before the nose wheel touches the ground can cause uncontrollable yaw, as steering with the nose wheel is the only way to maintain directional control in this situation.

While thrust reversal mode is used only for a fraction of aircraft operating time, its impact on design, weight, maintenance, performance, and cost is significant. Despite these penalties, reverse thrust is crucial for maintaining safety margins, enhancing directional control during landing rolls, and assisting in rejected take-offs and ground operations on contaminated runways with reduced braking effectiveness.