The landing experience is one of the most exciting stages of flight. If you pay close attention to a landing airplane, you would notice that as soon as the plane begins to touch down, a part of the jet engine opens up in a somewhat bizarre fashion as the plane comes to a gradual stop. This raises a question amongst aviation enthusiasts, why do jet engines open when landing?

Jet engines split open in order to slow down the speed of airplanes when landing. The opening of engines during landing is technically known as a thrust reversal mechanism, and diverts the airflow in the opposite direction. This helps reduce the speed of the airplane, and allows for landings on shorter runways. 

The landing speed of an airplane is about 190 mph. This speed, although in deceleration, is at the high end of the speed spectrum and could be difficult to handle on a landing strip that is just 6000 ft long.

The thrust reversal mechanism works to deflect the thrust away from the jet engine in the direction of the plane’s motion, thus causing the airplane to decelerate faster. As a result of this fast rate of deceleration, the thrust reversal mechanism allows the pilot to safely bring the speeding airplane to a gradual stop.

In this article, we will break down the working principles of thrust reversal in jet engines and explore other relevant principles that guide the landing of an airplane.

How Do Airplanes Slow Down When Landing?

An airplane blazing through the air at a speed of 150-200 mph could be difficult for pilots to handle. Sometimes, during landing, it feels as if the airplane is simply not interested in coming to a stop. In fact, most jet engines continue to generate thrust even when they are idle, therefore working directly against the deceleration of the airplane. 

Although an efficient brake system like in our automobiles can bring a jet engine to a stop in the case of an emergency, they produce sufficient heat that could melt the tires of airplanes and lead to disaster. As a result, airplanes are equipped with advanced mechanisms that work to decelerate their engines and bring them to a gradual stop. 

Apart from brakes, airplanes are brought to a steady stop through the combined efforts of aerodynamic ground spoilers and thrust reversers. Spoilers are designed on the wings of an airplane to generate drag, while thrust reversers adopt the thrust reversal mechanism to slow down the engines rapidly.

However, between these brake mechanisms, a critical component some commercial jets employ to slow down and achieve a safe landing is the thrust reverser installed in the engines of these airplanes to generate thrust in reverse.

What is Reverse Thrust?

As already stated, reverse thrust is also known as the thrust reversal mechanism. It is a control mechanism installed in jet engines that serves to change the direction in which air shoots out of the engine.

This mechanism should not be confused with a car’s reverse control that works to change the direction of the car because it is impossible for airplanes to abruptly drift or reverse direction in midair. 

However, the thrust reversal mechanism is adopted to assist pilots to reduce the speed of airplanes while landing them. By changing the direction in which air leaves the airplane, it allows the airplane to generate thrust in the opposite direction. As a result, the airplane slows down as it is brought to a gradual stop.

How Does Reverse Thrust Work?

Generally, all reverse thrust mechanisms work on the same principle that allows them to change the direction in which air leaves the engine.

However, the rate of the change in the direction of the air leaving the engine depends on the type of airplane.

In smaller airplanes, thrust reversers change the flow of air rapidly and by effect entirely, while lager airplanes experience a partial reversal in the flow of air as a result of a slower rate of change in the direction of air leaving the engine.

Most commercial jets are fitted with flaps on their engines that look like trap doors known as reversal doors. To achieve a change in the direction of airflow, the pilot opens up these reversal doors.

These doors act as guiding vanes in the midsection of the by-pass channel of the turbine and force the accelerating air to exhaust the engine in the opposite direction, therefore explaining why jet engines open when landing. 

Three Types of Thrust Reversal Systems 

Thrust reversal systems can be divided into three, namely:

• Clam-shell: Commonly used in turbofan engines, this system makes use of pressurized gas to activate and deactivate the reversal doors. It is also known as the cascade system.
• Target type reversal system: This type of reversal system adopts bucket type doors that are operated hydraulically to change the direction in which hot gas leaves the engine.
• Cold stream type reversal system: This reversal system type is activated by an air motor and is found on high bypass turbofan engines. In this system, doors are installed in the bypass duct of the airplane to reverse the flow of air that is accelerated by the engine, which does not pass through the combustion chamber. As a result, thrust is generated in the opposite direction.

Summary

Reverse thrust systems are essential for the safe landing of airplanes. Although these systems do not permit them to perform cool drifts like in our sports cars, they are necessary to decelerate airplanes before touch down.

The opening of jet engines during landing allows pilots to engage these thrust reverse mechanisms, in order to bring airplanes to a safe and steady halt.