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When you’re flying on a commercial jet, you naturally become very familiar with the sound and feeling of the powerful turbine engines. As you take off, you can feel their rumbling muscle rocketing the whole vehicle forward and up into the sky. When it’s time for the plane to land, you might expect that rumbling feeling to simply stop, the turbines deactivated. However, instead of that, you feel the distinct sensation of force being pressed against you, as though they were firing in reverse as the plane comes to a stop.

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Contrary to popular belief, this feeling of force is not the turbines spinning backwards. In fact, jet turbines can’t actually do that, or at least there’s no reason for them to do that. No, that feeling is the result of a special component of the jet engines aptly named the thrust reversers. It’s thanks to the thrust reversers that commercial jets can slow to a safe stop on the tarmac in a reasonable amount of time.

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Since a turbine engine generates force through rotation, you would expect simply spinning it the other way would create the reverse thrust a jet needs to slow down. The problem is that the internal components of a jet engine weigh thousands of tons and rotate extremely fast. It would take ages to manually slow their forward rotation to a stop, to say nothing of spinning the other way. This is where the thrust reversers come into play.

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Thrust reversers are small ports located on the outside of the engine, which open up into the engine’s outer layer. That outer layer normally pulls in cool air, which helps to spin up the turbofan and keep the internal fuel-combustion chamber of the engine a bit cooler. When the thrust reverser ports open, the cool air being pulled in from the rotation of the turbine begins shooting out in the opposite direction, generating drag. Basically, when it’s time for the plane to slow to a stop, the thrust reversers use the very air the turbine is pulling in to equalize its rotation and slow its forward thrust. It’s a deceptively simple mechanic, but one that’s vital for safe landings, as it lessens the strain placed on the landing gear wheels.

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The thrust reverser we’ve described here is known as a “cold stream reverser” because the ports open into the cold chamber of the engine. However, there are a couple of different configurations of thrust reverser that all do the same thing, but in slightly different ways.

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For example, in the earlier days of passenger aircraft, clamshell reversers were very common. This configuration had small, hinged doors that covered up the back of the engine exhaust to redirect the exhaust forward. There are also cascade thrust reversers, which use blocker doors that extend outwards, venting the exhaust forward through cascade veins. Some thrust reversers also open directly into the combustion chamber of the engine, rather than the cold layer. It’s the same effect, just with hot air instead of cool air.

Incidentally, if you were worried about thrust reversers opening in mid-flight, don’t. While the plane is in flight with its landing wheels tucked in, the thrust reversers are locked in place by powerful hydraulics. As long as the wheels are in, they literally can’t open. The only kinds of planes that can use thrust reversers mid-flight are military-grade aircraft, which may use them for high-flying, speedy landings in dangerous territory.

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