Reverse Thrusters

Have you ever sat by the window over the wing of an airplane, and seen the engine do some crazy transformer action when the plane lands, accompanied by deafening sounds and vibration? That's the reverse thruster braking the aircraft. In modern turbofan engines, 80-90% of the thrust is generated by the cool air bypass, not the compressor exhaust. Thus the bypass thurst can be deflected in any direction and the compressor will continue to rotate the engine. There are various configurations of reverse thrusters.


The petal reverse thruster shown above is used in the CFM56-5B engine, which I've seen on United's A319 and A320 aircraft. Here, a mechanical linkage blocks the cool air path while four doors surrounding the rear engine cowl fold open toward the front, allowing the air to exit at a 45 degree angle toward the front of the engine. When open, the engine resembles a flower hence the name.
The reverser above is called the cascade reverser. In this configuration, the whole rear engine cowl slides back, and vaned outlets similar to the vents in your car ventilation system direct the diverted air toward the front of the engine. This configuration is used on all modern Boeing aircraft, with engines such as the CFM56-7, the Rolls Royce RB211-535, as well as Pratt & Whitney engines.

There are other categories of reversers. The bucket, or target reverser, is similar to the petal reverser where vanes open at the rear of the engine and divert the cool air bypass. I believe, but am not sure, that the clamshell reverser is similar except it is situated at the very rear of the engine to divert both the cool air and compressor thrust. In modern high-bypass engines, since the bypass ratio can be 12:1, vectoring the compressor exhaust is not necessary.

I never knew engines could do this until this past weekend when I was watching a 737-300 being serviced. I spend airplane landings with my eyes shut, and the loud rumble of the reversers has always been just another noise death is making at the door. Comfortingly, all airplanes need to be able to land without the use of any reversers. The reverse thrust simply saves wear and tear on the landing gear and brakes. For fun, you can calculate the landing distance of a Boeing 737 here.

It might scare you that the engine keeping your plane up has a "backward" setting. Indeed, several accidents have been attributed to engine reverser malfunction. In 1978, reverse thruster malfunction on landing caused this accident. In 1991, in-flight deployment of one engine reverser caused this crash. Most recently, the Southwest Airlines accident where the plane skidded off the runway has been speculated to be caused by reversers failing to deploy.


All in all airplanes are pretty cool, and I fly them a lot, but they scare the hell out of me. For more information, read up on Turbine Basics and Aircraft Engineering, as well as the forums at Airliners.net.