Thrust Reversing

No, "back thrust", or thrust reversing as it is more commonly called, is not absolutely necessary after landing. For those unfamiliar with the concept, thrust reversing is the process of running an aircraft engine in such a way that it reverses the direction of the thrust and creates a drag force to oppose the aircraft motion. Thrust reversing is accomplished on propeller aircraft by rotating the propeller blades to produce drag, thereby slowing the aircraft, as it rolls down the runway. On jet aircraft, portions of the engine nozzle usually swivel around the end of the nozzle blocking the exhaust coming out of the engine. Both of these methods typically produce a drag force equal to about 40% to 60% of the thrust.

Conceptual diagram of a thrust reverser
Conceptual diagram of a thrust reverser

When you fly, you've probably noticed that the engines are loudest just after the aircraft lands. This fact is particularly true on planes with rear-mounted engines like the Boeing 727 or MD-80. The reason for this increase in noise is that the thrust reversers have been deployed and are blocking the engine exhaust. This effect causes turbulence in the exhaust flow that creates the increased noise.

Though they are not necessary for landing, the reason thrust reversers are so common is that they significantly reduce the distance needed to land an aircraft. Not only does this mean aircraft can land on shorter runways and at smaller airports, but it also greatly reduces the time needed to stop the aircraft before it can begin its taxi to the gate. The runway is cleared faster for other aircraft to land making the airport happy, the plane gets to the gate faster making the passengers happy, and the aircraft can be turned around for its next flight more quickly making the airline happy.
- answer by Jeff Scott, 7 January 2001

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