Thrust Reversers

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.

An Air-India Boeing 747-400 lands on a wet runway- Thrust Reversers deployed (note the small splits in the engine cowling)

Reverse thrust is the temporary diversion of a jet engine's output so that the exhaust gases are blown forwards rather than backwards. The resulting thrust force acts against the forward travel of the aircraft. Reverse thrust is used by many jet aircraft to help slow down just after touch-down, thus reducing wear on the brakes and enabling the aircraft to use shorter runways. It is also available on many propeller-driven aircraft through changing the pitch of the propeller blades to a negative angle, thereby directing air flow into the direction of travel.