Expert: Malcolm Dickinson - 9/29/2006

Question
Is this happening in the air a few miles before the airport? When standing outside and the airliners are coming in for thier landing the engines definitly rev up and change pitch before reaching the airport.
Thanks for the reply.
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Followup To

Question -
What is happening when during landing I here the engines change pitch and it sounds as if they are revving up?
Thanks,

Answer -
I assume you are referring to jet aircraft like those used by the airlines.

Jet aircraft are equipped with "thrust reversers" which are engaged by the pilot and allow the plane's engine to direct its thrust forward. This helps to slow the airplane down. If you are familiar with motorboats, it is the equivalent of putting the motorboat engine in reverse in order to slow down the boat's forward motion.

As soon as an airplane touches down, the pilot's first priority, safety-wise, is to keep the airplane on the runway, and his second priority is to slow the plane down as quickly as possible. To slow the plane down, he uses the brakes and also engages the thrust reversers. Once the thrust reversers engage, he increases the throttle (revving up the engines) to apply lots of reverse power to slow down as quickly as possible. It is standard operating procedure on all jets to do this. If you happen to be sitting near the engine, you can look out the window and see the thrust reversers engage. They are located at the back of the engine.

On turboprop airplanes, where the propeller can be turned into the opposite (reverse) position, the technique is the same. As soon as the plane is on the ground, the pilot puts the propeller into the reverse position, known as "beta," and gives power to the engine to help slow the plane down.

Hope this helps!
best,
Malcolm

Answer
Your question asked about a change in the pitch of the engine noise during landing. As I mentioned, thrust reversers are engaged after the airplane touches down on the runway.

When you hear airplanes increasing/decreasing their engine power while approaching an airport (prior to landing), that is just the pilot adjusting the power to keep the plane on correct air speed and descent to make a good landing. It is a normal part of flying, just as the driver of a car presses and releases the brake to varying degrees while pulling into a parking space.