Does anybody know what that loud growling-scraping-groaning plane noise is?

I've noticed this off and on with US Air and more recently with Spirit.

Right before they pull back to take off, there is this loud sound, kind of a "GRRRRrrrrrroooowwwwwwww-wow-wow" type of sound that occurs. It almost sounds like something is straining to do something and is not getting it done.

Anyone else notice this? Anybody know what it is?

TIA.

 

Not long ago on a Delta flight leaving Vegas the FA gave an announcement telling us it would happen. Cannot remember the reason though.

 

I only know this because I read a lot of aircraft forums, but the sound you hear is the hydraulic transfer mechanism that is running before the second engine is started. Most airlines these days only use one engine to taxi the aircraft to save fuel, so you'll hear this sound whenever only one engine is powering the aircraft.

Basically the mechanism transfers the hydraulic power to the other side of the aircraft so that the pilots can operate the controls on that side. With only one engine running, they would only have hydraulic power on one side of the aircraft. This mechanism allows both sides of the aircraft controls to receive power from just the one engine.

The sound will go away before you get to the runway because the second engine will have been started sometime between pushback from the gate and arrival at the takeoff runway. With the second engine producing its own power for that side of the aircraft, the transfer mechanism is no longer needed.

Most likely you were on an Airbus A320 or one of the aircraft in that family. They have done this forever, and it's actually a "good" sound to hear in the sense that the hydraulic systems are working as designed.

I hope this answers your question.

 

As pointed out, it is the hydraulic system's power transfer unit. Here is an article about it.

http://renewedpilot.com/2012/01/25/whats-the-loud-barking-noise-on-the-airbus-a320/

 

Thanks, gang.

 

I just thought it was the sound of the engines struggling...

 

Ha! Me Too! Great answer Hillwood!

 

It's the flux capacitor charging up.

 

NO, NO, NO, You don't know what you're talking about! It's the dilithium crystals getting the engines ready for warp drive.

 

Here is what it is, really:

http://www.tv.com/shows/the-twilight-zone/watch/nightmare-at-20000-feet-12708/

 

I always thought it was the wheels retracting ?

 

Aye, Captain, I'm givin' ya all she's got!

 

It's that invisible monster thing from the TV Series "Lost". Be careful, your plane may divert off the west coast and end up on a deserted island.

 

I thought this also. Always sounds and seems like something's gonna break.

 

I just asked a buddy who has the misfortune to fly for Ryanair in Europe and he said if the PTU is going at rotate (when they pull back) you should't be taking off! It should never have a loss of pressure when both engines are spooled up.

He also said it could be one of a load of things depending on the type of aircraft as they all make certain distinct ones. Do you know what type of aircraft it is?