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This question already has an answer here:

I've always wondered what is that noise. It sounds like a barking dog or an air drill. Sometimes you hear 4-5 "shots" but last time I flew the noise went on from before take-off to long after we were in the air.

It looks like they are tightening something. Am I correct?

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marked as duplicate by SentryRaven, David Richerby, Ralph J, Federico, Simon Aug 25 '16 at 9:07

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ This is the PTU, see reference in this question: aviation.stackexchange.com/q/137/572 $\endgroup$ – SentryRaven Aug 25 '16 at 8:29
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    $\begingroup$ Given that "barking dog" is the name everybody uses for this noise, Google would have found the answer for you straight away. Actually, I'm surprised the question we already have on the topic isn't high in the Google rankings. $\endgroup$ – David Richerby Aug 25 '16 at 8:44
  • $\begingroup$ I honestly expected to be told off for calling it "barking dog" :) So is it normal that the aircraft takes off with just one engine on? The answer made me a bit worried. $\endgroup$ – algiogia Aug 25 '16 at 11:53
  • $\begingroup$ No, but it's not uncommon that aircraft taxi with one engine running. Takeoff and landing are are with both engines, hopefully. $\endgroup$ – SentryRaven Aug 25 '16 at 13:26
  • $\begingroup$ What triggers the PTU (Power Transfer Unit)? $\endgroup$ – mins Aug 25 '16 at 19:30
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This sounds like the PTU, used to pressurize the hydraulic system when only one engine is running.

http://www.askthepilot.com/questionanswers/the-barking-dog-airbus-noises/

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    $\begingroup$ Somebody who doesn't know what the noise is probably isn't going to know what "PTU" stands for... $\endgroup$ – David Richerby Aug 25 '16 at 8:41

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