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?

  • $\begingroup$ This is the PTU, see reference in this question: aviation.stackexchange.com/q/137/572 $\endgroup$ Aug 25, 2016 at 8:29
  • 1
    $\begingroup$ 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. $\endgroup$ Aug 25, 2016 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, 2016 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. $\endgroup$ Aug 25, 2016 at 13:26
  • $\begingroup$ What triggers the PTU (Power Transfer Unit)? $\endgroup$
    – mins
    Aug 25, 2016 at 19:30

1 Answer 1


This sounds like the PTU, used to pressurize the hydraulic system when only one engine is running.


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    $\begingroup$ Somebody who doesn't know what the noise is probably isn't going to know what "PTU" stands for... $\endgroup$ Aug 25, 2016 at 8:41

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