If you’re an aviation enthusiast like the rest of us or you regularly pay attention to what’s happening during your flight, you may have noticed that the Airbus A320 produces a very audible “barking noise” while on the ground. Most of the time it’s heard prior to or during taxi departing from the airport. Those of you who have noticed it can probably agree with me in that it’s a very distinct noise, most similar to a dog barking but much more mechanical, like a motor failing to start. The noise comes from something called the power transfer unit, or PTU for short. What the PTU does is that it transfers power (hence the name) between the different hydraulic systems so that each individual system has an appropriate amount of pressure. What people normally find strange is that this PTU noise only heard on A320 aircraft, never on the Boeing 737 or Airbus A340. This is because the A320 has a unique hydraulic system that isn’t found elsewhere. The exact technical details of the A320’s hydraulics system is definitely beyond my knowledge and probably beyond your interest, but the A320 has three different hydraulics systems whereas the Boeing 737 only has two, and therefore the A320 needs to transfer pressure between its three systems more frequently than other aircraft and therefore it requires a more powerful PTU. And typically with heavy machinery the more powerful it is the noisier it will be, which is why the PTU on the A320 is audible from inside the cabin. The PTU activates mainly after Engine 2 (starboard) has been started, and since the A320 somtimes taxi’s on one engine to save fuel, the PTU can also be heard during taxi prior to approaching the runway when pilots would start the remaining engine in preparation for takeoff. Next time you’re flying on an A320, be sure to look out for the sound of PTU if you haven’t already, it’s definitely hard to miss and very cool once you know what you’re listening to.
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Article written by a real-world pilot: http://www.askthepilot.com/questionanswers/the-barking-dog-airbus-noises/