A pilot commuting to work one night had the misfortune of sitting next to me.
This gentleman flies the single-aisle Airbus series of aircraft. He (very patiently) answered all my nerdy questions about aviation and the airline industry during our short trip.
“Woof-Woof!” The Barking Dogs of Airbus
One of my questions pertained to certain sounds perhaps you’re familiar with. I told him I usually notice these odds sounds when one of my Airbus flights parks at a gate upon arrival. I compare them to air compressor guns rapidly change tires during auto races. They almost sound like the ramp crew is using a tool to open the luggage hold.
Some people find the sounds unnerving. And I think that’s perfectly understandable. But it turns out they’re harmless — and even have an affectionate nickname.
“You’re talking about the ‘barking dogs,'” my pilot seatmate said with a chuckle. He explained the sounds emanate from the power transfer unit (or “PTU”). Basically, it deals with hydraulic pressure.
Blogger, author, and airline pilot Patrick Smith does a nice job of putting into English the “barking dogs” noise:
Crews rarely make efforts to explain (the noise), leaving passengers befuddled and sometimes worried. Because the noise is akin to a motor repeatedly trying—and failing—to start, there’s often the assumption that something is malfunctioning.
What you hear is a device called the power transfer unit, or PTU, which is designed to ensure adequate hydraulic pressures during single-engine operations. To conserve fuel, it’s fairly routine for two-engine planes to taxi with an engine shut down. Each engine normally pressurizes its own hydraulic system, but with a motor not running, that leaves one system without a power source. That’s where the PTU comes in, helping left power the right, or right power the left. Since it is activated only when the pressure falls below a certain level, the PTU cycles on and off, on and off, on and off. Due to pressure fluctuations, the noise will sometimes continue even after both engines are up and running.
There are a couple of YouTube videos that explain the Airbus “barking dogs.” The first is from Simple Flying.
The second is from Captain Joe. (He wears only three stripes — but “Captain Joe” has a better ring than “First Officer Joe.” And he seems like a good guy, we’ll give it to him 😉 ). It’s very detailed; as my friend Larry would say, “it’s a little inside baseball.”
Those weird, whining, and sometimes unnerving sounds you hear on Airbus planes are nothing to worry about. They’re simply the Airbus “barking dogs.” They’re part of the plane’s hydraulic system. Best of all: you don’t need to give them any treats.
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I remember the first time I heard it, saying to the person next to me that it sounded like there were dogs underneath! Thanks for the explanation.