Aeronautical Randomness
What’s the loud barking noise on the Airbus A320?
January 25, 2012

Strange airplane noises concern people.  If you fly often, you’ve probably learned the common noises.  Most people recognize the engines spooling up or coming back towards idle.  Landing gear raising and lowering is also pretty discernible.  Very experienced fliers are also familiar with the rumbling and slight shaking associated with the speed brakes.

However, day after day, year after year, I continue to hear comments and questions about the strange noise from the belly of the Airbus A320.  If I’m riding in the back, I eavesdrop on the comments:

“What is that?”


“That’s one loud dog.  Maybe the owner should have sedated him before the flight.”

When I’m saying goodbye to guests at the end of the flight, at least one will ask about the strange noise.

The more enthusiastic people don’t even ask: “Call your mechanic.” they say. “There’s something seriously screwed up below the floor of the jet.”

The noise is “usually” heard briefly as the second engine is being started.  People more commonly notice it after shutdown at the gate.  It really does sound like a large angry dog stuck in a cage.  So, what is it?

The noise comes from the hydraulic system’s Power Transfer Unit (PTU).  That’s all the knowledge you need to impress your fellow passengers at the end of the flight.  But, if you want to know more, keep reading.

The A320 has three separate hydraulic systems which actuate the flight controls and other hydraulic systems on the aircraft.  Airbus, just to be different from Boeing, named them the green, blue, and yellow systems.

The green system is powered by an engine driven hydraulic pump.  The yellow system is powered by the other engine’s pump but can also be powered by an electric backup pump. (It can also be partially pressurized by a hand pump to operate the cargo doors… but, that’s not too relevant to this conversation.)  The blue system is powered by an electric pump… but, not associated with the PTU. (It does, however, have its own unique backup mechanism… again, beyond the scope of this post.)

The PTU is a reversible motor pump located between the green and yellow systems.  Hydraulic fluid from either system can drive the pump to pressurize the other hydraulic system.  Since the fluid drives the pump, the fluids from each system remain isolated.  They do not mix.

Each hydraulic system normally operates at 3000psi.  If the difference between the yellow and green system exceeds 500psi, the system energizes the PTU to pressurize the low system.  In other words, the green or yellow hydraulic pump has failed and is being pressurized from the other system through the PTU.

There’s a whole list of conditions that will inhibit the PTU from running…. but, here’s when you’ll “normally” hear it:

  • When starting the second engine, the PTU runs a self-test.  You will hear it either just before taxi or when we start the second engine on the way to the runway.
  • At the gate, especially if we taxi in on one engine.  During the taxi, the left engine is running and pressurizing the green system.  The yellow system is pressurized by the electric pump. When the pump is switched off at the gate, yellow pressure drops while the green stays pressurized as the left engine spools down. The PTU runs until the systems equalize.

When it runs, that’s when you’ll think there’s a dog trapped below.  Or, if you’re into logging, you’ll think someone is sawing a big tree.  Either way, it is definitely a noise unique to the Airbus.

So, go now… book your next flight on an A320.  When you hear the noise, impress your seatmates.  Or, grin and say “did you see the size of that dog they loaded into the pit?”

Airbus A320 Hydraulic System

Since 2010, I’ve been writing all kinds of articles about life in the airline business.  Feel free to look around The Blog or the Table of Contents and share the material with your friends.  Using the social media buttons below lets me know you were here and enjoyed what you read.

If you have a specific question, feel free to leave a comment or contact me.  I’m always happy to clarify anything about our profession.

About author

Renewed Pilot

I've endured a roller coaster career in the U.S. Aviation Industry. Currently flying the 737 on my third try with the same legacy carrier, I have also flown for a regional, fractional and start-up carrier. My piloting experience includes the 737, A320, 727, Citation Excel, Citation Bravo, Saab 340 and many light singles and twin engine aircraft. I reside in the suburbs of Nashville, Tennessee.

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There are 11 comments

  • David says:

    Here is the best video on youtube I could find of the noise – it really does sound like a barking dog! http://www.youtube.com/watch?v=9ffJgk1-tCA&feature=related

    • That’s it. The other noise you hear in the video is the yellow electric pump. Sounds like they were using the yellow system to pressurize the green system without either engine running.

  • Mick Pope says:

    Always thought it was PTU noise but didn’t understand why it kicked in. As an ex L1011 Mechanic I had heard the noise many a time when servicing the Hydraulic system, as it was the only way to pressurise A and D system when B and C were running on Electrical pumps/ATM’s with engines off. But these pumps and PTU’s were only kicked on in flight if the EDP’s had failed. They were switched on after all three engines had started (by the flight engineer) hence they were never heard in the cabin normally, as normal system pressure had already been achieved. Apparently you could fly the L1011 solely on Electrical pumps with the with the PTU’s engaged. However when we used it on the deck during flying control functional s. It would struggle to maintain system pressure across the two larger systems. So I would not want to test it out in flight, then again you have already lost all your engines by that point so what the hell.

  • Is there a button to make screenshot of LCD screen in aircraft?

  • […] What’s the loud barking noise on the A320? […]

  • Matt Duffin says:

    Thanks for this informative post! I’ve been wondering about this noise for years. I realise this is a comment on an old post, but I have a quick follow-up question…

    On a flight from Spain yesterday, I was seated opposite a Senior First Officer who was being relocated, so I took the opportunity to ask him about this noise.

    After cheekily telling me, “I fly Boeings, so it’s not something I have to listen to,” he told me that it was the sound of an ‘accumulator’.

    Is this the same thing as the PTU of your post, or was he incorrect?

    Many thanks,

    • Hi Matt…

      He was incorrect. An accumulator basically stores hydraulic pressure and a PTU transfers pressure from one system to another.

      The 737 also uses a PTU to assist leading edge devices when the normal system pressure is low. But, that’s not used as frequently as the A320 PTU.

      I have much experience in both Boeing and Airbus aircraft. Don’t ever let anyone trash one type if they’ve never flown it. Both are different, but great machines.

  • Noorul HASSAN Adnan says:

    I am on transition training of A320, want to clear the concept of Flt.Cont. computers ELAC, SEC, and FAC it’s working and changeover logic in case of system failure. Thanks and much appreciated.

  • Alex says:

    What cojos mean PTU sound during flight ? Engine restart or am I wrong ?

    Thank you

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