Ask a Pilot with Spencer: What’s That Barking Sound?

What is that barking sound? It's the Power Transfer Unit.

In our Ask a Pilot sequence, pilot Spencer Marker solutions certainly one of your aviation-related questions every week. See previous installments right here and submit your individual to

The query

Hello Spencer, what’s that mechanical sounding noise over the wing of an Airbus? It sounds form of like a barking canine.

—Neil H.

The reply

Hey Neil. Thanks for writing on this week. As a former Airbus pilot, I used to get a lot of questions from passengers about what precisely that noise is. Initially, the noise I assume you’re talking of is that this.

On this video from my buddies at PlaneSpottersBerlin, we hear the “barking canine” very clearly. Whereas this sound could appear uncommon for a lot of vacationers, its presence truly signifies a superbly functioning airplane. The noise originates from the plane’s Energy Switch Unit, or PTU, which is a element of the airplane’s hydraulic system. Let’s check out what precisely the PTU does, and why it makes that noise!

In its easiest kind, hydraulics is the usage of a liquid underneath strain with the intention to exert some mechanical pressure. In your automotive, hydraulic strain is distributed out of your brake pedal to your brake calipers to exert strain in your automotive’s brake rotors, slowing you down as you drive. In aviation, hydraulic strain is employed extensively all through the airplane. All the things from brakes and flaps to the touchdown gear and even the flight controls themselves makes use of hydraulic strain to assist transfer.

Within the first few many years of aviation, a pilot’s flight controls had been moved by a sequence of cables and pulleys woven all through the physique of the airplane. This direct connection was advantageous for lighter and slower shifting plane. Nonetheless, when airplanes grew to become bigger and far quicker, the pressure a pilot needed to exert to fly the plane grew to become too nice. Moreover, with the arrival of retractable touchdown gear, engineers needed to design a option to transfer these massive surfaces in a powerful wind with nominal effort from the flight crew. Hydraulic energy was seen as a option to accomplish each duties.

The primary plane that includes each hydraulically powered touchdown gear and hydraulically aided flight controls was the Lockheed Constellation. This four-engine, pressurized airliner first flew in 1943 and was thought of innovative for its time. The airplane would go on to be the flagship of airways like TWA and Pan Am (which launched the first-ever scheduled round-the-world service, Pan Am Flight 1, with the plane) till the introduction of jet-powered airliners within the following many years.

So how does an plane hydraulic system work? Nice query! An airplane’s hydraulic system works by utilizing a pump, which is most frequently electronically or engine pushed, to create very excessive strain in a fluid. Most trendy plane make use of a hydraulic strain of 3000psi, however some plane use strain as excessive as 5000psi! This strain is used to maneuver an actuator or piston that’s linked to the a part of the airplane the pilot needs to maneuver. Hydraulic actuators might be seen daily in heavy building tools, which permits the operator to maneuver the machine’s bucket or arm by merely shifting a joystick within the cab.

We use this fluid underneath strain to boost and decrease our touchdown gear, transfer our flight controls and steer the airplane on the bottom, and in quite a few different functions massive and small.

So how does the PTU match into the hydraulic system? Effectively, in trendy aviation, redundancy is of utmost significance. This fashion, if a element fails, there are two or three backups to make sure the protected continuation of the flight.

The Airbus A320 employs three separate hydraulic techniques. Every is pressurized by a special supply for redundancy. Two of the techniques are linked by our good friend, the Energy Switch Unit. The PTU hyperlinks the 2 techniques mechanically and is used as a backup in case a major pump fails. The PTU transfers hydraulic strain (not fluid) mechanically to make sure each techniques are correctly pressurized and functioning correctly. The facet impact of performing its job correctly is its signature sound!

So, I think about you’re considering that listening to the PTU in any respect means one thing has failed! That’s not the case. Within the A320, the PTU is examined mechanically whereas we begin the second engine to make sure it’s functioning correctly. This take a look at is what the video above is capturing. Different occasions the PTU might be heard embody throughout touchdown gear retraction or extension, when the load on the hydraulic system is the very best, and when the airplane arrives on the gate and the engines are shut down. In all of those situations, the PTU is working precisely because it ought to.

To sum up
Thanks for the query this week, Neil. Your query actually allowed me to look again on my time flying the A320 and a few of its extra distinctive attributes. To passengers, nothing is extra distinctive than the mechanical music of the PTU, which might be heard so clearly all through the cabin. Whereas the noise could also be disheartening to some, it’s music to my ears. Its mechanical whine tells me and the pilots that fly the Airbus daily that the PTU is functioning correctly, and able to do its job.

Thanks once more for the query! Have you ever heard the well-known Airbus “barking canine”? Share your tales beneath! And if anybody has a burning aviation query or one thing you desire to cleared up, drop us a line at to get your query featured in an upcoming Ask a Pilot column.




Spencer Marker

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