Why Do Certain Airbus Planes Emit A ‘Dog Bark’ Noise?

European manufacturer Airbus makes up one half of the industry-dominating Airbus-Boeing duopoly. The two companies’ aircraft compete directly against one another in various markets, and there is often little to separate the two. However, one distinct difference between Airbus and Boeing aircraft is the barking noise that can sometimes be heard on the former’s planes. But why is this the case, and where does it come from?

Airbus A320 Getty
Have you ever heard this strange, dog-like noise? Photo: Getty Images

Where does the noise come from?

The source of certain Airbus planes’ characteristic barking noise is a component known as the Power Transfer Unit (PTU). This part is an element of the aircraft’s hydraulic systems, and it facilitates the switching of power from one system to another in the event of a failure.

A PTU generally consists of a hydraulic pump, which is connected to a hydraulic motor with the help of a shaft. The Airbus A320 features a reversible or ‘bi-directional’ PTU. This setup allows either of its systems to help the other out in the event of a failure or loss of pressure.

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Spirit A321
The Airbus A320 family, including the A321, features a reversible PTU. Photo: Vincenzo Pace | Simple Flying

The barking noise, which sometimes sounds similar to that of a dog, occurs due to the nature in which the PTU functions. Namely, it does so by surging, which causes the component to repeatedly and suddenly spool up and down. This is what results in the barking sound, which can seem alarming at first. It often occurs on the ground, during engine startup and shutdown.

On which aircraft can you hear this noise?

According to Patrick Smith’s Ask The Pilot, the barking tends to occur on Airbus’s twin-engine jets. This would include the members of its A320 and larger A330 families. However, it tends to pertain to older designs. As such, you are unlikely to hear it on an A320neo family member.

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The new Airbus A320neo series has dampers to muffle the barking noise. Photo: Getty Images

The Points Guy reports that this is because Airbus came up with a solution to dampen the noise on its next-generation narrowbody family. It felt the need to do so as initial tests found that the noise was even louder than on previous models, according to Reuters. As such, it fitted the PTU’s pumps with hydraulic dampers near the engines and wing root.

No barking on Boeing aircraft

Aircraft produced by Chicago-based Boeing differ from their Airbus competitors in not emitting such a noise. This is because, despite also featuring a PTU, its hydraulic systems work in a different way to those of the European manufacturer.  This setup specifically consists of two hydraulic systems, as well as a standby.

Boeing 777X
Boeing’s hydraulic setup means that its aircraft do not emit the barking noise found on some Airbus designs. Photo: Getty Images

The PTU has less involvement in the Boeing setup, and will only intervene when the plane is airborne. It is a useful contingency that can provide the hydraulic systems with the necessary extra pressure if one of them drops.

It also powers the hydraulics concerning the aircraft’s slats. Overall, although the barking noise is certainly an alarming sensation upon first hearing it, it does not compromise the aircraft’s safety in any way.

Did you know the reasons for the barking noise sometimes heard on Airbus planes? Have you ever heard it yourself on your travels? Let us know your thoughts and experiences in the comments!


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