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, from a passenger's point of view, there is often little to separate them. 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?

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 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.

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 results in the barking sound, which can seem alarming initially, as if an engine is struggling to operate. It often occurs on the ground, during engine startup and shutdown. The PTU's position by the wing means those sitting towards the middle section of the aircraft are more likely to hear it.

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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 A330 family. However, it will only happen with older models (A320ceo or A330-200, for example), so you won't hear it on an A320neo or A330neo aircraft, which utilize a different PTU.

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. 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 Boeing differ from their Airbus competitors in not emitting such a noise. This is because, despite also featuring a PTU, its hydraulic systems work differently from those of the European manufacturer. This setup specifically consists of two hydraulic systems, as well as a standby.

A Boeing 737 MAX 7 flying in the sky.
Photo: Wirestock Creators | Shutterstock

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 why barking noises are sometimes heard on Airbus planes? Have you ever heard it yourself on your travels? Let us know your thoughts and experiences in the comments!

Source: Ask The Pilot, The Points Guy