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Video - What is this "barking" noise we occasionally hear on certain Airbus aircraft?

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When an aircraft of the A320 family (A318, A319, A320, A321) is on the ground, you may have heard a kind of noise that sounds like a dog barking, and apparently emanating from the baggage hold. Not to worry, there’s no stowaway pooch here; this very particular, and loud sound (video), is actually generated by the PTU, or Power Transfer Unit, located in the landing gear compartment.

On the A320 family, the flight control systems, nose wheel orientation and braking system are powered hydraulically. In fact, there are three independent hydraulic systems, one green, one yellow and one blue. The green and yellow circuits are pressurized by pumps mounted on the planes engines: engine 1 for the green circuit, engine 2 for the yellow. The blue circuit is a standby or backup circuit.

Whenever an A320 is moved to or from its parking area by a tractor, it starts its engines, one after another. When the first jet engine is operating, the PTU comes into operation automatically.

The PTU, which is a sort of pump, will then ensure the transfer of the hydraulic system pressure from the first engine to the second, to power everything dependant on the hydraulic system (flight control systems, the nose wheel orientation and braking system).

The barking noise we hear, therefore simply indicates that the pressurization of the second engine’s hydraulic circuit is operating. The PTU automatically switches off when the second engine starts. This phenomenon may also occur with the planes arrival on the tarmac when one of the two engines is cut by the pilot.

If this noise can only be heard on the A320 family aircraft, it is simply because the hydraulic system architecture on other aircraft is different. On aircraft such as the A350, each engine is equipped with two pumps (one for the green circuit, and one for the yellow), so there is no need for a Power Transfer Unit. On other aircraft, like the Boeing 737, the PTU is used only in emergency situations; which explains why it is heard rarely.

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