The Airbus A320 is one of the most popular narrow-body jets ever to fly. With more than 5,000 built and another 8000 orders, there is a high likelihood that you’ve either flown on one before or will fly on one soon. It’s a nice ride. The jet is wider than the 737. For pilots, the plane is also fly by wire, has a side stick instead of a yoke and even a fold-out tray table in each crew position. There is one very peculiar thing about the jet. Before takeoff when the engines are starting and after landing (typically at the gate), the jet makes an odd barking noise. Why is that?
Yep, it sounds like a barking dog
For the first time traveler, they might think that their jet is carrying a pack of race dogs in the cargo compartment below. This video above from DAR 4K does a nice job of capturing the noise. We can assure you. It not a dog (or pack of them). It’s actually a noise made by the PTU or power transfer unit.
The PTU transfers power from one hydraulic unit to another. In the A320, there are actually three hydraulic systems. Two hydraulic systems are powered by each engine with a third system powered by an electric backup pump. The PTU provides hydraulic power across the systems. The hydraulics are used in flight to power the control surfaces, landing gear deployment and retraction, nose wheel steering, breaking system on the ground and movement of the flaps and slats. The system’s power is shared but fluid is not. This setup ensures that even with an engine off, you still have full hydraulics available.
The reason that you hear this sound when departing the gate is because typically only one system is running. After arrival, typically one engine is shut down. That means that the system is utilizing the PTU to ensure that the entire hydraulic system stays pressurized.
It’s a great system that works and ensures that pilots have access to all controls at all times. But it can be a bit annoying, particularly if your are sitting in rows 15-20!