What are the "barking sounds" on an A320?

If you have ever traveled on an Airbus A320 aircraft, you must have heard a very peculiar sound before takeoff or after landing, a sound that is associated with a dog barking, and which seems to announce that something is wrong with the aircraft. Well, there's no need to be scared, if you hear that noise it means that the PTU is working. But what is PTU and how does it work?

Well, PTU is the acronym for Power Transfer Unit, and this system is part of the aircraft's hydraulic system, but to understand how the PTU works, we need to understand how the Airbus A320's hydraulic system works.

Audio from inside an A320 during Taxi and the "barking sound."

The Hydraulic System

There are 3 hydraulic systems completely independent of each other inside an Airbus A320; each system is identified with a color; we have the Blue, Yellow, and Green systems. Each system is pressurized separately from the other. The Blue system uses an electrical pump to pressurize its system. The Green and Yellow systems, which are the most used systems on the aircraft, use pumps located in each aircraft engine to pressurize their systems. Each pump's function is to pressurize each system up to approximately 3000 psi and maintain constant pressure to ensure the functioning of the hydraulic systems.

PTU operation and the Barking Sound

The PTU is a system located between the Green and Yellow systems, and hydraulic oil from either system can rotate the pump to pressurize the other side in the event of a failure, and the 2 systems never mix. It is activated when one of the systems has a pressurization failure and there is a pressure difference of 500 psi between systems. At this moment, the PTU comes into action automatically, transferring hydraulic power from the operating system to pressurize the lower pressure system, maintaining a pressure of 3000 psi in both systems. The PTU also facilitates the smooth and efficient operation of the landing gear mechanism. It ensures that the landing gear can be safely extended or retracted during takeoff and landing, contributing to the overall safety and performance of the aircraft.

The sound we usually hear that resembles a dog barking is the PTU operating and balancing the hydraulic pressures between systems. As the system is operating in a vacuum, it causes this unique noise.