The barking dog on Airbus 320 ✈️ location The Airbus A320 Power Transfer Unit (PTU) is a mechanical device that transfers hydraulic power, not fluid, between the aircraft's green and yellow hydraulic systems to maintain pressure when one system is low, preventing fluid transfer and creating the famous "barking dog" sound, especially on the ground when one engine is running and the other system is idle. It activates when the pressure difference between systems exceeds about 500 PSI, ensuring back