PTU — Power Transfer Unit — makes a loud, rapid “woof-woof-woof” / barking noise when it operates. Crews and ramp workers started calling it the “barking dog” long before it ever made it into training slang. Here’s what’s going on: What the PTU does The PTU connects two hydraulic systems (e.g., Green and Yellow on Airbus) It transfers power, not fluid If one system has pressure and the other doesn’t, the PTU kicks in to balance things Why it sounds like a dog The PTU is a hydromechanical device