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Passengers on the Airbus A320 might notice a unique "barking" sound during boarding and taxiing. This noise, also heard on the A330 but not on Boeing 737 aircraft, is caused by the Power Transfer Unit (PTU) conducting self-tests to ensure hydraulic system redundancy. The PTU, a critical hydraulic device, maintains a pressure of 3000 psi, transferring power between systems for functions like landing gear operation and flight control surfaces.
The PTU operates automatically by detecting pressure differences between systems. It provides backup power to the Green and Yellow hydraulic systems in the Airbus A320's three-system design: Green, Yellow, and Blue. Each system is pressurized differently; engine-driven pumps pressurize the Green and Yellow systems while an electric pump pressurizes the Blue system.
The "bark" results from rapid valve cycling during PTU self-tests to equalize pressure. Although passengers may find it unsettling, this noise indicates normal operations as the PTU achieves high pressure through quick cycles. The barking occurs mainly after pushback when both engines start or during single-engine taxis if needed for balancing pressure.
