Resident pilot 121pilot, a captain for a major U.S. airline, authors a new column on Live and Let’s Fly called Ask Your Captain. His mission: demystify the flight deck and an answer any question you may have on the topic of flying.
Q: The Airbus A320 fleet has a fuel pump system that sounds like barking dogs for passengers in the back. Is there a reason why Airbus does this but Boeing doesn’t? Sometimes I hear it after we land too, wouldn’t it be sufficiently evened out after a flight?
A: So that barking dog noise isn’t a fuel pump it’s a hydraulic pump. The Airbus has three independent hydraulic systems (Green, Blue, and Yellow) and each system has two independent methods by which it can be pressurized. The Blue system is pressurized by an electric pump and in an emergency can also be powered by a Ram Air Turbine (RAT) which is a small propellor that drops from the belly of the airplane.
Green and Yellow are normally pressurized by engine-driven pumps though Yellow also has an electric pump for ground operation when the engines are off. The normal backup to these engine-driven pumps is another pump called the Power Transfer Unit or PTU. The PTU automatically turns on when it senses more than 500 psi of differential pressure between the Green and Yellow systems.
Ok great you’re thinking, but what on earth is that barking dog noise? Well, when you shut the engines down at the gate especially if you’ve taxied in on one engine, the PTU naturally sees this differential pressure happening and starts trying to equalize pressure between the two systems until the pressure bleeds off. The noise it makes sounds, as you’ve noted, like a barking dog down there in the cargo hold which is why we in the cockpit also refer to it as the barking dog. If you listen carefully, you’ll also hear it during engine start as the system turns itself on as part of its self-test logic.
Have a question for the captain? E-mail him at ask121pilot at yahoo dot com and you may see your question appear in a future column!