It’s common to hear a loud, repetitive whirring sound emanating from the floorboards of Airbus planes. Sometimes it’s a high-pitched whine; other times it’s a stilted WOOF, WOOF, WOOF, like the noise a very agitated dog might make.
This pertains to twin-engine Airbus models: the A320 series (includes the subvariants A319 and A321) and the larger A330. In the United States, the largest operators of these types are Delta, United, JetBlue, and US Airways. Almost every frequent flyer has encountered this sound at one time or another. Crews rarely make efforts to explain it, leaving passengers befuddled and sometimes worried. Because the noise is akin to a motor repeatedly trying—and failing—to start, there’s often the assumption that something is malfunctioning.
What you hear is a device called the power transfer unit, or PTU, which is designed to ensure adequate hydraulic pressures during single-engine operations. To conserve fuel, it’s fairly routine for two-engine planes to taxi with an engine shut down. Each engine normally pressurizes its own hydraulic system, but with a motor not running, that leaves one system without a power source. That’s where the PTU comes in, helping left power the right, or right power the left. Since it is activated only when the pressure falls below a certain level, the PTU cycles on and off, on and off, on and off. Due to pressure fluctuations, the noise will sometimes continue even after both engines are up and running. It also does a self-test when the starboard engine is started, so you’ll hear it then as well. Some Boeing aircraft also employ a PTU, but the operation is slightly different and it doesn’t bark like a dog.
Another noise peculiar to Airbus models is a shrill, prolonged whine heard at the gate prior to departure and again after landing. This is an electric hydraulic pump used to open and close the cargo doors.