I had written here that I thought it might be the flap motors, but then I looked up exactly what a PTU was, and found this:
I frequently ride aboard Airbus planes. During taxi, either prior to takeoff or after landing, it’s common to hear a loud whirring sound emanating from beneath the floorboards. It’s a robotic “WHOOF, WHOOF, WHOOF” – like the noise a very angry and deranged Great Dane might make.
This is a very common query, and pertains to later-generation, twin-engine Airbus models – namely the A320 series (includes the sub-variants A318, A319 and A321) and the much larger A330. Almost every frequent flier 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. It also does a self-test when the right engine is started, so you’d hear it then as well, regardless of hydraulic pressures.
That’s the short answer, which ought to appease the typical nervous or curious rider. For gearheads, or those wishing to impress (or bore) their seatmates, here’s a description by correspondent Dave English, Airbus A320 pilot and aviation writer:
“The A320 has two main hydraulic systems that operate flaps, landing gear, flight controls, cargo doors and brakes. These are called Green and Yellow. This has nothing to do with the color of the hydraulic fluids; they’re just useful labels. The Green system is powered by the number 1 (left) engine-driven pump. The Yellow system is powered by the number 2 (right) engine-driven pump. Should we lose or depower an engine-driven pump, there is a neat device called PTU – a two-way reversible motor pump that allows the Green system to pressurize Yellow, or Yellow to pressurize Green. When you taxi with only one engine running, the PTU kicks in to power the opposite side. There is no fluid transfer, keeping each system otherwise independent.”
English points out that the PTU racket is not audible from the cockpit. “But,” he admits, “it’s annoying as heck in the main cabin.” It’s anybody’s guess why Airbus didn’t come up with quieter machinery, and most non-Airbus aircraft employ a different layout altogether.
In the United States, the largest operators of A320s and/or A330s are JetBlue, Northwest, U.S. Airways and United. To reduce PTU noise, some airlines ask crews to run a separate electric pump during single-engine taxi.
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