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Curious Flyers Want to Know... What the Heck is that Noise!

Posted by Patrick Smith  September 10, 2013 12:24 PM

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Q: 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.

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.

It happens on 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.

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.

Q: Sometimes, while the plane is accelerating for takeoff, there's a repetitive, rhythmic thumping from below: bang-bang-bang-bang, all the way down the runway, like we're hitting a string of potholes. A friend tells me this is an indication of flat spots on the plane's tire, or a tire that isn't inflated properly.

Another good reason to ignore advice from your friends. What you're hearing is the plane's forward landing gear -- its nose tires -- hitting the recessed lights along the runway centerline. These centerline lights are inlaid flush with the pavement, but they're not that flush and almost always you can feel them.

One technique is for the pilot to track a few feet off-center. The takeoff roll is seldom perfectly straight, however -- especially during strong crosswinds -- and so the bumps might start and stop, start and stop.

Q: Why is engine power cut back shortly after taking off? Takeoff is the scariest part of flying to me, and suddenly, only seconds after leaving the ground, it feels like the plane is falling.

Planes routinely use more thrust than is necessary to take off, and the output of the engines is routinely drawn back to what we call "climb thrust" or "climb power" after reaching a thousand feet or so. This saves wear and tear on the engines, reduces noise on the ground, and keeps the jet from overspeeding (there are speed limits, yes, varying with altitude and/or the departure procedure being flown).

The sounds and sensations of this cutback are sometimes quite noticeable, but trust me the plane is not descending, or even decelerating. It's simply not climbing as sharply, and the rate of acceleration is reduced.


These questions, and dozens more, can be found in Patrick Smith's new book.

Book Cover Thumbnail


NOTE: Portions of this Q&A originally were published in the magazine SALON.


This blog is not written or edited by or the Boston Globe.
The author is solely responsible for the content.

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About the author

Patrick Smith is an airline pilot, air travel columnist, author, and host of In his spare time, he has visited more than 80 countries and always asks for a window seat. He lives in Somerville. More »

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