Weird noises you hear on larger planes

Sometimes you hear this before the plane is started up, but almost without fail after they’ve taxied and shut down the engines. This might be specific to the Airbus 319/320’s I tend to fly more often, but the sound sounds almost like a short drilling sound. It happens several times in succession and there doesn’t seem to be anything I can see on the wings that is coordinated with it.

I always thought it might’ve been something with the baggage door but I really don’t know. Again, it sounds like a drilling noise… almost always after they shut down the engines.


(I figure I’d make the topic open ended so that this thread can be reused years from now when someone else has a noise-related question :wink:)

That sound is the PTU self test; the A320 and A330 series are both known for a particularly loud PTU.

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.

The BA 146 is very similar to the Airbus problem. The hydrolics that opperate the flaps is located above the cabin ( 146 is a high wing configuation ) and is very loud when the flaps are retracted after take off and extending for landing.

Air Canada always made a point in the pre-flight to passengers that they should not be alarmed and that the noise was part of the normal operation of the aircraft.

as per your request.

Thanks for the reply. Northwest most certainly does not run the electric pump (as I think more and more about it they do cycle that right before take off usually) while they taxi. I actually be on a 320 next week so I’ll be sure to listen closer.

Thanks again!

Could you shrink the pictures so that we don’t have to navigate the page left and right? I like the tail.

PTU is right on. You can hear the baggage doors also but it’s higher pitched and very fast. No to mention very quit.

I visit this site to help ease my somewhat irrational fear of flying. Within the last year I flew Jet Blue from Long Beach to Las Vegas and that whirring sound scared the bejeezus out of several passengers. The sound occurred repeatedly, and no one from Jet Blue bothered to explain it. Another passenger mentioned the hydraulics, which sounded good to me, especially since the Flight Attendants didn’t seem all that concerned. What made an impression on me was the Attendants knew many people were concerned about the noise, yet they walked right by without saying anything.

If it was during approach or climb out, it was probably the flaps. I’m sure it was some sort of hydraulic actuator, though.

We had just backed up out of the gate and were sitting on the runway waiting to get in line for take-off.

It could be flaps or any other sort of device that’s running then. With a modern plane with so many moving parts, you’re bound to hear some strange things sometimes. They used to bother me when I was younger, but if you go into the flight expecting to hear some strange sounds as part of the flight, it won’t bother you as much.

There are also programs operated by individual airlines that introduce reluctant flyers to the mysteries of flight, including all of the sounds generated by APUs, hydraulic systems, screw actuators, etc.

One of the best known is that offered by Virgin Atlantic, but there are others, including Southwest Airlines (SWA).

On a few occasions, I’ve noticed a humming sound with a regular pattern of “ka-chunk” sounds while taxiing, with it being especially noticeable on the 717 (around rows 18-22). I’ve been told this is the pump pressurizing the cabin. My wife was less than comforted the last time we flew and I tried to explain that to her.

Next time tell her it’s just road kill coyotes. (or in ohio, much more likely to be 'possums!).

Thanks for the suggestion, jreeves… It was in Florida though, so it could have been any number of critters. :smiley:

cfjames gave the answer already. It’s the PTU balancing the hydraulic pressure, and it’s unique to the Airbus 320 family.

The sound from a BAe146/Avro RJ when the flaps are extended is aerodynamic not mechanical.

Oh Florida- gators, or those cockroaches that crawl up the bathtub drain if you don’t leave the plug in place. they’d cause a nice Ka-chunk.

Could even be Snakes on the plane!?

<My humble apologies, but I just couldn’t resist.>