Sawing Off The Wings: What’s That Sound Beneath The Plane?

As I was sitting patiently in Long Beach waiting to exit the plane and listening to that weird sawing sound that I hear every time I fly on an Airbus A320, I was wondering two things:



1) Why does it sound like Paul Bunyon is attacking my plane with a drywall saw, and



2) Who is going to be the first person to make some stupid joke, like “Oh no, Paul Bunyon is attacking our plane with a drywall saw!” Would it be the young woman to my right in 20D — we’ll call her “Cathy” — with whom I was fiercely trying not to make eye contact for the duration of the flight, for fear that I would be not only subjected to the inordinate flow of words streaming from her mouth, but also socially obliged to smile and nod at the relevant pauses in her soliloquy? Or would it be the entrepreneur five rows back in 25D with whom I—by virtue of sitting within five rows of him—was forcefully included as an eavesdropper in his conversation in which he reminded the listeners several times that, for a guy like him, $2 million dollars was not a lot of money?



I was betting it would be him. But it turns out I was wrong. It was the guy in 16C who looked far too discerning to make such an unbecoming joke. I was a little mad that it was 16C. I really wanted it to be 25D; I wanted to say silently to myself—in a cold, measured voice as if I were discovering the identity of Batman for the first time—“I knew it would be you,” when the ostentatiously affluent entrepreneur took it upon himself to be “that guy.”



By the way, $2 million dollars is not a big deal for 25D. So in an attempt to rid Airbusses everywhere from the same overused charades in which the actor comically drives his hand forward-and-back in time with the sawing sound, and in which he thinks he is the first passenger ever to do this, I called Airbus to find out what exactly was happening to cause this sound.



(Don’t worry, it’s a perfectly normal sound).



The plane has a system called the “Power Transfer Unit” (PTU) that maintains hydraulic pressure in the two primary hydraulic systems. The sound, it turns out, is unique to the Airbus A320 family of planes. Airlines all over the world—including British Airways, easyJet, jetBlue, Lufthansa, and US Airways—all use this plane. So chances are you’ve heard this sound, and chances are you’ve heard the same jokes that I have…maybe even in German or British! Banter!



The first woman I spoke to at Airbus, who sounded like she was working in a 1950’s office cooled with a dusty chrome oscillating desk fan somewhere along Route 66 in Texas, was noticeably alarmed by my question. Ironic, considering Airbus is a European aerospace consortium that is a world leader in a pretty technical field of engineering.



“Someone was trying to saw the wings off!?" she asked with dismay as if I had just told her that the Mayan calender was in fact correct and the the world would end in 2012. "Did you report it to the captain? Maybe you should report it to the airline!"



I could almost see the concern and confusion in her face over the phone. “No,” I kindly clarified, “It just sounded like they were trying to saw the wings off. I think it is just a normal sound and I’m just trying to find out what it really is.” She transferred me to a man in another office far away from Route 66, who was more than happy to give me a detailed explanation: "It typically happens when you’re doing a single-engine taxi, because there is only one hydraulic pump running on one engine, but it has to power both sides. You’ll also hear it if you’re powering things like the landing gear on the approach, since the landing gear is powered by only one hydraulic pump."



When you hear the sawing, the PTU is cycling to make sure that there is equal pressure on either side. Since the landing gear is powered by one hydraulic system, and the other one is sitting idle, the pressure will drop on one, triggering the PTU to go on and pressurize both systems. If you’re sitting over the wing, it’s the loudest, because the PTU is right underneath the wingbox.



So now I’ll wait with eager anticipation to give my calculated yet cheeky retort to the first person who thinks about making a joke about the maintenance crew sawing off the wings of the A320: “Actually the sound, unique to the Airbus A320 family of planes including not only the A320, but also the 318, 319, and 321, emanates from the Power Transfer Unit, a system which equalizes the hydraulic pressure between each of the two primary hydraulic units…but good joke! I’ve never heard that one before!”



Flying sometimes puts me in a bad mood.