If you don’t fly very often, there will be plenty you see and hear that you might not understand. We’ve gone over some of them in the past:
- The secret code words and phrases airline crew use during the flight (including the ones they never tell you about)
- What that white vapor is that you sometimes see on planes
- What happens when your plane flies with a broken APU
There are also all different kinds of sounds that happen on a plane when you’re flying through the air. Most people who fly don’t know what the vast majority of the sounds are (save for, say, announcements over the PA), but that’s OK because most don’t sound particularly scary. But there’s always been one sound that I have no idea of what it is. It almost sounds like a barking dog, or maybe something whirring or grinding? Or almost like if you were pumping a noisy set of brakes. But it also sort of sounds like a motor that’s trying to start, but failing to do so…and that part always makes me a little leery of what’s going on.
I can also say it always sounds like it’s below us – so not on the wings or something like that? And it’s never a continuous sound – you usually hear it as the plane gets ready to leave the gate, or sometimes when it’s on its way to the runway. And yet other times, you never hear it at all.
So I started searching, and lo and behold, I found a brief explanation of the sound:
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OK – he mentions hydraulics. So with that keyword, I started Googling. Here’s what I discovered:
The barking/on-off motor sound comes from a device called a power transfer unit (PTU for short, not to be confused with the APU). The PTU ensures that there’s adequate hydraulic pressure during single-engine operations (when piloting a 2 engine plane, pilots will often taxi with one engine shut down because it conserves fuel).
Each engine typically pressurizes its own hydraulic system, but when one motor isn’t running, one system doesn’t have a power source. That’s where the PTU comes in – it helps move left power to the right or right power to the left (depending on which engine has been temporarily turned off).
The PTU is activated only when the pressure falls below a certain level. And since the pressure changes constantly, the PTU continually cycles on and off, on and off, on and off. And that’s why it sounds like a barking dog.
Oh, and the reason you sometimes don’t hear the sound at all? That’s because it only happens with the Airbus A320 family of aircraft (that would include A318, A319, A320 and A321 planes).
Some Boeing aircraft apparently have a PTU as well, but the way their system works is different from the A320 system, so it doesn’t make the same “barking” noise.
So it’s a normal sound and nothing to be concerned about – even though it SOUNDS like it should be!
H/T: Simple Flying, Pilot George, Daily Mail, et al
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