A320neo: What does that "screwdriver" sound mean, where does it come from?

Dear all,

What I always wanted to know (but never dared to ask any pilot ;)): after turning on engine 1 and waiting for, say, 30 seconds there is this “screwdriver” sound for like 5 seconds. Also in real airplanes (so it is great that I am also hearing this in FS 2020 :)). Like first a high pitched frequency which quickly descents to lower frequencies, and then followed by short wrrrmm wrrmmm wrrmmm wrmmm (I think 4 times) sounds which sound like someone would close a doort or a hatch with an electric screwdriver.

Where does that sound come from, what does it mean?

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If i recall correctly that is the hydraulic system powering up. It is often described as a “barking” sound.

I got you correct with this…?

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Actually it’s the PTU (Power Transfer Unit), called “the dog”. Cause it barks. The PTU is a system which provides extra pressure to the hydraulic. A bit like a battery. If the main hydraulic circle goes out (failure), the system has to switch to the next hydraulic system. This short moment, when switching, the system has no pressure. There comes the PTU. It provides pressure to the system, while switching. So no rudder etc. is without pressure this short moment. The sound is, when the PTU get’s itself under pressure.

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Ahh, great - so I was about 50% right :smiley:

This should even be the moment when the airlerons start to “live” and raise in their neutral position, right?

The hydraulic circles themselves are normally already under pressure before engine start (at gate). They are fed by an electrical system (run by external power or APU), when engines are not running.

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Hm. Do you know if this is obligatory? I remember from my last trip (in real life) that while already sitting in my seat the ailerons were hanging down; I am quite sure they just raised during push-back/engine start.

Aren’t there cases where the blue system isn’t pressurized before engine startup? Like, when APU Bleed is off? Just curious.

I would assume that depends from airline to airline, from plane to plane (Boeing, Airbus, etc.). Normally you would set the hydraulic under pressure when at gate and all work around the plane is finished. >> This is mandatory cause if the hydraulic has a leak, it could hurt someone around the plane (because of the pressure).

If I’m not mistaken, it uses a turbine powered pump operated by the hydraulic flow in the running system.
The barking noise, I believe, is the relief valve on the system operating to maintain proper pressure
as the pump starts.

This one nicely explains this barking Sound

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Thanks for asking this. I’ve always wondered the same but didn’t know how to describe it. LOL.

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Actually, the correct answer appears right at the top when you either type “a320 barking sound” or “a320 strange noise” into Google.

Can you hear this in the cockpit? I have never heard it but have always wondered if it is included in the sim.

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If I’m not wrong, FBW altered the PTU sound in their mod as it’s mostly been heard from back in the cabin.

At least I’m sure I’ve read that on their Github. Not sure if it’s in the current build as I haven’t listened.

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Thanks! I’ll listen out for it. I love these small touches.

Yes. But then others here in the forum would have missed the opportunity to get to know about this barking sound. Besides, it didn’t even occur to me that this sounded like a dog (while that is the perfect description of it!), so my search with “a320 wrrmm wrmmm wrrrrm” would most likely have miserably failed :wink:

And my assumption is still that there are people (real pilots perhaps even) here in the forum who are happy to help to answer noob questions like mine, too.

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This wasn’t meant to sound rude. I just wanted to encourage people to even type weird sounding questions into Google.

Like “who’s the weird guy in police acedemy?” Boom - Bobcat Goldthwait.

Google got really good in that kind of things. That’s all I wanted to express.

It’s like a little, tiny dog.

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Like, hellhound-tiny.

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When I start the engines in the FBW A320, I don’t hear this sound. Is it there and I’m missing it?