Noise while boarding?

Hello, this probably sounds like a dumb question but…

So you know when you’re boarding a flight and way before the engines are started you can hear white noise. Is that just the air-conditioning or is it something different?

I know it’s a dumb question but I’m trying to get it in IF. Also I didn’t know whether to out this in general or RWA so I just chose general. Thanks!


I think it is the APU maybe not entirely sure tho


Hello! I don’t think it is that but if you want a boarding noise you can get IF Pax on Ios and Android!


I have it but I’m thinking I could play aircon sounds on loop in iTunes? As well as IFpax, and inflight assistant could have the ultimate realism 🤣


It’s not a certain aircraft. Just in general.

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Oh wait, you mean to get it in IF…

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Watch this. That sort of engine noise.

I’m more curious to what it is. Cause once I know what it is. Replicating it in IF is easy :)

It is the APU. Since the engines aren’t powering the plane.


@Infinite_Flight_Dude is correct, it is the APU. And when your in flight movie or music gets paused for a quick while, I am pretty sure it is the crew switching off of ground power.

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Oh ok now I need to find a sound of one :D Thanks!

Its APU. (auxiliary power unit) its a miniature version of the actual jet engine fitted at the back of the aircraft. its main functions are to provide electric power to aircraft systems and generate bleed air for air conditioner and main engine start. its shut down once the main engines are up and running and turned on again after landing.


Wish those systems were in infinite flight…

If you’re on about real life it’s something to do with Hydraulics

You can vote for this here:


No I wouldn’t be so sure that it would be the APU. A few weeks ago I was on a flight on a B777-300ER and whilst the boarding was finishing all the lights and the noise went out. A PA has been made telling us that the external power supply from a generator in the airport (LFPG) failed.

The APU is usually turned on shortly before pushback but not all the time during the boarding so I think the noise you are talking about is mostly coming from the air-conditioning system.

Edit: @C150_driver made a very detailed and instructive answer to your question below :)

@A-FitzGerald that depends on airlines. in apu or using AGPU. that is auxiliary ground power unit. even if its ground power they make the same noise as mechanism is same. they have a jet engine in it.

Humming sound: the sound of the APU generator OR the GPU. Electrical systems on aircraft operate at 400 Hz. I tested this one day to confirm during work on the ramp with a scope on my phone. As long as the plane has electrical power, which is pretty much as long as the door is open (cuz what would anyone do on a dark plane? Lol), you’ll hear this. Battery won’t be switched on unless there is standby power available, otherwise you’re discharging the battery with no trade off.

Hissing sound/whir: Recirculation fans. Only works as long as the plane has electrical systems running. Usually used in conjunction with the yellow tube we hook up to the fuselage. Make sure to put the probe on the jetway inside the cabin somewhere otherwise you’ll fault the AC unit and the tube will blow unconditioned air which feels miserable lol.

As long as either the BATT, APU GEN, or EXT PWR is on, you’ll hear this stuff. Once the yellow tube is disconnected, you’re looking for the APU BLEED and PACKS to be switched to ON to provide cool or warm air for the cabin.

One last thing… louder humming sound, usually closer to departure time: that’s the hydraulic system being actuated by the ground crew (or pilots up front), given they have to switch on an auxiliary hydraulic system, such as in the MD-80. The Airbus also sometimes uses an AUX HYD system when at the gate, and it will make a woo-woo sound. I think MX usually is the one using it. Note, this is NOT the PTU I’m talking about, which makes that characteristic barking sound.

Source: I work around and on Airbuses and MD-80s about 35-40 hours per week.


Does it sound almost like a barking dog?

yes the ptu kicking in will make a barking sound. you’re most likely to hear it in the a320 when sitting in front rows. right after pushback.