What's that change in noise? on the B737
 

helloo,

i was wondering if anyone could help me

Imagine that the APU is running on the B737 (the pilots have just switched it on, as they will be starting the engines within the next five minutes, let's say) Imagine the noise in your head of the apu running.....

Now it's either when the APU bleed is switched on, or when the APU bleed is already on and then both packs are positioned into the off position that there is the change in noise.... ?? it's kind of like a noise of shooting air.. it's like the noise of the apu has changed...

does anybody get what i'm on about?


thanks

Hmm, I suppose its the airflow towards the packs from the APU? I'm not really getting you I think ;) Can you specify the circumstances a bit more? :ok: When the packs go offline for engine start it can only really get more quiet in the cabin... The isolation valve will open when any pack or engine bleed valve is closed, that might create a noise of sorts? (I'm talking 737-3/4/5 here, not sure about the NGs)

I think the noise you are referring to is due to the operation of the APU surge control valve, dumping the excess air from the APU load compressor to ambient, as the bleed demand has changed through selecting the air conditioning packs to off.

IAW

I don't know the 737, but a lot of APU's are not running at 100%, except during engine start. The A300 had this noise change also, normally turning at around 97%, but when the aircraft got inputs for an engine start (as you said through pack shutoff or so) the APU revved up to 100%.
This together with a change in surge valve and load compressor vanes can cause that noise change. Generally the APU making sure it can deliver the air for a start.

I suppose the sound you are referring to is the excess air being dumped overboard by the surge control valve. It happens when the pneumatic demand goes from high to low. I.E: air con packs off prior to eng start, eng start completed.
It could also be the APU air inlet taking in more air when the pneumatic demand goes from low to high. But I doubt so, as the former's "shooting air" noise is more pronounced.

If the surge control valve is not working, you will hear a more startling sound of "gun shots". It will help if you could pinpoint exactly when you heard the sound.

That is a misconception. Most places, the APU will be started after landing, before reaching the stand, and will run throughout the turnround to provide both electric power and air for the air con packs. A few airports have strict rules requiring use of ground electric power to minimise noise and emissions. Just before engine start, the air con packs will be turned off so that all the APU air supply can be directed to each engine starter in turn. This will produce a change in the air noise in the cabin - and in the flight deck. Is this what you hear?

i'm trying to find videos of this "noise change" but i'm having no luck :(
maybe you could help?

that's the sound of the apu running before the 'noise change' that I had in mind
YouTube - Boarding passengers Boeing737 Ryanair airport of Dinard