A320 Avionics ventilation
 

This is probably the most confusing thing about the A320 in my view - challenging the very essence of logical thought. It strikes me that very simply, you can boil it down to this...


If you press either the Blower or Extract buttons, you send the system to closed config (outside valves close up and circulation air is cooled against the skin), and AIR CON air is now used (in additon) to help cool the avionics.


That being the case (and at the risk of asking a stupid question), why isn't AIR CON air ALWAYS used, if available?

If that's the most confusing thing about the A320 to you, you're doing pretty good I'd say, but I digress...

Are you basically asking why there is not an automatic logic to put the system in closed configuration as soon as the PACKs come online or am I misunderstanding you?

Fair point but I can think of at least two possible explanations.

Performance wise I think it's always a goal to use as little bleed air as possible i.e. lower demand on the packs. Therefore the less conditioned air that needs to be supplied to any system, the better SFC.

Maybe redundancy and system separation has been another factor in the design? Smoke QRH drill comes to mind where you can alternate between normal cooling and alternate aircond cooling by overriding fans (as you mention yourself). It is easier to identify aircond/equipment smoke if systems are separate (e.g. cabin crew senses smoke, but not visible in cockpit will indicate aircond affected)

A quick look at the Boeing suggests (if I read correctly) that indeed aircond air is used to cool the equipment. Any Boeing-thoughts on why the different designs?

Hi fidelio,
If the skin was in bright sunlight & with no breeze, you would end up warming the circulation air.

Everything about the A320 design was optimised to make the aircraft economical to operate so minimising any extra air demand would help with the overall economy.
The avionics vent system is over engineered but think of the heat produced by all of those 1980s era computer boxes and also the basic flight control of the aircraft is totally reliant upon them.
I was told by an old avionics engineer that one of the early operators had an avionics vent problem in flight and ended up with only one of the fbw computers operating.

Ok thanks for all the thoughts guys... Some in retesting ideas. Indeed, it must predominantly be for performance/economy.

Just to be clear then, as long as we have the APU bleed on for the turnaround (ie aircon system on) then there should be no problem for avionics cooling turning the extract to override (to save our hearing), as long as it doesn't breach limits in the MEL (max 1h up to 38 degrees which, let's face it, is going to cover you for Europe).

Sure... Which is why there is a temp and time limit in the Mel.

Are you basically asking why there is not an automatic logic to put the system in cl
 

Yes I was, but your answers have clarified things for me thanks

A320s produced in the last year or so have a modified system that keeps the cooling in closed loop on the gate for much longer than the older ones. Here in Stockholm it stays closed all the time! There is a temp sensor in the duct that goes to open loop at 38deg (I think), unlike the original design that opened with skin temp plus 5 deg.

Hi Swedish Steve
I noticed this behaviour too, but the manuals have not been changed on the new MSNs... have you got any reference for that?

Fidelio,

At least in my MEL, up to 38C there's no limit.
39 to 45 C the limit is 3 hours.

Tiennetti
 

DSC-21-30-20. NORMAL OPERATION, OPEN-CIRCUIT CONFIGURATION

(At the end, below the graphics)

Note: In some cases, the opening of the skin air valves can be delayed even if the skin temperature is above the on-ground thresholds: This is to avoid condensation phenomenon when the temperature inside the avionic compartment is too cold. :ok: