Actually it happens in Airbus, when you fully close the TRIM AIR VALVE!
Then the air with same temperature is diverted to all the zone FROM THE PACKS
In that instance only as you mentioned the ACM provides the right temperature (ACM directly provides the temperature control!)
But if you AFT CBN is cold! you have to supply hotter air by opening the TRIM Air Valve because it is always easy to get a little hot! air stream from the engine and mix with pack air to supply!
hope this helps!
Last edited by code0; 31st October 2012 at 15:42.