Hello to all, when you hook up external air why can't you keep both packs on to supply air to the cabin? The psi goes to 0 and when you turn one pack off psi goes to about 20psi. Is this a real effect on the real aircraft? Thank you :-wave

Packs are always swtiched to off during engine start, whether using APU BLEED AIR or an Air Start Unit. This is to supply maximum airflow to the engine during start up. Tha Packs supply air conditioning to the aircraft. The Recirc Fans can be left on during engine start - these will recirculate the air in the cabin.The only times that the lack of APU ( no packs then ) is a problem is on hot days, where the cabin heats up real fast. You then need to hook up an air conditioning unit to the aircraft. http://www.smartcockpit.com/b767/B767%20Aircond.PDFhttp://www.smartcockpit.com/b767/B767%20PNEUMATICS.PDF

Mark,I think the original post was regarding using external air AND packs at the same time. I suppose you could use 1 pack to keep the cabin conditioned but the external air source isn't really that powerful and using 2 packs drains the pressure from the external air source units.With regards to Recirc Fans during engine start, they are also switched off...but just automatically.Next time you rotate the GND START watch the recirc fan switch illuminate "INOP" (or similar). This is to reduce electrical loads on the APU so that it can produce as much wind as possible :).Charsobk

Cheers Ben.Not only that, airport authorities hate the air start units - they make some racket. Heck, I know they ain't powerful - that's why we need 2 for the 76.

"With regards to Recirc Fans during engine start, they are also switched off...but just automatically"Yes, the Recirc Fans are supplied by the two utility busses, which are shed during an engine start. Actually, when you start the second engine, the opposite side is not shed!Regards,Mark

Because the external air module can't supply both packs at once. There is not enough air to support both packs and that is why you have to shut off one for a minimum of 20 psi. It really depends on the output pressure of the external air module but most can only support enough for one pack to operate.

Thanks AAL for confirming my answer...Cheers[br][/img][/font][/b]