dual bleeding
 

Dual bleeds - any a/c. We all know the theory but what are the actual effects on the APU and a/c system? In the stories I've heard nothing happened in the few seconds it existed ie. reconfiguring after bleeds off t/o - obviously to be avoided but it doesn't really blow out of the tail as groundschool instructors might suggest?

Ta

you obviously turn one pack ON, then APU OFF and then pack 2; that is what I was taught on the 737 groundschool! db16:confused::bored:

The Apu has a Check Valve prior to the pneumatic crossover duct. You can get dual bleed light from # 1 eng and Apu bleeds on or #2 eng with isolation valve and APU open also.

Basically just a light to point out you finger troubles with switching, the check valve will save your ass, if it's working that is :ooh:.

db16
Are you talking about after start or for take-off?

At idle power after start you would probably have all 3 bleeds selected on (depending on aircraft type) but the system should be designed to cope with that.

Surely for a bleeds off take-off it would be eng bleed 1 & eng bleed 2 as with the packs off you would be unpressuised so there would be no point in having the APU bleed on in the first place

Any reason in particular, why you would not use APU for pressurisation on a non-bleed takeoff?

With regards to the order of config, I personally don't trust the isolation valve to do its job properly, hence my actions are(after non-bleed t/o):

Isolation valve closed
#2 engine bleed on
APU bleed off
#1 engine bleed on
Isolation valve auto

I cases where isolation valve was left in auto, I have experienced dual-bleed light coming on briefly during re-config. No damage done however.