View Full Version : B737NG Packs & Apu bleed

9th Jun 2019, 07:36
Dear all,

I am new on aircraft. I checked documents and saw the real procedures in the cockpit. But I haven't decided on one thing yet which is very simple.

Is there a flow to follow when both packs off, iso valve open and apu bleed off and we want to operate the packs with bleed air. In which order we should configure the switches and what's the logic behind that ?

Do you operate firstly the packs auto then apu bleed on or vice versa or mixed order ?


9th Jun 2019, 16:38
Makes no difference anyway. In absence of any bleed air pressure (i.e. engines OFF and APU bleed air switches in OFF position), the pack valves will not open anyway when the PACK switches are moved to AUTO, since they are pneumatically operated.

In the preflight procedure, Boeing recommends a “top down approach”: packs first, then isolation valve, then engine bleeds, then the APU.

Air conditioning PACK switches – AUTO or HIGH


Engine BLEED air switches – ON

APU BLEED air switch – ON

Verify that the DUAL BLEED light is illuminated.

Viv Savage
22nd Nov 2022, 19:38
Sorry to hijack the thread, but I was just wanting to discuss a hypothetical in-flight situation. If we were to lose both bleeds or packs in flight there is no provision in the QRH to use APU Bleed to keep pressurisation. I am guessing that this is possible below FL170 by just selecting the Bleed to “ON”?

70 Mustang
23rd Nov 2022, 10:51
While reading the air systems chapter.