B777 APU Bleed
 

Hello everyone,

i'm wondering why, on the B777, when the pack and APU bleed settings are set to Auto, bleed is immediately drawn as soon as the APU starts up? I know from our Airbuses that you're supposed to wait at least 3 minutes (to give some warmup time and to avoid fumes). Why does Boeing apparently not care?

Greetings
Michael

It's not that Boeing does not care, it is that many Airbus aircraft have a design flaw that they attempt to partially mitigate with this additional operational procedure. The A300, A320 (including NEO it appears?), A330, A340 and APU installations are particularity susceptible to inlet air contamination because the air inlet scoop is mounted in the belly. Then, it has oil vents and drains that are installed just behind and uphill of the air intake scoop. Since gravity continues to act in a downward direction, it ingests any fluid that happens to come out of the drain mast and run down and forward while the airplane is parked. It was moved to the upper fuselage on the A350 to provide improved fluid ingestion protection. Here's a really good instagram reel where an mechanic shows some of the fluid protection changes that Airbus has made over the years on the a320 to try to mitigate the issues with the design of this installation. At the end of the video he says that the A321XLR has moved the APU inlet to the upper fuselage, and shows some computer graphics of it, but I can't find any pictures of this change in-service, and the Airbus Airport Planning document does not show it that way.

https://www.instagram.com/reels/DLGKRXKBuBX/

https://www.aircraft.airbus.com/site..._a321_1223.pdf

Also, an Airbus tech article on APU air contamination from external leakage.

https://www.aircraft.airbus.com/site...-04/FAST52.pdf

As I understand it, the "Cobra" mod is still in development and not yet installed on a production aircraft.

Cobra?

Described near the end of this short video; haven't found anything super authoritative: https://www.instagram.com/reel/DLGKRXKBuBX/

To put things somewhat into perspective, this 3 minute warm up time that Airbus prefers, is only a recent development. It used to be one minute, or even no warm up time at all if I recall correctly. And indeed, the 3 minute rule has more to do with fumes as I understand it.