Originally Posted by
Kapitanleutnant
Wondering why Boeing would not simply utilise the APU to run BOTH packs and therefore air conditioning in front AND back cabins. APU Air is good up to what... FL220? Simply use APU to power both packs and maybe turn off the bleeds (Don't know if that affects any anti icing required... no FCOM handy)
K
As stated above -
PACKS OFF::Bleeds both open, L and R supplying C1 and C2 demand pumps
APU to PACK:: L Bleed off, R Bleed supplying C2 Demand pump.
Your question was pertaining to why APU to Pack doesn't supply BOTH packs and additionally why not turn off eng bleeds for extra desired thrust?
Apu to pack function (apart from other good reasons) considers a single pack takeoff by APU Bleed.
Kindly ASSUME an APU failure in such a takeoff roll, BOEING has ensured that your both packs DONT TAKE ANY bleed from any ENGINE. How? Right pack is off, left Eng bleed is OFF.
This ensures performance weights/calculations complied with using FULL ENG THR with no PACK Extraction.
If APU were to supply both packs as you questioned, an APU failure would somewhat invalidate/deviate from calculated PERF weights.
Why ENG bleeds can't be off WHILE APU supplies both packs and demand pumps as you questioned, Assume APU failure and there is no BLEED supply for HYD RESERVIORS pressurization /air for ADPs / 'maybe' Gear Retraction / 'maybe' RTO Braking
I wish I could back this explanation with a legal document, I was studying the same issue and this is what I concluded.
Any corrections to above is welcome
Regards