So far the starter theory might make sense. We have tried all suggestions here. My question is under what conditions would the APU bleed valve modulate to closed uncommanded upon creating a bleed demand?
So even if switches like hyd pumps, anti ice and opposite isol valve were in correct or incorrect positions ( no idea what cross bleeds are on 74!) what would cause the APU Bleed valve to modulate closed? Is it to protect the APU from excessive load?
If so then turning one APU Gen, hyd pumps, packs, one isol valve and anti ice off would ensure minimum load...not so? Tried that still goes closed. Weird thing is you can only open it again if you completely de power the aircraft or use an air cart for start then shut APU down - subsequent APU start allows APU bleed valve to open until you try and start again?
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