Originally Posted by
slacktide
Looking at the airplane's pneumatic diagram, one way the engines are tied together is through the 10'th stage bleed manifold, but there is a check valve that should isolate them. A bad 10th stage bleed check valve could cause a compressor stall and engine flameout at low power settings when you start the APU, which could account for an engine failure in this phase of flight. Having 2 check valves on 2 different engines fail on the same flight? That seems extraordinary unlikely. It was a quick turnaround at OSU so they would not have had time to do any maintenance, so a common mode maintenance failure seems unlikely.
A similar issue (corroded 5'th stage check valve sticking open due to extended duration storage) was causing multiple in-flight engine shutdowns at top of descent on the 737NG after bunches of them were poorly stored at the beginning of COVID. It is complete pure luck that there were no dual engine flameouts.
https://www.federalregister.gov/docu...pany-airplanes
no, you dont transition the bleeds(its manually done in the 604/5) until APU is up and running and also switching 10ths off is approved for ops if you have no APU (limitation of pulling two bleed sources at same time off engine) if you want to use Anti-ice for takeoff or landing