Originally Posted by
Dave Therhino
That youtube video has some inaccurate and confusing information.
It does appear to, but it is also obvious that the guy is working, and trying to reverse-engineer, with incomplete information. There is a follow-up video which makes it a lot clearer, both to the viewer and to the presenter - because he was provided with (and shows) a more complete diagram and description of the system. Because the videos aren't edited much (if at all) you can see how he ends up on the wrong track, basically assuming that there is a control valve in addition to the check valve in the input from stage 5, whereas in fact (if I understand it right) the input from stage 5 is effectively controlled only by differential pressure with the check valve being the only valve (doing what check valves do, unless it sticks open...).
But yes, if it sticks open you will almost always end up with a shutdown, and if it sticks on both engines it's likely to be a forced landing. The AD was issued because the first few flights after a storage period have a risk of dual engine shutdown that is quite high.
How did the AD come to be issued though? Is the stuck valve a known risk that was previously thought to be acceptable because 737s were not usually parked up for months, or has there actually been an incident due to this (maybe only on one engine)? Engines need to be regularly run-up on parked aircraft - is that enough to trip this failure mode, has it happened on the ground perhaps?
EDIT: now been pointed out, thanks, that I missed the reports of multiple single-engine events being the trigger, as noted in the AD. Must RTFAD slowly and carefully before posting, not skim read :-)