Originally Posted by
safetypee
The other threads have been closed due to lack of information, but perhaps this is the critical point in getting the 737 Max back in service.
There still is no clarity as to what exactly contributed to the incorrect value of AoA.
I'm not sure it matters - AOA vanes fail and I haven't seen, yet, any figures to indicate Max vanes (or AOA systems inc adiru) are failing at higher rate than expected. It would be interesting to check if they are and if there is a common cause, and hopefully the investigations will, but I'm not sure it's part of any fix.
I do recall finding a document (from Boeing?) which implied that left AOA vane on Max is notably closer to the air bridge (and therefore maybe more vulnerable) - that may be enough to account for the left-side bias.
If ET AOA had also failed like Lion - to +20 degrees but tracking right side, then I'd say (not that my opinion is worth anything) a wiring investigation and fleet inspection would be warranted, but it didn't. It looks like a different failure mode and I would agree with others that the vane likely departed the aircraft in the ET case. If it was me, I might just want to look in more detail at the Sunwing Max where adiru (left, again) was replaced following AOA issues - since that plane is still in one piece.
But in the end I still come back to the first point - AOA vane failures happen (I've seen mtbfs of <100k flight hours quoted) and even if only (say) one in ten failures result in errant MCAS (probably generous)
and if 99% of crews are well trained enough to cope with errant MCAS (probably
very generous), the resulting crash rate is
still too high. The rate at which AOA vane failure leads to uncommanded AND trim needs to be drastically reduced - fix (or remove) MCAS.