Originally Posted by
A Squared
As nearly as I can tell, you theory is that Boeing just changed the name from STS to MCAS and that whatever happened to the Lion airplane could happen to other 737 model prior to the MAX . [...] Point being, Boeing probably had pretty sound reasons for saying that the problem was exclusive to the 737 MAX -8 and -9, and not including the NG and prior models.
Boeing probably had pretty sound (to Boeing) reasons for not telling anyone about MCAS, those reasons probably look like "WTF" to everyone else, but that's with the benefit of hindsight.
I would caution that the AD is quite specific that it addresses an issue caused by erroneously high AOA input, if AOA is
not an input to STS on previous models then this issue can't happen on them. That doesn't mean that STS on NG doesn't have the capability to dump you into the ocean, just that it cannot do it because of one dud AOA input, and the runaway stab trim procedure is there for the other cases. It is also possible that STS runaway on NG and earlier would require multiple sensor or ADIRU failures making it a lot less likely - I still don't get how only one sensor failure can upset MCAS, I can't (or don't want to) believe they took a direct output from
one sensor into the stab trim.
My personal theory on STS/MCAS
naming is that MCAS was invented and named in engineering/design to meet Part 25, and the existing STS was the cheapest/simplest implementation option (
possibly just software change and an additional input on the same unit). The MCAS acronym (definition only) sneaked out into some versions of the documentation but then someone decided that if it was effectively just an existing system, STS, why document it separately - it's all STS which is already covered. Further down the line someone decided that if the additional STS documentation was about an area of the envelope where only test pilots would go, why worry anyone else with it. So it became undocumented.
Add to the above a very slight suspicion that treating it as a separate new control system just might have opened a certification can of worms that Boeing wanted to avoid (as someone else suggested earlier in thread).