Originally Posted by
FullWings
What I find interesting, is whenever MCAS is being discussed (re the Lion Air crash and possibly this one as more data comes to light), there are a lot of posts saying that if they’d just done the trim runaway checklist, everything would have been OK. Yes, probably. But what would have led them to that checklist? If you took off in a 737 MAX on a flight where nothing went wrong and you didn’t crash (rare, I know
) and manually flew the cleanup, would you get a bit of trim from MCAS as the last of the flap went in, making it something “normal” and “expected"?
If you had read all the manuals, then got a stall warning shortly after clean, you’d expect the MCAS to do a bit of trimming, yes? So “normal” behaviour? It’s also intermittent and you can trim back and forth yourself, so hardly a “runaway” in the traditional sense; in fact not in the sense of the QRH either, which describes a runaway stab as “continuous”.
So, unless you are prepared to do the runaway stabiliser drill every time you see the trim wheel moving and it wasn’t initiated by you (which would lead to lots of very short flights), how do you tell in a limited timeframe whether MCAS is trying to save your life or trying to kill you...?
MCAS is only supposed to trim the aircraft at "very" high AOA, not during the normal flight profile, so you should never see an MCAS trim input. If you get a stall warning shortly after T/O it should be obvious if it is a real stall or a false warning, based on known aircraft performance, pitch and power (if not more, training required).
MCAS will add 2.5 deg AND trim over a 10 second time frame and stop, unless the pilots resets by manual trim, that will stop the MCAS input for 5 sec and if the high AOA (real or false) is still present add another 10 sec 2.5 deg input (and so on). Not on the 737 myself, but often on the jumpseat, I don't think STS will ever give you a 10 sec trim input, and STS trim after T/O would normally be ANU, as it will try to keep the speed constant while the pilot wants to accelerate. STS and MCAS are
AFAIK the only systems able to run the trim on the 737. I also don't think a 737 pilot who is hand flying and trying to accelerate would miss an un-commanded (by him) 10 sec AND trim input. If in doubt just keep blipping the trim every few seconds, and MCAS will do nothing, no need to immediately disconnect the trim (but obviously follow the QRH with regard to trim issues).