Originally Posted by
Jetthrust
Hi Mick. I would suggest the difference in view is due to a difference in how you interpret the second last sentance, in the description of MCAS's operation. ...
Thanks for that summary. FCeng84 seems to have the inside dope on MCAS so I'll happily defer to his explanations. I would make a couple of points though:
1. Neither the FAA nor Boeing were particularly forthcoming in their initial correspondence to operators on this matter. Neither the FAA Emergency AD or the Boeing FCOM Bulletin mentioned MCAS. Boeing's bulletin simply stated that
'In the event of erroneous AOA data, the pitch trim system can trim the stabiliser nose down in increments lasting up to 10 seconds.'
That statement neatly sidesteps the fact that on the B737, AOA data had never previously been used to directly inform '
the pitch trim system'.
2. Having had at least another three days to come up with a clear description of MCAS the best that Boeing could manage was still somewhat ambiguous. What the subsequent Boeing correspondence fails to make clear is that:
a. In the absence of crew trim inputs, MCAS will only make one nose down trim adjustment of up to 2.5 units.
b. MCAS is apparently not interested in overcoming the attitude condition that triggered it - having made its initial nose down trim adjustment the trigger condition AOA can persist and (so long as there is no crew trim input) MCAS will do nothing further.
c. MCAS also apparently makes a subsequent nose up trim adjustment to return the stabiliser to the previously trimmed position.
3. Boeing fails to make it clear that a trigger for MCAS to continue trimming in more nose down trim is a countermand from the crew. Frankly, I think that is astounding. The system will ignore the persistence of the trigger condition after it makes its initial nose down trim adjustment but it will make a further nose down trim adjustment if the crew countermands it! Moreover, it will do that each and every time the crew countermands it. That is a very clear cut case of an automated system that is
designed to override a specific crew command. To borrow an image from Gums, when the crew looks to wind off some of the automatically commanded nose down trim, HAL says, '
I'm afraid I can't allow that, Dave.'