Originally Posted by
FCeng84
It is surprising to me that the details of the MCAS functionality have not come to full focus here within PPRUNE yet. MCAS inserts airplane nose down stabilizer as a function of Mach number and how far beyond a trigger AOA value (a function of flight condition) the airplane has gone. When AOA reduces to below the MCAS sctivation threshold MCAS runs the stabilizer airplane nose up the same amount so that it is back where it started. If the pilot commands manual electric trim while MCAS is active, MCAS is “reset” based on the assumption that the pilot has taken over responsibility for pitch trim and will bring the stabilizer to the proper trimmed position.
The only way to get MCAS to insert more than one increment of airplane nose down trim is for all of the following to occur:
1. High AOA is detected leading to one MCAS increment of stabilizer
2. Manual pilot electric trim command is detected thus causing a reset of MCAS
3. Pilot does not fully command stabilizer to its trimmed position
4. High AOA is once again detected leading to another MCAS increment of stabilizer
For an engineering description (going by your username) this seems oddly vague.
- Do all these things have to happen exactly in order? That would mean MCAS would never do more than one trim increment unless the pilot uses the manually-operated electric trim in between? How do we know that?
- Where does the information come from that MCAS undoes its nose-down input when AoA returns below activation threshold? STS does something like that, but I have not seen anything to that effect about MCAS.
- How would MCAS determine the "trimmed position" of the stabilizer? And would MCAS really never operate if the stabilizer were moved to this "trimmed position" (assuming there is an engineering definition)?
- Is there really only an "activation threshold" AoA value (even if dynamically calculated)? Is rate-of-change of AoA not taken into account? I know that many stick-pusher activation systems do take RoC into account.
- It still seems odd that 2. and 3. above would be required for 4. to be considered a sufficient condition for activation.
Originally Posted by
FCeng84
MCAS will not command more than one increment of nose down trim without either having taken out the first one (via the same amount of nose up trim) or having sensed manual pilot electric pitch trim.
So you say, but how do you know? And if that is so, why weren't operators told?
Airbus famously has these "FOR INFO" sections in their FCOMs (e. g. ground spoiler activation conditions), which give more in-depth information about the inner workings and algorithms, that is not part of any exam, but very useful to know. Boeing manuals are lacking something similar and are restricted to what is considered "need to know" (on what basis?).
Cheers,
Bernd