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AerocatS2A
8th Apr 2019, 14:34
This has probably been answered in the massive Ethiopian thread but I doubt Iíll have any luck tracking down a minor diversion such as this.

When MCAS (the original version) is activated it trims down for approximately 10 seconds. If the pilot does not trim up against the MCAS, ie, he just lets it do its thing and holds the column back physically, will the MCAS input another 10 seconds worth of trim after a 5 second pause? If this is not the case is there an official Boeing document that supports it?

FlightDetent
8th Apr 2019, 15:04
Same question: If allowed to finish, and with the high AOA still persistent, is MCAS done or will start again?

Goldenrivett
8th Apr 2019, 15:53
will the MCAS input another 10 seconds worth of trim after a 5 second pause?
If allowed to finish, and with the high AOA still persistent, is MCAS done or will start again?
Yes. See 737 MAX - MCAS (http://www.b737.org.uk/mcas.htm)

"The MCAS design at the time of these accidents would trim the Stabilizer (http://www.b737.org.uk/flightcontrols.htm#Stab_Trim) down for up to 9.26 seconds (2.5 deg nose down) then pause for 5 seconds and repeat if the conditions (high AoA, flaps up and autopilot disengaged) continued to be met. If the pilots used electric pitch trim, it would only pause MCAS for 5s; to deactivate it you have to switch off the STAB TRIM CUTOUT switches."

wiedehopf
8th Apr 2019, 17:07
I remember reading somewhere that it would stop after running twice when thumb trim switches were not used. (5 deg nose down trim)

Don't think i'll find it though, one of the posts by FcEng84 i believe.

hans brinker
8th Apr 2019, 20:07
Yes. See 737 MAX - MCAS (http://www.b737.org.uk/mcas.htm)

"The MCAS design at the time of these accidents would trim the Stabilizer (http://www.b737.org.uk/flightcontrols.htm#Stab_Trim) down for up to 9.26 seconds (2.5 deg nose down) then pause for 5 seconds and repeat if the conditions (high AoA, flaps up and autopilot disengaged) continued to be met. If the pilots used electric pitch trim, it would only pause MCAS for 5s; to deactivate it you have to switch off the STAB TRIM CUTOUT switches."

incorrect, it needs to be “reset” by trim input before it will trim again.
go through FcEng84 post history.

Loose rivets
8th Apr 2019, 22:48
One of the biggest issues to me was the redatumization of MCAS upon reset. This would allow the inputs to sum and account for the screwjack going to full travel.

I've raised this issue on R&N with no real response. It seems so important.

Another mystery is the clear statement way back of MCAS reestablishing the old setting after actioning. I just don't know how to search that in the vast number of posts.

Reading everything FcEng84 has written is good advise.

ZeBedie
8th Apr 2019, 22:55
That these two accidents have been allowed to happen doesn't show the FAA or indeed EASA in a good light. I wonder if the MCAS system would have got past the old UKCAA/ARB?

MurphyWasRight
8th Apr 2019, 23:42
One of the biggest issues to me was the redatumization of MCAS upon reset. This would allow the inputs to sum and account for the screwjack going to full travel.

I've raised this issue on R&N with no real response. It seems so important.

Another mystery is the clear statement way back of MCAS reestablishing the old setting after actioning. I just don't know how to search that in the vast number of posts.

Reading everything FcEng84 has written is good advise.

One source of confusion is the mixing/confusion of descriptions of MCAS 'normal operartion' and with a failed (positive) AoA sensor.

What I read was that in normal operation absent any pilot trim inputs it would back out it's input once the AoA returned to a lower value.
Any pilot trim inputs cancel entire the sequence (further ND trim and the restore), basically reset to initial state.

The flaw in the design is that in case of a hard AoA fail the action is to trim nose down and then do nothing -until- it sees a pilot trim input which reset it to initial state.It then waits 5 seconds and begins again.

It it then again detects high AoA and repeats ND with no 'memory' (preserved state) of prior actions. This is the root cause of extreme ND trim.

In ET case you can see this action, including the ~10S attempted ND trim after the trim cutout (no effect).
It then does nothing until after the trim is re-enabled and 5 seconds after the ineffective short pilot trim inputs.

The Boeing 'fix' description includes a limit on total trim applied as well as using both AoA sensors. It is ambiguous on what constitutes an occurrence though

Roj approved
9th Apr 2019, 02:48
Yes. See 737 MAX - MCAS (http://www.b737.org.uk/mcas.htm)

"The MCAS design at the time of these accidents would trim the Stabilizer (http://www.b737.org.uk/flightcontrols.htm#Stab_Trim) down for up to 9.26 seconds (2.5 deg nose down) then pause for 5 seconds and repeat if the conditions (high AoA, flaps up and autopilot disengaged) continued to be met. If the pilots used electric pitch trim, it would only pause MCAS for 5s; to deactivate it you have to switch off the STAB TRIM CUTOUT switches."

So is it fair to say, if the MCAS activates on erroneous data, and you put flap out, it would stop the MCAS system activating?

If this is the case, this would have been a handy piece of information for the flight crew as they fought the system.

737 Driver
9th Apr 2019, 03:45
So is it fair to say, if the MCAS activates on erroneous data, and you put flap out, it would stop the MCAS system activating?

If this is the case, this would have been a handy piece of information for the flight crew as they fought the system. More to the point, NOT RETRACTING THE FLAPS would have avoided the subsequent series of events that led to the accident. An AD had been published describing the conditions under which MCAS was activated, and in theory the crew should have been aware. Prior to the second accident, there was a growing consensus at my airline that the best way to operate the MAX until a long-term fix was implemented was to keep the flaps extended until the A/P was engaged on all departures. If a potential AOA problem was detected before then, then don't retract the flaps at all. Why the Ethiopian Captain elected to clean up with a known stick shaker is one of the great mysteries of this accident.

IliasP
16th Jul 2022, 07:40
More to the point, NOT RETRACTING THE FLAPS would have avoided the subsequent series of events that led to the accident. An AD had been published describing the conditions under which MCAS was activated, and in theory the crew should have been aware. Prior to the second accident, there was a growing consensus at my airline that the best way to operate the MAX until a long-term fix was implemented was to keep the flaps extended until the A/P was engaged on all departures. If a potential AOA problem was detected before then, then don't retract the flaps at all. Why the Ethiopian Captain elected to clean up with a known stick shaker is one of the great mysteries of this accident.
That is a very good point ,Ihavent thought about that!
Iíll implemented

DaveReidUK
16th Jul 2022, 13:42
Hindsight is a wonderful thing ...

vilas
16th Jul 2022, 14:35
More to the point, NOT RETRACTING THE FLAPS would have avoided the subsequent series of events that led to the accident
Chicken and egg. How would anyone know MCAS is going to activate unless you retract flap? Once activated yes you can stop by extending again.