Originally Posted by
FCeng84
Several people have asked MCAS clarification questions in response to some of my recent posts. Rather than respond to them individually I offer the following as hopeful a fairly comprehensive description of the MCAS system that has been in the 737MAX fleet to date. Note that this does not reflect any of the changes about to be released. Hopefully Boeing will provide clear detail of those soon.
As always, if you still have questions after reading and trying to absorb the following please ask.
1. MCAS was designed to command airplane nose down stabilizer in response to high AOA up to an authority limit of 2.5 degrees for Mach less than 0.4 with lower authority at higher Mach numbers. If the pilot does not make any pitch trim commands, once AOA goes low MCAS will run the stabilizer in the airplane nose up direction back to the location from which it started.
2. MCAS is activated when all of the following are true:
a. Flaps are up
b. Autopilot is not engaged
c. Sensed AOA is above the MCAS activation AOA threshold
3. Once activated, MCAS will not command more than one increment of airplane nose down stabilizer motion until it has been reset. MCAS will be reset by either:
a. Pilot pitch trim command followed by a period of 5 consecutive seconds with no pilot pitch trim command.
- The assumption is made that pilot activation of pitch trim will be closely followed by continued pilot use of pitch trim to return the airplane to a column neutral pitch trim condition. MCAS seeing no further pilot pitch trim for a period of 5 seconds is interpreted as indication that the pilot has achieved column neutral pitch trim.
b. Return (by MCAS) of the stabilizer to its starting position per (1) above.
- Having returned the stabilizer to its pre-MCAS event starting point it is assumed that the airplane is back to a column neutral pitch trim condition.
4. Pilot pitch trim input at any time during the MCAS sequence as described in (1) above will stop MCAS stabilizer motion and end the current MCAS event while immediately moving the stabilizer in the direction of the pilot command.
a. If pilot pitch trim input is issued while MCAS is running the stabilizer airplane nose down, that motion will stop and the stabilizer will immediately move in the direction of the pilot command.
b. If pilot pitch trim input is issued while MCAS is running the stabilizer airplane nose up, that motion will stop and the stabilizer will move in the direction of the pilot command.
c. If pilot pitch trim input is issued after MCAS has completed its airplane nose down motion but prior to MCAS acting to take that motion out (as a result of return to low AOA), the stabilizer will immediately move in the direction of the pilot command.
d. It is assumed that the pilot issuing a pitch trim command is indication that the pilot is taking over the pitch trim task and will return the airplane to a column neutral pitch trim condition.
As a result of 1 through 4 above, given an AOA sensor that is failed so as to give an erroneously high reading (similar to what data appears to indicate occurred during the Lion Air accident flight), the following MCAS related scenarios can occur:
A. MCAS will activate (if flying manually) as soon as the flaps are retracted to up. Note that the stick shaker will have activated as soon as the airplane lifted off the ground regardless of the takeoff flap setting.
B. If the pilot does not make any pitch trim inputs, MCAS will run the stabilizer airplane nose down for one MCAS increment (as much as 2.5 degrees over approximately 10 seconds if Mach is less than 0.4). Without pilot pitch trim input, MCAS will not command further stabilizer motion in either direction. There is plenty of pitch control authority via the elevator using the column to counter the pitch disturbance generated by one MCAS increment of stabilizer motion.
C. If during or after the MCAS stabilizer motion per (A) above the pilot gives a pitch trim command the stabilizer will immediately start moving in the direction of the pilot command.
a. If the pilot pitch trim commands drive that stabilizer back to a column neutral pitch trim condition then the airplane will be back where it started from. In this event, provided sensed AOA is still high, MCAS will activate again once it sees a period of 5 seconds with not pilot pitch trim input. This process will repeat itself with the stabilizer never getting further than one MCAS increment out of trim. It appears that this sequence was repeated 20 or more times by the Lion Air accident flight pilot before he handed control over to his first officer.
b. If the pilot pitch trim commands are activated, but do not drive the stabilizer back to a column neutral pitch trim condition it is possible that successive activation of MCAS triggered by high AOA signal and ineffective pilot pitch trim inputs will lead to the stabilizer moving progressively further in the airplane nose down direction. It appears that this is what took place on the Lion Air accident airplane once the first officer took over control.
As for the reference in media reports this week regarding the crew’s need to intervene within 40 seconds of errant MCAS response to an AOA signal failed high, the following sequence of events would have to occur to so compromise pitch control power in that amount of time:
(a) The flight crew would have to allow a full increment of MCAS airplane nose down stabilizer motion to go in over 10 seconds without interrupting that motion via pilot pitch trim input.
(b) The flight crew would then have to have made a very short pitch trim command that triggered MCAS to reset,We but did not re-establish anywhere near a column neutral pitch trim condition.
(c) After a 5 second pause wherein MCAS inferred that no more pilot pitch trim indicated the airplane was back to column neutral pitch trim, the flight crew would have to allow a second full increment of MCAS airplane nose down stabilizer motion to be inserted over another period of 10 seconds. Once again the crew would have to allow this stabilizer motion to go in without interruption via pilot pitch trim command.
(d) Once again, there would have be a repeat of (b) with a short, ineffective pilot pitch trim command.
(e) The final 10 seconds of this proposed 40 second sequence would be allowing another errant MCAS airplane nose down stabilizer motion increment to go in without interruption by pilot pitch trim command.
To summarize, getting in trouble over the course of just 40 seconds requires the crew to not trim when then should and make two very short, ineffective trim inputs following each of the first two MCAS stabilizer motion increments. This represents a worst case scenario.
We have full Stall ID in the case of JT610 by a margin of some 10 degrees. Stall ID on the 737ng triggers: Stick shaker, Feel pressure increase and trim actions very similar to MCAS (though with the Yoke cutout switches active). So given we don't have a clean MCAS case (MCAS should fix the pitch moment curve before stall), what is the MAX logic at Stall ID? Does MCAS replace the 737ng Stall ID trimming and is then the MCAS logic as you described unchanged?