Originally Posted by
Rananim
posted by Babilu
Explanation on MCAS
Note that MCAS overrides pilot stick input...first automatic override of a pilots input to a primary flight control in their commercial design history to my knowledge.Previously all anti-stall devices merely warned and aided the pilot to recognize and recover from a stall(eg,PLI,EFS,autoslats,shakers)
That leaves either a)counter-trimming or b)selecting BOTH trim switches to CUTOUT as a way for the pilots to override MCAS triggered by false ADIRU data.
Boeings AD mandates (b) as the approved procedure but hindsight is 20/20.If the pilots dont recognize it as runaway trim due i)startle factor ii)trim is not continuous but in 9 second stages iii)false stickshaker masks trim and degrades pilots alertness then they are left with (a)
Pilots have spoken about overriding STS with main electric trim.All B737 pilots have done this.But STS uses AP stab trim motor(presumably B/U in the MAX) whilst stick trim uses main electric(presumably PRI in MAX).A pilot flying manually cant trim down and pull on the stick at the same time...not unless the override switch is in override.What if MCAS is using the same trim motor as the one actuated by stick trim?MCAS description says it uses lower trim rate at higher mach and vice versa.So it may well be using PRI motor at low altitudes and B/U at higher altitudes.In which case there is a possibility that the simultaneous command of opposing trim in the same trim motor could lead to motor stall and burnout.MCAS is trimming down using PRI motor because a/c is at low altitude and low mach.Pilot pulls on stick to no effect.Pilot then trims up using stick trim(PRI) causing clutch engagement and motor stall.Who wins?MCAS or the pilot?How many reversals can this trim motor withstand before failure?Any engineers please comment.
The underlying concept of Boeings commercial design has always been that the pilot has the final say.The input to the stick by the pilot overrides any device or warning system controlled by HAL.For Boeing to have deviated from this principle means there were serious concerns about MAX's pitch characteristics.In shooting for glory and lower fuel burn they have sacrificed their soul at the altar of greed and complacency.When the pilot pulls the houses must get smaller.....always.
Above comments do not relieve a pilot from exercising airmanship and killing ALL HS trim when the pitch control of his/her aircraft is in doubt.
Rananim:
”....Sistering STS with MCAS and only one trim drive? If Trim is flight critical, (the crash suggests so), where is drive redundancy? How is one drive isolated from the other(s) such that opposite inputs would (not) stall a (single) motor?...”
post number
1514
just sayin.