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Old 11th Apr 2019, 17:10
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Originally Posted by DaveReidUK
Do you mean the speed at which MCAS trims was increased, or the duration that it runs for, or both ?
Authority (per increment / reset) was definitely increased from the original design of 0.6 degrees to 2.5, I don't know whether this was done by increasing speed or duration or both - the speed change wouldn't do it alone, but might have been required to "correct" the stick force fast enough.

Stab trim does go through a gearbox, between motor and jackscrew, and there are at least two clutches (nose up/down - not sure if ap/manual is also separate or is a speed change).

From NG AMM chapter 27:

When the flaps are up, the switch is open and low speed trim is engaged. Low speed trim moves the stabilizer at 0.2 units per second. When the flaps are not up, the switch closes and sends a signal to the stabilizer trim actuator to engage high speed trim. High speed trim moves the stabilizer at 0.4 units per second.
During autopilot operation, the stabilizer trim speed changes. When the flaps are up, the low speed trim is 0.09 units per second. When the flaps are not up, the high speed trim is 0.27 units per second
What is also known is that MCAS trims at the flaps down autopilot speed - i.e. the "High speed". I don't know if this was a change at some stage due to finding low speed was insufficient for certification - it might have been. I also know how it was implemented (which is as I guessed it might be) - by blocking the flaps-up signal on MCAS engage.

References - elec trim functional diagrams below. One is from NG AMM, one is from an internet source but appears to be the equivalent diagram from MAX AMM. A whole lot looks to have changed with the wiring to shoehorn the MCAS function in...
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Last edited by infrequentflyer789; 11th Apr 2019 at 17:11. Reason: twypo
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