Originally Posted by
Double07
Please excuse me if I am repeating something that has been discussed earlier in this thread, but from the schematic for the “Horizontal Stabilizer Trim Control System – Functional Description – Electric Trim” (see PPRUNE thread entitled “737MAX Stab Trim architecture”, post #194), one can see that when MCAS is active, then STS in inactive. And when MCAS is inactive, then STS is active.).
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... Could this help to explain the failure of the ET302 pilot to trim back to a fully neutral trim after an MCAS trim event?
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This means that there is no way to turn off all automatic trim functions while keeping the manual electric trim operative..
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This would make the 737 MAX operate more like the 737NG aircraft, allowing full use of manual electric trim at all times. Why has this not been done?
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Other than it's existence and a few things posted here I have no knowledge of sts, however does not look to be a significant factor in ET case.
There is one brief NU automatic trim on the trace at 05:40:00 after the AP disconnect followed (or even interrupted by) MCAS for 9 seconds, this could be an STS input.
The only possible impact of this is that the pilot may have heard trim starting and glanced down to see normal looking (NU) trim, which was quickly reversed by MCAS.
Had not thought of that before but likely not significant, in any case it was short and "the right way".
BTW: The penultimate Lion Air crew wrote up the defect as "STS trimming in wrong direction".
Correct on cut out switch change from NG to MAX.
No one has offered an 'official' sounding reason for the change.
The most likely case is that training had shifted to always using both and it simplified some aspect (cert?) of MCAS to not have a seperate auto only cutout, but both switches were retained for commonality.