Originally Posted by
Loose rivets
Back to the switches for a moment. The RH switch wiring shows a little more complexity than simply a series backup toggle. Bear in mind, a million posts back, someone said each switch was 'dual gang' - but even so, the toggle lever could just fail* Anyway two it is. If only . . . if ONLY, Boeing had said, Now, this is what MCAS is, and this is what it does, If you don't like it, the right hand switch will turn it off. Leave the left one on if the wheels have stopped spinning and you want to be able to use thumb trim. IF only.
As it is, the left one still cuts all power to the horizontal stabilizer motor/clutches - even if we'd dearly love our thumb trim switches to still work.
*
Based on the electrical drawings posted on this forum, the LH switch has 3 contacts while the LH has two contacts. Both contacts on the RH are in series with the RH contacts supplying 28 VDC to the whole circuit, the 3rd contact on the RH switch enables the brushless drive. So any of the switches will cut 28 VDC to the circuit. I cannot see how MCAS or STS can be disconnected without cutting power to the whole circuit. There is now way to keep ONLY thumb controls.
I do not accept the justification that two in series provide a better fail mode in case one contact is stuck, fused, etc. If that would be a design concept all switches should have a back up, and thousands of earlier 737 are unsafe. IMHO either CB are the fail safe in case contacts are fused. One CB cuts 115 VAC and the other cuts 28 VDC.
It can also be noted that one contact from the LF switch provides a cut out signal to FCC, which seems that was not used by the SW, since the FDR traces show MCAS activation with switches in CUT OFF position.
Would be interesting to know the number of occurrences of fused contacts on those switches....