Originally Posted by
DaveReidUK
Or, more accurately, it neither validates nor invalidates the theory - we simply don't know.
But the fact remains that if it turns out that the FDR isn't showing the thumb switch position, then it contravenes the Annex 6 requirements.
Seems pretty clear comparing the NG & MAX wiring diagrams posted on this thread that the actual switch position is not captured by a sensor but rather the switch output is recorded, and on the MAX the stab trim cutouts cut power upstream of the switches (downstream on the NG), thus on a MAX there would be no output to record.
Boeing have been building aeroplanes for long enough that one would hope they know what an FDR is supposed to record.
Indeed, however I suspect they may be relying on the asterisk -
"to be recorded if an information source for the parameter is used by aeroplane systems and/or flight crew to operate the aeroplane". If the system has been cut-out then it's not being used to operate the a/c and therefore need not be recorded... Trim cutout switches themselves are not (I don't think?) recorded, therefore they are not themselves being considered a control input, but more as a handily positioned circuit breaker. Start pulling CBs and all sorts of mandatory parameters will cease to be recorded.
Not my logic or interpretation, but what I think Boeing may be relying on. I do think it could be questioned though, and if I was Boeing I would want to have that interpretation of the requirements to have been fully documented and signed off by FAA during the design process - because coming up with it after the fact is a lot less convincing.