Originally Posted by
MurphyWasRight
For the NG ....
Can probably make the case that non auto instigated trim runaway is very unlikely (common english, not attempting to use certification levels) so having manual electric trim available would suffice.
No doubt that the operational statistics on the NG prove that this is very unlikely.
My concern is that the system have over achieved in the sense that the low runaway rate is not a result of the Design Assurance Level (DAL C instead of A), but undocumented good engineering :-) This can have unexpected consequences that a minor design update (e.g. the trim motor drive electronics, or implementing MCAS) can change the statistics drastically, despite being within specified limits.
The risk of a trim runaway to it’s stops, is the combined risk of trim system falling, and the pilots falling to react correctly in time.
CS 25 paragraphs 671/672 does not include the possibility of including the pilots reaction time into the equation.
Either the system by design have a low failure rate.
Or the pilots must be able to handle a full runaway without requiring exceptional piloting skill or strength.
It does however not exclude the solution of flipping switches to engage a standby/alternate system.