Originally Posted by
Judd
Try restraining a fast turning stab trim by hand and you risk losing skin. The restraining is for a wheel that is coasting due airloads (?) not electrically moving. That is my understanding, anyway
Judd,
A couple of things to consider, the stab "should" not drive (coast) the jackscrew --- they are all square (Acme) threads, stab air loads only locks the nut tighter. Where "coasting" has happened, there was gross wear well beyond limits, negating the fundamental "no back drive" feature of an Acme thread.
Again, in theory, when power is applied to the stab trim motor(s) the brake releases, with no power to the stab trim motors (from whatever source) the brake(s) is on., but it just reinforces the basic engineering of the screw jack.
You are supposed to be able to arrest a main electric runaway by hitting the spinning wheels with your palm, allegedly there is enough slack in the cables for you to then grab and break the handles out --- I have my doubts, im my cases, the cutout switches worked.
As I recall, the stab brake breakout force with no power (manual trimming) makes movement via the handles on the trim wheels a matter of some serious physical effort.
As I said before, I have never done this for real on a B737, only a B707 ( all the aircraft with fully hydraulic/electric controls are quite different) including having two real runaways (both nose down, Murphy's Law) , and as I see it, right to this day, the B737 hardware is fundamentally the same. as the original B737, despite modern "digital" autopilots and associated software systems
Tootle pip!!