Originally Posted by
walkon19
Uhh- not too sure where you get the worm drive screw jack bit - but the 737 series ( and others 767,777, etc - use a double recirculating ball " NUT " on the course threaded screw jack. Which has about a 90 percent efficiency and can therefore be back driven absent a braking system. While the electric motor MIGHT be overloaded at the extreme position (AND) and high speed, it is more likely that the manual (trim wheel- handle system ) is simply unable to produce enough torque absent superman along with the large number of turns required per degree and all by ONE person.
Walkon - you make a very good point about jackscrew torque. I think the information available tells us:-
- For an aircraft not too far out of trim the trim wheels can just about overcome the Max stab actuator ie slip its clutch and turn the jackscrew (dependent on the strength of pnf)
- For an aircraft which is severely out of trim air loads on the stabiliser and hence the jackscrew are expected to be too large to be overcome by use of the trim wheels (hence the the 'porpoise' technique)
Since the stab clutch can be overcome by the trim wheels but the out of trim stabiliser forces can't doesn't this mean that in some circumstances the torque to turn the jackscrew will be higher than the actuator clutch slip torque and so manual electric trim will not work.
If this is the case then it may explain what was going on between the pilots in the final stages of ET302 when they seem to have tried re-selecting the cutout switches but could not get manual electric trim to work.
I'm conscious that the above contains assumptions about the detailed workings of the stab actuator particularly its clutch / torque limiting mechanism (it's certainly much more than a motor). Does anyone have more information about it?