Originally Posted by
Europa01
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?
One possible problem rarely mentioned re Manual Trim Wheel IMHO is this you used the term "PNF" whichI assume meant pilot NOT flying .. However, somewhere in the regs as I recall ( not sure ) is the requirement that emergency situations must be able to be handled by ONE person. Now add to that the ergonomic issue- Something ( MCAS or other ) suddenly trims AND for several seconds- with much bells and whistles going off. Eventually at some position between IN trim- level flight and AND- cutouts are used- refert to manual. - But AND may have resulted in negative G. So while ONE person is light in seat, nose is somewhat down, yoke is probably pulled back, ONE then has to lean forward, unstow handle, and wind Trim wheel maybe a dozen turns by providing a constant XX lbs force for the full revolution(S). I suspect even IF the PNF ids asssigned the task, providing even 10 pounds force full circle would be difficult if not impossible. - And guess what - that problem is NOT unique to MAX ! Avation week as I recall monitored and wrote about a real test flight using the yo yo technique starting at 12 to 15 K and 200 -250 kts. Took about 8k feet to get back to level flight .. and the pilots KNEW what to expect and do. Just how far out of trim they started with is not mentioned.
perhaps someone can provide a link- this was in April- May time period.