747 PSX response to Trim Unscheduled

[torrence]

I've been trying to understand how the 747 responds to "unscheduled trim" malfunctions, using this situ - simple takeoff from PHNL climbing to FL370, crossing MKK (Molokai) at 5000 ft.

Situ file: https://www.dropbox.com/s/aw5t6pqgkgj3mi5/PHNL%20takeoff%20stab%20trim%20testing.situ?dl=0.  This has an Airframe Malfunction set for ALT 5000 ft -  Stab Trim Runaway - Stoppable (Severe) - auto-cutout fails. 

Still working on this, but my first tries on this, following th 747 FCOM, is a bit confusing to the say the least.  After malfunction, following checklist works ok - Both Auto Cutout switches to Cutout, both 2and3, auto-throttle OFF, allows manual control.  OK but checklist then specifies trying to try restoring by setting Auto Cutout 2 to ON, if doesn't work, then Cutout 2 OFF then Cutout 3 to ON and seeing if that works instead.  Not sure what works beyond this point . 


In all of these cases, the Stab Trim tape keeps creeping the wrong direction after going to flight deck.


Confused in LA (not an uncommon condition).

Torrence
















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[Hardy Heinlin]

It's stoppable by setting both switches to CUTOUT.

When both have been cutout, and one is reset to ON, the trim may stay at the current position. But as soon as you push a trim switch again, the runaway will continue, and both need to be cutout again.

In this malfunction scenario the runaway always start after trim switch activity. This scenario is also modelled in the big sims.


|-|ardy

[torrence]

It's stoppable by setting both switches to CUTOUT.

Agreed - and, at least in PSX, the autothrottle can then be engaged and seems to operate normally.  What had me looking at the one Cutout on, one off cases was the procedure in the FCOM that I was looking at, which seemed to imply failures that could be dealt with beyond getting both switches to CUTOUT.
Cheers
Torrence

[torrence]

Further comment - I don't know near enough about what things could actually cause stab trim runaway.  The recent -MAX accidents seem likely to relate mostly  to AOA inputs and the MCAS issues.  I seem to remember that earlier accidents of other types, where trim control was involved, were due to mechanical or other issues. I had assumed, perhaps wrongly, from the 747 STAB TRIM FCOM info, that some fault cases might result from one or more hydraulic malfunctions and might be resolved by isolating which of the two hydraulic systems involved with the trim ( 2 or 3 ) is involved.

Cheers
Torrence

[Hardy Heinlin]

It's also important that a switch, when it's commanded to open, really isolates the electric contact.


Cheers,

|-|ardy