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Old 7th Nov 2018, 19:46
  #777 (permalink)  
Rananim
 
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The STS(which btw I agree is not technically an anti-stall device but we can view it as such as it will indeed trim down if you do approach a real stall in manual flight) would be disabled by selecting only the AP STAB trim cutout switch to CUTOUT.Boeing use bulletins to get vital information out immediately.The reminder to do the RUNAWAY STABILIZER NNC covers this runaway STS..They may refine procedures checklists later...or may not.This is probably what the commander of the previous flight did...he saw unwanted trim and he disabled both trims..fair enough.That wasnt the time or place to work out why/how he was getting unwanted trim...he just needed to disable all trim.He can trim the stabilizer manually once the trouble-shooting has ended.
RUNAWAY STABILIZER NNC...in the Boeing,unlike the Airbus where it is quiet,the trim wheel makes a lot of noise.That noise(if its continuous and not momentary) is actually the first and best indication of a runaway stabilizer,well before the out of trim condition develops.The pilot disengages automation and if it still continues he puts both trim switches to cutout.If that doesnt do it,the other pilot grasps and holds the flying pilots trim wheel.The point being is the pilot has good aural and tactile warnings of a runaway.However,in a UAS with stick shaker operating,those warnings may be diluted...and lets not forget this was an AP trim motor "runaway"(slower,quieter.more insidious)
Re manual vs auto flight...I think this is just a case of lost in translation.Boeing confirm that STS trim occurs in manual flight only and that it will trim to the stops(as the AMM quite clearly says it will) unless disabled.The AMM also says very clearly that the AUTOPILOT will trim it to the stops regardless of which AP you engage providing the FCC controlling the stab trim signal is being fed corrupt ADIRU data ...so theyre saying use manual flight which is what the UAS checklist calls for anyway.
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