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Yea, runaway trim is a failure pilots are aware of. That big wheel spins whenever the trim changes so they would notice the huge wheel turning, unprompted, whi

Yea, runaway trim is a failure pilots are aware of. That big wheel spins whenever the trim changes so they would notice the huge wheel turning, unprompted, while pulling back on the yoke. This doesn't seem like the whole story though. They clearly were able to control it for a while and were probably making trim inputs so why the sudden descent? They probably didn't turn off the trim motors as the article suggests but should have been able to keep re-trimming the aircraft indefinitely.

WalterBright 8y ago
There's definitely more to the story, as the information we have just doesn't add up. Investigation of the wreckage should clearly show what position the stab trim was at. It's a large nut on a jackscrew, the disintegration of the airplane should leave that intact. The nut/jackscrew is built out of incredibly strong steel.
- tyingq 8y ago
  Maybe this? Pilots have long been taught that pulling back on a 737's control column can arrest that condition – a fix pilots call a "breakaway", says the APA. Indeed, American's runaway stabiliser checklist, dated 10 July, says, "stabiliser trim commands are interrupted when the control column is displaced in the opposite direction". But the APA has now learned that the 737 Max is apparently different. https://www.flightglobal.com/news/articles/airline-industry-rushes-to-understand-nuances-in-737-453602/ https://www.flightglobal.com/news/articles/airline-industry-...
  - Syconious 8y ago
    Some aircraft have yoke aft limit switches that disable electrical stab trim I believe. As has been stated above, stab trim runaway is common knowledge and an emergency procedure (EP) that is trained for. Arresting the descent with yoke while activating the electrical stabilizer trim cutout and having the pilot-not-flying manually rotate the stabilizer wheel in the opposite direction or at least attempting to hold it from adding more trim if cutout is not working. Many pilots will position a knee or shin on the stabilizer wheel so they are aware when trim is being run. The yoke itself holds a trim adjustment mechanism so the pilots can adjust trim while retaining arrest authority on the yoke as well, though in the case of runaway trim the cutout would be used and manual operation of the wheel would be required. I believe another commentator mentioned the jack screw, this will very likely have survived the crash and investigators will be able to tell exactly where the nut was positioned (trim setting) when impact occurred. With stab trim runaway being such well trained for EP [assumed as its common in my circles] I would be surprised if that were the root cause unless pilot error is attributed to suspecting another fault mode. I am not familiar with commercial aviation as much, is it common place to transition from the critical phase of flight during take off to autopilot (AP) climb at such a low altitude? If they were not on some sort of AP mode I can't see how they wouldn't notice the trim being added as a serious issue. -Sorry I had meant to post this as a response above and not directly to your post
    - sokoloff 8y ago
      Most APs are certified for use from 400' above-ground-level on departure and on a complex departure procedure or in weather, that's not an uncommon engagement level. There's a fair number of people who think that pilots are relying on "George/Otto" too much and should do more hand flying, but I'd imagine if you peered into 100 airliners on departure, that 95 or more of them are on autopilot somewhere below 5000' AGL.
- vilhelm_s 8y ago
  I guess it's a drawback of the news cycle, but it seems kindof premature to start doing detailed 3D renderings of the 737 cockpit when basically nothing is known except one update to the plane manual. If we just wait a few month, the investigation report will no doubt clarify exactly what the pilots needed to do.
deleted 8y ago
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WalterBright 8y ago

There's definitely more to the story, as the information we have just doesn't add up. Investigation of the wreckage should clearly show what position the stab trim was at. It's a large nut on a jackscrew, the disintegration of the airplane should leave that intact. The nut/jackscrew is built out of incredibly strong steel.

tyingq 8y ago
Maybe this? Pilots have long been taught that pulling back on a 737's control column can arrest that condition – a fix pilots call a "breakaway", says the APA. Indeed, American's runaway stabiliser checklist, dated 10 July, says, "stabiliser trim commands are interrupted when the control column is displaced in the opposite direction". But the APA has now learned that the 737 Max is apparently different. https://www.flightglobal.com/news/articles/airline-industry-rushes-to-understand-nuances-in-737-453602/ https://www.flightglobal.com/news/articles/airline-industry-...
- Syconious 8y ago
  Some aircraft have yoke aft limit switches that disable electrical stab trim I believe. As has been stated above, stab trim runaway is common knowledge and an emergency procedure (EP) that is trained for. Arresting the descent with yoke while activating the electrical stabilizer trim cutout and having the pilot-not-flying manually rotate the stabilizer wheel in the opposite direction or at least attempting to hold it from adding more trim if cutout is not working. Many pilots will position a knee or shin on the stabilizer wheel so they are aware when trim is being run. The yoke itself holds a trim adjustment mechanism so the pilots can adjust trim while retaining arrest authority on the yoke as well, though in the case of runaway trim the cutout would be used and manual operation of the wheel would be required. I believe another commentator mentioned the jack screw, this will very likely have survived the crash and investigators will be able to tell exactly where the nut was positioned (trim setting) when impact occurred. With stab trim runaway being such well trained for EP [assumed as its common in my circles] I would be surprised if that were the root cause unless pilot error is attributed to suspecting another fault mode. I am not familiar with commercial aviation as much, is it common place to transition from the critical phase of flight during take off to autopilot (AP) climb at such a low altitude? If they were not on some sort of AP mode I can't see how they wouldn't notice the trim being added as a serious issue. -Sorry I had meant to post this as a response above and not directly to your post
  - sokoloff 8y ago
    Most APs are certified for use from 400' above-ground-level on departure and on a complex departure procedure or in weather, that's not an uncommon engagement level. There's a fair number of people who think that pilots are relying on "George/Otto" too much and should do more hand flying, but I'd imagine if you peered into 100 airliners on departure, that 95 or more of them are on autopilot somewhere below 5000' AGL.
vilhelm_s 8y ago
I guess it's a drawback of the news cycle, but it seems kindof premature to start doing detailed 3D renderings of the 737 cockpit when basically nothing is known except one update to the plane manual. If we just wait a few month, the investigation report will no doubt clarify exactly what the pilots needed to do.

deleted 8y ago

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