3 ms·

Afaik, the crashes were caused by the software mistakenly believing the plane was in a stall due to a single faulty sensor and pulling down the nose - while the

by hooby 3y ago

Afaik, the crashes were caused by the software mistakenly believing the plane was in a stall due to a single faulty sensor and pulling down the nose - while the pilots had absolutely no clue what was going on (because they never were told the software could do this), and had no way to disable/counteract the software. I believe the updated software now uses both sensors and only acts if both sensors agree (which is actually standard for such systems and should have been done right from the start) - and can be disabled by the pilots, who are now aware of it's function. But please don't trust my word... I'm no aviation export by a long stretch. For a more qualified take on this, done by an actual pilot and pilot trainer, I recommend checking out "Mentour Pilot" on Youtube. He's got a number of videos on the 737 Max and the reported problems at Boeing.

Sohcahtoa82 3y ago
> and had no way to disable/counteract the software. Both pilots pulling up on their yokes couldn't override the system?
- procflora 3y ago
  Another armchair pilot here, also recommending Mentour Pilot's channel, but hopefully I can summarize it well enough. In this case no, because the way the MCAS system operated was to actuate the stabilizer trim which changes the angle of the entire huge tail plane (the horizontal part of the tail). For the most part the yoke controls the elevator (a smaller flap on the trailing edge of the tail plane), so the pitch control authority available in the yoke movement is not enough to override the adjustment to the tail plane trim MCAS was making. Once MCAS moved the trim far enough, you couldn't raise the nose with the yoke alone. To correct it, pilots needed to deactivate the trim adjustment motor ("stab trim cut-out" you might have heard about) and manually re-trim the tail plane by folding out a literal crank and spinning a wheel by their legs in the cockpit[1]. The concept of doing this was not new with the MAX (similar procedures around trim system malfunctions existed), but what was new was that MCAS existed in the first place, that it could repeatedly activate if the erroneous inputs were still present, and that it could move the trim quite far before pilots realized what was happening. As I understand it that is pretty different than a traditional runaway trim malfunction for which they'd had training for. I believe that looks more like a stuck switch making a constant movement of the trim until you hit the cut-out. If you cut it back in, either it immediately starts moving again and you know you have a problem or it doesn't and leaves you wondering. [1](https://www.trbimg.com/img-5d0ba28b/turbine/ct-biz-wp-boeing-737-max-cockpit-controls-20190620-2du2u45m7bdvnmvqhv3dqfxa6a https://www.trbimg.com/img-5d0ba28b/turbine/ct-biz-wp-boeing...)
  - Sohcahtoa82 3y ago
    MCAS uses trim rather than elevator? That seems insane, though I suppose trim has more authority than elevator...but yeah...in the time it takes for the pilot to notice the trim has moved itself to some insane level, they've been nose down for so long that they couldn't possibly recover.

by hooby 3y ago

Sohcahtoa82 3y ago

> and had no way to disable/counteract the software. Both pilots pulling up on their yokes couldn't override the system?

procflora 3y ago
Another armchair pilot here, also recommending Mentour Pilot's channel, but hopefully I can summarize it well enough. In this case no, because the way the MCAS system operated was to actuate the stabilizer trim which changes the angle of the entire huge tail plane (the horizontal part of the tail). For the most part the yoke controls the elevator (a smaller flap on the trailing edge of the tail plane), so the pitch control authority available in the yoke movement is not enough to override the adjustment to the tail plane trim MCAS was making. Once MCAS moved the trim far enough, you couldn't raise the nose with the yoke alone. To correct it, pilots needed to deactivate the trim adjustment motor ("stab trim cut-out" you might have heard about) and manually re-trim the tail plane by folding out a literal crank and spinning a wheel by their legs in the cockpit[1]. The concept of doing this was not new with the MAX (similar procedures around trim system malfunctions existed), but what was new was that MCAS existed in the first place, that it could repeatedly activate if the erroneous inputs were still present, and that it could move the trim quite far before pilots realized what was happening. As I understand it that is pretty different than a traditional runaway trim malfunction for which they'd had training for. I believe that looks more like a stuck switch making a constant movement of the trim until you hit the cut-out. If you cut it back in, either it immediately starts moving again and you know you have a problem or it doesn't and leaves you wondering. [1](https://www.trbimg.com/img-5d0ba28b/turbine/ct-biz-wp-boeing-737-max-cockpit-controls-20190620-2du2u45m7bdvnmvqhv3dqfxa6a https://www.trbimg.com/img-5d0ba28b/turbine/ct-biz-wp-boeing...)
- Sohcahtoa82 3y ago
  MCAS uses trim rather than elevator? That seems insane, though I suppose trim has more authority than elevator...but yeah...in the time it takes for the pilot to notice the trim has moved itself to some insane level, they've been nose down for so long that they couldn't possibly recover.