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My understanding is that it's a bit of both. This Slate article[1] described it as both a design flaw and an effort to paper over that design flaw by tweaking

My understanding is that it's a bit of both. This Slate article[1] described it as both a design flaw and an effort to paper over that design flaw by tweaking the autopilot system. >Boeing swapped out the engines for new models, which, together with airframe tweaks, promised a 20 percent increase in fuel efficiency. In order to accommodate the engine’s larger diameter, Boeing engineers had to move the point where the plane attaches to the wing. This, in turn, affected the way the plane handled. Most alarmingly, it left the plane with a tendency to pitch up, which could result in a dangerous aerodynamic stall. To prevent this, Boeing added a new autopilot system that would pitch the nose down if it looked like it was getting too high. Because of the changes to the autopilot system it functions differently than pilots expect it to during an emergency situation on this particular plane. [1] https://slate.com/technology/2019/03/ethiopian-air-crash-where-did-boeing-go-wrong-with-the-737-max.html https://slate.com/technology/2019/03/ethiopian-air-crash-whe...

msbarnett 8y ago
Yeah, my understanding is essentially, the changes made the Max 8 not behave aerodynamically like other 737s. This would normally mean that pilots would need to requalify on it as a new plane type, in order to legally fly one. But that's more expensive for Boeing's customers, so to make it more attractive, they decided to "paper over" the differences in behaviour with software. Fly it like any other 737, and if it starts to stall in conditions where other 737s wouldn't, the software corrects things! Except, except, like all abstractions, this one's leaky -- and they've only shifted the difference in behaviour from "stalls at angles that would surprise a 737 pilot" to "noses-down in ways that would surprise a 737 pilot" along with "can't be muscle-overridden the way the autopilot on other 737s can be".
- cmurf 8y ago
  It's not the autotrim that you can't fight. The plane can be trimmed (elevator surface) in such a way that pilot yoke input (stabilizer surface) cannot fight against. That is common to all 737's, and all pilots know about it. It is not that the autotrim feature can't be fought. What's new in the MAX is the autotrim doesn't terminate when the pilot gives back pressure on the yoke if MCAS senses high angle of attack, it will do an uncommanded nose down (every I think 10s). This is to compensate for the MAX engine nacelle which provides a meaningful amount of lift to cause a pitch up moment in high angles of attack, unlike previous 737's. I wonder whether the stall behavior between 737's is slightly dissimilar or very dissimilar without MCAS, between 737NG and 737 MAX? And then quantifying that difference. I also seriously wonder if it's even slightly more likely for a pilot to induce a stall as a result of not knowing about the natural uncorrected stall behavior of the new design compared to previous designs. MCAS is not my concern, it's how the plane flies without it that I wonder about. Also, WSJ is reporting that the MCAS software update mandated by the FAA in an airworthiness directive was delayed due to the nearly five week government shutdown. I'm already imagining the tweets...
amyjess 8y ago
> To prevent this, Boeing added a new autopilot system that would pitch the nose down if it looked like it was getting too high. The biggest problem here is that the new autopilot system relies on a pair of Angle of Attack sensors. Those sensors are prone to damage, and if the sensor is damaged, the erroneous information it reports can engage the new autopilot system. One of the reasons why Southwest thinks it's immune to this problem, by the way, is because they paid extra for a system that displays an alert when the Angle of Attack sensors disagree with each other, so pilots are aware of what's wrong and can take action to stop it. Most other carriers, including Ethiopian and Lion Air, didn't. Also, other autopilot systems can be disengaged by pulling back on the stick. This one can't: you have to manually disengage the system. Pilots were never informed of any of this because Boeing wanted to maintain the fiction that the 737 MAX is no different than previous versions of the 737.

msbarnett 8y ago

Yeah, my understanding is essentially, the changes made the Max 8 not behave aerodynamically like other 737s. This would normally mean that pilots would need to requalify on it as a new plane type, in order to legally fly one. But that's more expensive for Boeing's customers, so to make it more attractive, they decided to "paper over" the differences in behaviour with software. Fly it like any other 737, and if it starts to stall in conditions where other 737s wouldn't, the software corrects things! Except, except, like all abstractions, this one's leaky -- and they've only shifted the difference in behaviour from "stalls at angles that would surprise a 737 pilot" to "noses-down in ways that would surprise a 737 pilot" along with "can't be muscle-overridden the way the autopilot on other 737s can be".

cmurf 8y ago
It's not the autotrim that you can't fight. The plane can be trimmed (elevator surface) in such a way that pilot yoke input (stabilizer surface) cannot fight against. That is common to all 737's, and all pilots know about it. It is not that the autotrim feature can't be fought. What's new in the MAX is the autotrim doesn't terminate when the pilot gives back pressure on the yoke if MCAS senses high angle of attack, it will do an uncommanded nose down (every I think 10s). This is to compensate for the MAX engine nacelle which provides a meaningful amount of lift to cause a pitch up moment in high angles of attack, unlike previous 737's. I wonder whether the stall behavior between 737's is slightly dissimilar or very dissimilar without MCAS, between 737NG and 737 MAX? And then quantifying that difference. I also seriously wonder if it's even slightly more likely for a pilot to induce a stall as a result of not knowing about the natural uncorrected stall behavior of the new design compared to previous designs. MCAS is not my concern, it's how the plane flies without it that I wonder about. Also, WSJ is reporting that the MCAS software update mandated by the FAA in an airworthiness directive was delayed due to the nearly five week government shutdown. I'm already imagining the tweets...

amyjess 8y ago

> To prevent this, Boeing added a new autopilot system that would pitch the nose down if it looked like it was getting too high. The biggest problem here is that the new autopilot system relies on a pair of Angle of Attack sensors. Those sensors are prone to damage, and if the sensor is damaged, the erroneous information it reports can engage the new autopilot system. One of the reasons why Southwest thinks it's immune to this problem, by the way, is because they paid extra for a system that displays an alert when the Angle of Attack sensors disagree with each other, so pilots are aware of what's wrong and can take action to stop it. Most other carriers, including Ethiopian and Lion Air, didn't. Also, other autopilot systems can be disengaged by pulling back on the stick. This one can't: you have to manually disengage the system. Pilots were never informed of any of this because Boeing wanted to maintain the fiction that the 737 MAX is no different than previous versions of the 737.