5 ms·

Airspeed is required for safe flight. The failure on that flight was detected immediately, it just couldn’t be handled. AoA on a 737 MAX is not required for saf

Airspeed is required for safe flight. The failure on that flight was detected immediately, it just couldn’t be handled. AoA on a 737 MAX is not required for safe flight and the system just needs to refrain from taking any action if it fails.

phkahler 7y ago
But MCAS was added because the plane doesn't handle well in some situations.
- Filligree 7y ago
  It handles differently, yes. That could be handled with a bit of training.
  - anticensor 7y ago
    It will not satisfy the current handling requirements, hence either making it completely unairworthy or a special category with vastly different handling from usual passenger-rated airplanes.
- cameldrv 7y ago
  I haven't heard of it ever activating except in the incident/accident flights. It's required for certification, but you would either have to be mishandling the plane or get in some extreme weather for MCAS to activate. Think about it like the Antilock brakes on your car. Suppose the wheel position sensor fails. It's fine if the car puts up a warning light and says that you don't have antilock brakes anymore. You can drive fine without them until you can get them fixed with a minor safety impact. It's not fine if the wheel position sensor fails and this causes the car to slam on the brakes going 65mph down the highway.
  - inferiorhuman 7y ago
    I haven't heard of it ever activating except in the incident/accident flights. You wouldn't hear about it unless the activation was triggered by egregious pilot error and you're scouring aviation news sites. It's not fine if the wheel position sensor fails and this causes the car to slam on the brakes going 65mph down the highway. Been there, done that. It's an unpleasant failure mode, but it is survivable.
  - CydeWeys 7y ago
    ABS isn't the best example because it does prevent lots of accidents in its own right, including a >50% prevention rate of some types of accidents in rainy, snowy, or icy weather. The overall fatal accident reduction is 15% for cars and 27% for trucks and light trucks. Source: https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/811182 https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/... And, anecdotally, I've had ABS kick in in some occurrences for which I was very thankful.
    - perl4ever 7y ago
      The first thing I read in your link seems to be the opposite of what you wrote: "ABS has close to a zero net effect on fatal crash involvements." Further down, there is a chart which seems to show that ABS is associated with a huge increase in fatal accidents in inclement weather, and a large increase in side impact accidents, both fatal and non-fatal. Maybe there's something obvious I'm missing?
    - cameldrv 7y ago
      Having a safety system that rarely fails to prevent an accident is much different statistically than a safety system that rarely causes an accident. Suppose the crash probability on a normal flight is 1/1E7, but without MCAS it's 100x more dangerous, or 1/1E5. Suppose MCAS failure probability is 1/1E6, the probability of an additional crash due to the failure of MCAS is 1/1E11, which is acceptable. The problem is that in practice, the crash probability if MCAS fails is empirically 2/3 instead of 1/1E5, because MCAS actually causes the crash rather than merely failing to prevent a crash.
- volkl48 7y ago
  Doesn't handle well is not equivalent to "is uncontrollable", though.
  - phkahler 7y ago
    If I recall, it made it more prone to stall in certain conditions. I'd like to call that hazardous, but without test pilot feedback I'm not sure how to classify it. My recollection from articles a few months ago is that they added MCAS not due to simple differences in handling, but due to a potentially hazardous difference in handling.
    - darkpuma 7y ago
      Mentour Pilot on youtube, a 737 instructor, has a few videos about it. The gist is that in any plane when you're close to stall, you want to pitch down and increase airspeed over the wings. Depending on the precise circumstances you do this by pitching down and throttling up. However in the case of every 737 and in fact any aircraft with engines slung below the wings, the act of throttling up the engines causes the airplane to pitch up. The reason is because the engines are not in line with the center of mass. Imagine rowing a row boat with only a single oar. This is something all 737 pilots are trained for, so they have to balance that pitch up from the engines throttling up by pitching the plane down more than they would otherwise need to. The precise relationship between throttling up and pitching up was changed when the 737 was modified to create the 737 MAX. Namely, on the 737 MAX the pitch-up is more extreme. It is not so extreme that it makes the airplane a bad design, but it is extreme enough that pilots need to not expect the legacy 737 behavior. Had pilots been trained for the performance characteristics of the 737 MAX specifically, it would have been perfectly fine. But they weren't, and instead MCAS was meant to paper over the difference so pilots could be kept in the dark (which is cheaper but borderline homicidal...) To put this another way, in some hypothetical universe where the 737 MAX was the first 737 ever, it would have been introduced without MCAS and pilots would have said it handles like a dream. Then, when the 737 NG was introduced after the 737 MAX, there might have been a reverse-MCAS system implemented to make the NG handle like a MAX. That reverse-MCAS system may have then failed catastrophically.
- roywiggins 7y ago
  It sounds like in normal flight you could cope without it, not easily enough that you'd trust an entire fleet without it, but enough that in the rare case of a detected AoA sensor failure, you could land the thing in a controllable way.
- darkpuma 7y ago
  It handles fine, except it doesn't handle like a Classic/NG 737. This is a problem because pilots were trained to expect it to handle like a Classic/NG. The discrepancy between reality and pilot expectations is the gap MCAS was meant to fill, such that it wasn't necessary to correct pilot expectations.

phkahler 7y ago

But MCAS was added because the plane doesn't handle well in some situations.

Filligree 7y ago
It handles differently, yes. That could be handled with a bit of training.
- anticensor 7y ago
  It will not satisfy the current handling requirements, hence either making it completely unairworthy or a special category with vastly different handling from usual passenger-rated airplanes.
cameldrv 7y ago
I haven't heard of it ever activating except in the incident/accident flights. It's required for certification, but you would either have to be mishandling the plane or get in some extreme weather for MCAS to activate. Think about it like the Antilock brakes on your car. Suppose the wheel position sensor fails. It's fine if the car puts up a warning light and says that you don't have antilock brakes anymore. You can drive fine without them until you can get them fixed with a minor safety impact. It's not fine if the wheel position sensor fails and this causes the car to slam on the brakes going 65mph down the highway.
- inferiorhuman 7y ago
  I haven't heard of it ever activating except in the incident/accident flights. You wouldn't hear about it unless the activation was triggered by egregious pilot error and you're scouring aviation news sites. It's not fine if the wheel position sensor fails and this causes the car to slam on the brakes going 65mph down the highway. Been there, done that. It's an unpleasant failure mode, but it is survivable.
- CydeWeys 7y ago
  ABS isn't the best example because it does prevent lots of accidents in its own right, including a >50% prevention rate of some types of accidents in rainy, snowy, or icy weather. The overall fatal accident reduction is 15% for cars and 27% for trucks and light trucks. Source: https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/811182 https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/... And, anecdotally, I've had ABS kick in in some occurrences for which I was very thankful.
  - perl4ever 7y ago
    The first thing I read in your link seems to be the opposite of what you wrote: "ABS has close to a zero net effect on fatal crash involvements." Further down, there is a chart which seems to show that ABS is associated with a huge increase in fatal accidents in inclement weather, and a large increase in side impact accidents, both fatal and non-fatal. Maybe there's something obvious I'm missing?
  - cameldrv 7y ago
    Having a safety system that rarely fails to prevent an accident is much different statistically than a safety system that rarely causes an accident. Suppose the crash probability on a normal flight is 1/1E7, but without MCAS it's 100x more dangerous, or 1/1E5. Suppose MCAS failure probability is 1/1E6, the probability of an additional crash due to the failure of MCAS is 1/1E11, which is acceptable. The problem is that in practice, the crash probability if MCAS fails is empirically 2/3 instead of 1/1E5, because MCAS actually causes the crash rather than merely failing to prevent a crash.
volkl48 7y ago
Doesn't handle well is not equivalent to "is uncontrollable", though.
- phkahler 7y ago
  If I recall, it made it more prone to stall in certain conditions. I'd like to call that hazardous, but without test pilot feedback I'm not sure how to classify it. My recollection from articles a few months ago is that they added MCAS not due to simple differences in handling, but due to a potentially hazardous difference in handling.
  - darkpuma 7y ago
    Mentour Pilot on youtube, a 737 instructor, has a few videos about it. The gist is that in any plane when you're close to stall, you want to pitch down and increase airspeed over the wings. Depending on the precise circumstances you do this by pitching down and throttling up. However in the case of every 737 and in fact any aircraft with engines slung below the wings, the act of throttling up the engines causes the airplane to pitch up. The reason is because the engines are not in line with the center of mass. Imagine rowing a row boat with only a single oar. This is something all 737 pilots are trained for, so they have to balance that pitch up from the engines throttling up by pitching the plane down more than they would otherwise need to. The precise relationship between throttling up and pitching up was changed when the 737 was modified to create the 737 MAX. Namely, on the 737 MAX the pitch-up is more extreme. It is not so extreme that it makes the airplane a bad design, but it is extreme enough that pilots need to not expect the legacy 737 behavior. Had pilots been trained for the performance characteristics of the 737 MAX specifically, it would have been perfectly fine. But they weren't, and instead MCAS was meant to paper over the difference so pilots could be kept in the dark (which is cheaper but borderline homicidal...) To put this another way, in some hypothetical universe where the 737 MAX was the first 737 ever, it would have been introduced without MCAS and pilots would have said it handles like a dream. Then, when the 737 NG was introduced after the 737 MAX, there might have been a reverse-MCAS system implemented to make the NG handle like a MAX. That reverse-MCAS system may have then failed catastrophically.
roywiggins 7y ago
It sounds like in normal flight you could cope without it, not easily enough that you'd trust an entire fleet without it, but enough that in the rare case of a detected AoA sensor failure, you could land the thing in a controllable way.
darkpuma 7y ago
It handles fine, except it doesn't handle like a Classic/NG 737. This is a problem because pilots were trained to expect it to handle like a Classic/NG. The discrepancy between reality and pilot expectations is the gap MCAS was meant to fill, such that it wasn't necessary to correct pilot expectations.