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This seems to be the part that everyone's missing. "Oh, this rarely happens" is ambiguous, we don't have any data on how often MCAS operated correctly during f

This seems to be the part that everyone's missing. "Oh, this rarely happens" is ambiguous, we don't have any data on how often MCAS operated correctly during flights. Also, in events that 'rarely happen' one of which is approaching a stall (according to the FAA's report), is it safe to disable the system in the actual scenario when encountered? It doesn't seem so. I can't see how both "Need this system for extremely rare scenario" and "Disabling this system because scenario is extremely rare is okay" can both be true.

tgsovlerkhgsel 6y ago
> I can't see how both "Need this system for extremely rare scenario" and "Disabling this system because scenario is extremely rare is okay" can both be true. The benevolent interpretation is: a) the plane being in a situation where the system is required is rare, but frequent enough that the risk wouldn't be acceptable without MCAS. b) MCAS breaking is rare, but frequent enough that we can't allow it to fly the plane into the ground when it does (as has been demonstrated). c) Both things happening at the same time is so exceedingly rare that a crater is an acceptable outcome, just like we accept that all engines simultaneously failing at the same time during take-off will likely result in a crash. (compare: https://commons.wikimedia.org/wiki/File:FAA_8040.4B_Risk_matrix.svg https://commons.wikimedia.org/wiki/File:FAA_8040.4B_Risk_mat...) That said, I hope EASA takes a very close look instead of blindly trusting the FAA again...
- linuxftw 6y ago
  These are fair points, but the FAA report used the wording "extremely rare" (or similar) instead of a concrete quantity, and I find that extremely reprehensible. As we all know, "extremely rare" needs to be quantified. There are 44k flights in the US alone every day (pre COVID). > MCAS breaking is rare This is the part I'm not particularly inclined to agree with. The scenario is no longer breaking, it's breaking OR disabling. There are a host of new conditions that will result in MCAS being disabled according to the report. Some of these conditions disable MCAS for the remainder of the flight, and the way that I read the scenarios, it's certainly possible for these conditions to stack together. Reading the report gives the impression that the FAA is completely incompetent. They didn't specify any rate of failure or disabling of MCAS, and the 'failure scenarios' that are proposed to be carried out in the near future are lacking in exposing these disabled scenarios. Also, the FAA stated in the report that the plane handles similar to the 737 NG when STS is disabled; however, the MCAS automation disables MCAS and STS. There is no information about the rate at which STS has been disabled in flights in NGs, and no comparison on how disabling STS during the 'near stall' portion of a 'rare event' behaves in those planes. They wrote this long-winded document that's almost entirely devoid of meaningful factual detail. It reads like a Boeing PR piece if I've ever seen one.

tgsovlerkhgsel 6y ago

> I can't see how both "Need this system for extremely rare scenario" and "Disabling this system because scenario is extremely rare is okay" can both be true. The benevolent interpretation is: a) the plane being in a situation where the system is required is rare, but frequent enough that the risk wouldn't be acceptable without MCAS. b) MCAS breaking is rare, but frequent enough that we can't allow it to fly the plane into the ground when it does (as has been demonstrated). c) Both things happening at the same time is so exceedingly rare that a crater is an acceptable outcome, just like we accept that all engines simultaneously failing at the same time during take-off will likely result in a crash. (compare: https://commons.wikimedia.org/wiki/File:FAA_8040.4B_Risk_matrix.svg https://commons.wikimedia.org/wiki/File:FAA_8040.4B_Risk_mat...) That said, I hope EASA takes a very close look instead of blindly trusting the FAA again...

linuxftw 6y ago
These are fair points, but the FAA report used the wording "extremely rare" (or similar) instead of a concrete quantity, and I find that extremely reprehensible. As we all know, "extremely rare" needs to be quantified. There are 44k flights in the US alone every day (pre COVID). > MCAS breaking is rare This is the part I'm not particularly inclined to agree with. The scenario is no longer breaking, it's breaking OR disabling. There are a host of new conditions that will result in MCAS being disabled according to the report. Some of these conditions disable MCAS for the remainder of the flight, and the way that I read the scenarios, it's certainly possible for these conditions to stack together. Reading the report gives the impression that the FAA is completely incompetent. They didn't specify any rate of failure or disabling of MCAS, and the 'failure scenarios' that are proposed to be carried out in the near future are lacking in exposing these disabled scenarios. Also, the FAA stated in the report that the plane handles similar to the 737 NG when STS is disabled; however, the MCAS automation disables MCAS and STS. There is no information about the rate at which STS has been disabled in flights in NGs, and no comparison on how disabling STS during the 'near stall' portion of a 'rare event' behaves in those planes. They wrote this long-winded document that's almost entirely devoid of meaningful factual detail. It reads like a Boeing PR piece if I've ever seen one.