3 ms·

>So the pilot knows what's happening and take corrective action Except that now we have a plane that behaves differently compared to the plane the pilot actual

>So the pilot knows what's happening and take corrective action Except that now we have a plane that behaves differently compared to the plane the pilot actually is trained for (the whole reason to implement MCAS in the first place). So now the plane comes down because it stalls where it should not according to the training of the pilot. Gee, makes me confident to fly...

VBprogrammer 8y ago
Apologies, I keep trying to quash this one but it keeps reappearing. The reason MCAS is needed is because the engine nacelle generates lift at high angles of attack. This meant that there was a lack of positive control required to increase angle of attack. I understand this as if you need to pull with 10KG of force to get a 10 degree angle of attack, then you need to pull with at least 20kg to get to a 20 degree angle of attack. I'm not going to claim that's 100% accurate but I believe it's close enough. This is a generic requirement of part 25. So had the 737 Max been a clean sheet design it would have been a requirement. It's not just a compatibility layer to make it behave the same as old aircraft. It's a fix to make it meet the requirements of certification. Training only came into it at the point that Boeing declined to add it to the difference training, charitably, because they felt the system was covered by existing procedures or, less charitably, because they wanted to downplay any differences to make the aircraft more appealing.
- beefield 8y ago
  No need to apologize, I am happy to be corrected. But that still means that if you disengage MCAS, you have a plane that flies out of certification specs (and by extension, out of the training), then?
  - VBprogrammer 8y ago
    Absolutely. I feel that the motives which led to the situation are quite important though in the greater scheme of things. To phrase it another way, without MCAS the 737 Max didn't meant the requirements of certification and so wouldn't have gone into production. The alternative wasn't to train pilots into how to handle the new stall characteristics. Of course, the region of flight that MCAS was intended to activate in is also a bit of an edge case. Without MCAS we may have seen an aircraft flown into a stall, most likely around a critical phase of flight (landing or a go-around), but that would have involved a pilot ignoring numerous warnings (stick shaker activation, 'stall stall' oral alerts etc) and you'd be better categorised as pilot error than directly related to the handling characteristics. Unfortunately, due to a terrible system design, MCAS appears to be activating in regions of flight outside of the design intent and with greater authority than it should ever have been given.
  - mannykannot 8y ago
    I guess that, for systems that are not considered critical for safety, disabling them if they are failing in a way that creates additional problems is the better option. This calculus depends on how useful they are when working, how much trouble they cause when failing, and how likely they are to fail in that way. In some cases, I imagine the option to disable comes with the requirement to land as soon as safely possible, or to not fly again until fixed.
- mannykannot 8y ago
  This is precisely the issue, as far as I know, but I have had this tangential question tugging at me: The 737 already has an Elevator Feel and Centering Unit which "provides simulated aerodynamic forces using airspeed (from the elevator pitot system) and stabilizer position" [1]. I wonder why, if the issue is stick forces, this would not be the place to make a change. The answer is probably in the details of the system, but I also wonder if the reasoning was in part that, by changing the trim, the MCAS system is simultaneously and proactively reducing the possibility of a stall. [1] https://www.737ng.co.uk/B_NG-Flight_Controls.pdf https://www.737ng.co.uk/B_NG-Flight_Controls.pdf page 8.
  - VBprogrammer 8y ago
    I've asked myself the same question. And the only plausible answer I can come up with is that the engineers working on the 737 Max view it the same way I view a lot of legacy code. They barely understand how it works; are kind of surprised that it does work; and have no inclination to try to change it.

VBprogrammer 8y ago

Apologies, I keep trying to quash this one but it keeps reappearing. The reason MCAS is needed is because the engine nacelle generates lift at high angles of attack. This meant that there was a lack of positive control required to increase angle of attack. I understand this as if you need to pull with 10KG of force to get a 10 degree angle of attack, then you need to pull with at least 20kg to get to a 20 degree angle of attack. I'm not going to claim that's 100% accurate but I believe it's close enough. This is a generic requirement of part 25. So had the 737 Max been a clean sheet design it would have been a requirement. It's not just a compatibility layer to make it behave the same as old aircraft. It's a fix to make it meet the requirements of certification. Training only came into it at the point that Boeing declined to add it to the difference training, charitably, because they felt the system was covered by existing procedures or, less charitably, because they wanted to downplay any differences to make the aircraft more appealing.

beefield 8y ago
No need to apologize, I am happy to be corrected. But that still means that if you disengage MCAS, you have a plane that flies out of certification specs (and by extension, out of the training), then?
- VBprogrammer 8y ago
  Absolutely. I feel that the motives which led to the situation are quite important though in the greater scheme of things. To phrase it another way, without MCAS the 737 Max didn't meant the requirements of certification and so wouldn't have gone into production. The alternative wasn't to train pilots into how to handle the new stall characteristics. Of course, the region of flight that MCAS was intended to activate in is also a bit of an edge case. Without MCAS we may have seen an aircraft flown into a stall, most likely around a critical phase of flight (landing or a go-around), but that would have involved a pilot ignoring numerous warnings (stick shaker activation, 'stall stall' oral alerts etc) and you'd be better categorised as pilot error than directly related to the handling characteristics. Unfortunately, due to a terrible system design, MCAS appears to be activating in regions of flight outside of the design intent and with greater authority than it should ever have been given.
- mannykannot 8y ago
  I guess that, for systems that are not considered critical for safety, disabling them if they are failing in a way that creates additional problems is the better option. This calculus depends on how useful they are when working, how much trouble they cause when failing, and how likely they are to fail in that way. In some cases, I imagine the option to disable comes with the requirement to land as soon as safely possible, or to not fly again until fixed.
mannykannot 8y ago
This is precisely the issue, as far as I know, but I have had this tangential question tugging at me: The 737 already has an Elevator Feel and Centering Unit which "provides simulated aerodynamic forces using airspeed (from the elevator pitot system) and stabilizer position" [1]. I wonder why, if the issue is stick forces, this would not be the place to make a change. The answer is probably in the details of the system, but I also wonder if the reasoning was in part that, by changing the trim, the MCAS system is simultaneously and proactively reducing the possibility of a stall. [1] https://www.737ng.co.uk/B_NG-Flight_Controls.pdf https://www.737ng.co.uk/B_NG-Flight_Controls.pdf page 8.
- VBprogrammer 8y ago
  I've asked myself the same question. And the only plausible answer I can come up with is that the engineers working on the 737 Max view it the same way I view a lot of legacy code. They barely understand how it works; are kind of surprised that it does work; and have no inclination to try to change it.