4 ms·

The MCAS is designed to prevent an aerodynamic stall; a situation where the plane is pointed too far up and as a result starts heading straight for the ground.

The MCAS is designed to prevent an aerodynamic stall; a situation where the plane is pointed too far up and as a result starts heading straight for the ground. It does do by detecting the plane pointing too far up, and pushing it back down. If it did not function when the plane was descending rapidly it would not be fit for its designed purpose.

deleted 8y ago
[deleted]
AceyMan 8y ago
> The MCAS is designed to prevent an aerodynamic stall [...] This has become a trope and is flatly incorrect. MCAS was cooked up to maintain the handling characteristics required by the certification specifications. The standard requires stick resistance to increase as the critical (stall) angle of attack is approached. The MAX violated that requirement due to the aerodynamics of the new engine cowls, so MCAS was Boeing's (?crap) implementation of "artificial feel" to meet the spec. Yes, while the spec exists presumably to prevent the pilots from themselves taking the ship into a stall, MCAS doesn't change wing or aircraft performance in a material way that would be meant by "prevent a stall."
- linuxftw 8y ago
  There seem to be a handful of people spreading this talking point here. In every authoritative source, MCAS is described as anti-stall system.
  - AceyMan 8y ago
    I believe the critical passage is here in CFR 14 §25.175¹ (2) With the landing gear retracted at low speed, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 VSR1, nor speeds greater than the minimum speed of the applicable speed range prescribed in paragraph (b)(1), nor speeds that require a stick force of more than 50 pounds)[...] The crucial phrase is "stick force curve", which is the characteristic MCAS was created to tweak. As far as 'authoritative sources' go, I'll just mention the Gell-Mann amnesia effect.² (me: non-active aircraft dispatcher) ¹—https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se14.1.25_1175 https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se1... ²—https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect?wprov=sfla1 https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect?wprov...
    - linuxftw 8y ago
      > The crucial phrase is "stick force curve", which is the characteristic MCAS was created to tweak. How do you know this? Haven't seen that reported anywhere but in user comments here.
  - salawat 8y ago
    Define authoritative source. When speaking to the uninitiated, anti-stall system gets the point across. When speaking reguspeak, and actual aerodynamics, MCAS is a compensatory system that is meant to allow the plane to be certified airworthy. It is the same class of system as a mechanical stick-pusher (though even less effective since all it does is changes the yoke pressure required to bring the plane to a stall). It's not a meme sadly, which makes this all the more tragic in hindsight. Sibling poster has quoted the CFR.
    - linuxftw 8y ago
      > MCAS is a compensatory system that is meant to allow the plane to be certified airworthy Where did you get this information? My understanding is the airframe has the potential to nose up due to extra lift provided by engine placement (eg, lift not commanded by stick pull). MCAS is used to prevent this stall condition during high AoA.
      - salawat 8y ago
        From an understanding of control systems. One of the most important things to get right with a control system is to maintain some level of consistency throughout the entire range of the system you're actuating. Get on your computer, and move your mouse around. Now go into the settings and muck with your mouse sensitivity/acceleration curves, and try to drop your cursor on a particular file. Odds are, you'll have some degree of difficulty until your brain adjusts to the new settings. Same thing is relevant with airplane yokes. The force required to deflect a control surface (and therefore the plane) in an airstream is a result of the forces imparted on the surface + plane while by the impinged airstream. Now, a force response curve should be smooth and predictable. Some distance of deflection on the yoke at one position should yield approximately the same level of deflection from the yoke at another position. If there is a sudden change in the number of degrees of AoA you get per degree of deflection, this is a highly undesirable flight characteristic. For a pilot's perspective, I recommend the D.P. Davies Interview Podcast from the Royal Aeronautics Society, specifically the Boeing episode where he describes his certification flights of the Boeing 727-300. That plane had a similar interaction with high lift devices at high AoA, making the plane more eager to pitch into a stall when you got to a certain point AoA-wise. This was in direct violation of this CFR. As Davies puts it, "You can't certify a plane that wants to stall itself!" The 727 was eventually certified though when they added a mechanical stick pusher which would shove the stick forward when the pilot was getting close to that flight regime to make sure they didn't stall. MCAS, by all indications, serves a similar purpose, it detects current AoA and trims the plane to counter the extra lift so that the deflection of the elevator at the top of the AoA curve has to fight the nose-down trim, which to a pilot would feel like the plane takes the same amount of force to deflect for those last few degrees. Basically, to use the mouse analogy, it's bumping down the sensitivity of the yoke to compensate for the extra sensitivity created by the lift at high angles of attack. All of this is fine and dandy at 17 or so degrees AoA. Not so much though in attempted level flight with a busted AoA sensor. Much of the art behind control systems comes from translating technological activity into humanly processable control schemes. That CFR is a common sense guideline basically specifying that a transport plane must have a predictable response curve. It reminds me of the way nuclear reactors work, which kinda made it click. There they make criticality dependent on neutrons donated by fission byproduct decay, allowing for control rod situations, and thereby criticality level changes to take place over minutes instead of seconds. Plus, look at the name Maneuvering Characteristics Augmentation System. The important takeaway is it doesn't really do squat to make stalls not happen beyond making it increasingly impossible for the pilot to get enough oomph out of the control surfaces to get the plane in stall and stay there. I don't know if there is also a stick pusher for MAX aircraft to help recover from a stall.

deleted 8y ago

[deleted]

AceyMan 8y ago

> The MCAS is designed to prevent an aerodynamic stall [...] This has become a trope and is flatly incorrect. MCAS was cooked up to maintain the handling characteristics required by the certification specifications. The standard requires stick resistance to increase as the critical (stall) angle of attack is approached. The MAX violated that requirement due to the aerodynamics of the new engine cowls, so MCAS was Boeing's (?crap) implementation of "artificial feel" to meet the spec. Yes, while the spec exists presumably to prevent the pilots from themselves taking the ship into a stall, MCAS doesn't change wing or aircraft performance in a material way that would be meant by "prevent a stall."

linuxftw 8y ago
There seem to be a handful of people spreading this talking point here. In every authoritative source, MCAS is described as anti-stall system.
- AceyMan 8y ago
  I believe the critical passage is here in CFR 14 §25.175¹ (2) With the landing gear retracted at low speed, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 VSR1, nor speeds greater than the minimum speed of the applicable speed range prescribed in paragraph (b)(1), nor speeds that require a stick force of more than 50 pounds)[...] The crucial phrase is "stick force curve", which is the characteristic MCAS was created to tweak. As far as 'authoritative sources' go, I'll just mention the Gell-Mann amnesia effect.² (me: non-active aircraft dispatcher) ¹—https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se14.1.25_1175 https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se1... ²—https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect?wprov=sfla1 https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect?wprov...
  - linuxftw 8y ago
    > The crucial phrase is "stick force curve", which is the characteristic MCAS was created to tweak. How do you know this? Haven't seen that reported anywhere but in user comments here.
- salawat 8y ago
  Define authoritative source. When speaking to the uninitiated, anti-stall system gets the point across. When speaking reguspeak, and actual aerodynamics, MCAS is a compensatory system that is meant to allow the plane to be certified airworthy. It is the same class of system as a mechanical stick-pusher (though even less effective since all it does is changes the yoke pressure required to bring the plane to a stall). It's not a meme sadly, which makes this all the more tragic in hindsight. Sibling poster has quoted the CFR.
  - linuxftw 8y ago
    > MCAS is a compensatory system that is meant to allow the plane to be certified airworthy Where did you get this information? My understanding is the airframe has the potential to nose up due to extra lift provided by engine placement (eg, lift not commanded by stick pull). MCAS is used to prevent this stall condition during high AoA.
    - salawat 8y ago
      From an understanding of control systems. One of the most important things to get right with a control system is to maintain some level of consistency throughout the entire range of the system you're actuating. Get on your computer, and move your mouse around. Now go into the settings and muck with your mouse sensitivity/acceleration curves, and try to drop your cursor on a particular file. Odds are, you'll have some degree of difficulty until your brain adjusts to the new settings. Same thing is relevant with airplane yokes. The force required to deflect a control surface (and therefore the plane) in an airstream is a result of the forces imparted on the surface + plane while by the impinged airstream. Now, a force response curve should be smooth and predictable. Some distance of deflection on the yoke at one position should yield approximately the same level of deflection from the yoke at another position. If there is a sudden change in the number of degrees of AoA you get per degree of deflection, this is a highly undesirable flight characteristic. For a pilot's perspective, I recommend the D.P. Davies Interview Podcast from the Royal Aeronautics Society, specifically the Boeing episode where he describes his certification flights of the Boeing 727-300. That plane had a similar interaction with high lift devices at high AoA, making the plane more eager to pitch into a stall when you got to a certain point AoA-wise. This was in direct violation of this CFR. As Davies puts it, "You can't certify a plane that wants to stall itself!" The 727 was eventually certified though when they added a mechanical stick pusher which would shove the stick forward when the pilot was getting close to that flight regime to make sure they didn't stall. MCAS, by all indications, serves a similar purpose, it detects current AoA and trims the plane to counter the extra lift so that the deflection of the elevator at the top of the AoA curve has to fight the nose-down trim, which to a pilot would feel like the plane takes the same amount of force to deflect for those last few degrees. Basically, to use the mouse analogy, it's bumping down the sensitivity of the yoke to compensate for the extra sensitivity created by the lift at high angles of attack. All of this is fine and dandy at 17 or so degrees AoA. Not so much though in attempted level flight with a busted AoA sensor. Much of the art behind control systems comes from translating technological activity into humanly processable control schemes. That CFR is a common sense guideline basically specifying that a transport plane must have a predictable response curve. It reminds me of the way nuclear reactors work, which kinda made it click. There they make criticality dependent on neutrons donated by fission byproduct decay, allowing for control rod situations, and thereby criticality level changes to take place over minutes instead of seconds. Plus, look at the name Maneuvering Characteristics Augmentation System. The important takeaway is it doesn't really do squat to make stalls not happen beyond making it increasingly impossible for the pilot to get enough oomph out of the control surfaces to get the plane in stall and stay there. I don't know if there is also a stick pusher for MAX aircraft to help recover from a stall.