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MCAS is not there because of maintaining pilot type rating. The plane's airworthiness certificate depends on certain behavior while approaching stall and the n

by janoc 7y ago

MCAS is not there because of maintaining pilot type rating. The plane's airworthiness certificate depends on certain behavior while approaching stall and the new larger and more forward engines on the MAX changed the behavior of the machine in such way that would make it very difficult for the crew to bring the nose down at high angles of attack and high thrust. The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has been added to help with this - to help the crew push the nose down in such situation by using the stabilizer trim. The older NG doesn't have this issue because the engines are both smaller and more to the back, so the pitch-up moment is smaller, even though it is still present. This is an inherent issue with underslung engines, all planes with engines mounted under and ahead of the CoG have this issue. >If it is the case that 737 MAX is not actually airworthy without a functioning MCAS then Boeing has fucked up an order of magnitude worse than even they appear to have at this point. That's BS. By that logic no Airbus or any fly-by-wire plane could ever be certified - they actually require the computer augmentations to fly. Even the 737 NG has a similar trim system that automatically trims the plane depending on speed, angle of attack and what not.

VBprogrammer 7y ago
> The engines on the MAX are forward of the CoG It doesn't make much difference, most of what you've said is accurate to the best of my knowledge, however the thrust can be as far forwards of the CoG as it likes, that doesn't increase the pitch up moment. Having them lower may increase that moment and them having more power too. But the main issue on the Max is airodynamic lift generated by the nacelles at high AoA.
- acqq 7y ago
  > the main issue on the Max is airodynamic lift generated by the nacelles at high AoA. This simplifies the physics so much that it's wrong. It is true that there are texts attributing everything just to the "nacelles", but the engineering doesn't work the way you argue: First, you can't put the engines on the plane without some casing for them (you have to have nacelles). Second, if you'd try to modify the casing of the engines (the said nacelles) to have other aerodynamic properties, you'd interfere with other design goals (e.g. you can increase the fuel consumption which was the most important design goal for the MAX). Third, the position of the engines definitely does matter for the behavior like "nose up" as long as the rest of the plane has some already determined form, which in this case is what has to be kept to have the plane being 737 and not something completely different. In short: the 737 MAX is Boeing's equivalent of VW's diesel cars. Its behavior was determined by the prescribed design goals (by those who sold the plane): Boeing simply directly lied, selling something that can't be achieved given the "sold" design characteristics. Just like the diesel cars can reduce nitrogen oxides production, but then they are less "efficient" than as they were sold (they need more refills, or use more fuel, i.e. cost more than advertised). Likewise, the 737 MAX-like plane could have been safer, but then Boeing could have not sold it as a plane for which there's "no need for pilot's re-certification and retraining." At the end, Feynman conclusion of his Shuttle investigation report holds: "For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." Note also: https://www.theguardian.com/business/2019/apr/15/former-head-of-volkswagen-could-face-10-years-in-prison https://www.theguardian.com/business/2019/apr/15/former-head... ("Former head of Volkswagen could face 10 years in prison -- Martin Winterkorn and other executives charged with fraud over emissions test cheating") I'm quite sure Boeing's "heads" won't even be charged. And no, the "nacelles" aren't the main issue. It's the fraudulent selling of the planes which can't deliver what they were sold for. Fraud. (P.S. It's "aerodynamic")
  - JudgeWapner 7y ago
    I wonder if thrust vectoring could be of use here? I know it's primarily a military technology now, but maybe the MAX can be saved by having engines that slightly divert their thrust in very specific situations to offset this undesirable effect?
    - zaroth 7y ago
      For this to be stable it would seem very elegant if you could design this in such a way that it occurred in equilibrium with any undesirable lift generated by the nascelles. As supposed to an active measure controlled by electronics based on a separate AoA sensor, if somehow the lift caused the engine the thrust vector in opposition to the lift as a matter of physics. I mean, in theory there’s nothing to say inherently such a design would be more reliable than triple redundant sensors and a bunch of code, but that would be pretty neat.
- zepearl 7y ago
  >But the main issue on the Max is airodynamic lift generated by the nacelles at high AoA. I agree. I did not find the original article I read about it but e.g. this article ( https://moneymaven.io/mishtalk/economics/boeing-737-max-unsafe-to-fly-new-scathing-report-by-pilot-and-software-designer-ed5jwi2s8kCuGTAgFXR0GA/ https://moneymaven.io/mishtalk/economics/boeing-737-max-unsa... ) contains a statement similar to what I read - a bit more aggressive and I did not actually read the article and I do not know the website (so yeah, it might be a bad and unconfirmed source). Anyway, just focusing only on the nacelles and AoA here is the statement: >>because the engine nacelles were so far in front of the wing and so large, a power increase will cause them to actually produce lift, particularly at high angles of attack. So the nacelles make a bad problem worse. >>I’ll say it again: In the 737 Max, the engine nacelles themselves can, at high angles of attack, work as a wing and produce lift. And the lift they produce is well ahead of the wing’s center of lift, meaning the nacelles will cause the 737 Max at a high angle of attack to go to a higher angle of attack. Summarized, I understand this as a looping chain reaction: the higher the AoA the higher the lift produced by the nacelles, which in turn (without auto-trim by MCAS or active pilot correction) will increase the AoA which in turn will increase the lift, etc... .
  - zepearl 7y ago
    I admit of not being able to understand this: >>>because the engine nacelles were so far in front of the wing and so large, a power increase will cause them to actually produce lift, I thought that it would make the plane "tilt" upwards, but the quote mentions "lift". Does this happen because the engines, being in front of the wing and as well very near to it are "sucking" air that would otherwise flow as well >>over<< the wing, therefore creating a lower-than-usual pressure above the wing, therefore making the airplane climb/generate lift?
    - mannykannot 7y ago
      It has nothing to do with power. Even cylindrical bodies produce some lift when their axis is not aligned with the airflow, and all engine nacelles do this. If they are forward of the CofG, this produces a pitch-up moment that increases with increasing angle of attack, and therefore reduces stability. The further forward they are, the more the leverage and the stronger the effect. With rear mounted engines, the effect produces a nose-down moment, which contributes to the longitudinal stability. See the links in my other post: https://news.ycombinator.com/item?id=19904685 https://news.ycombinator.com/item?id=19904685
lutorm 7y ago
The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has been added to help with this - to help the crew push the nose down in such situation by using the stabilizer trim. The forward-backward position of the engines have nothing to do with their resultand pitch torque. What matters is that they are below the CG. The engines being forward matters because it shifts the center of pressure forward, giving an inherent pitch-up tendency that becomes more pronounced at high angles of attack. And like you say, all planes with underslung engines have this issue to some degree. Maybe the issue with the MAX is severe enough to cause certification problems but the stall recovery procedures for the NG apparently already call for reducing pitch angle before going to full power or you may not be able to get it down.
- mannykannot 7y ago
  The fore/aft position has a lot to do with the AofA-dependent effect, as it results from the approximately cylindrical engine nacelles producing some lift when their axis is at a positive angle of attack (fuselages have a similar effect [1].) On airplanes with rear-mounted engines, this effect on their nacelles increases the longitudinal stability [2]. The high/low position matters for the power-dependent effect, and furthermore, what matters in steady-state is the pitch moment created by misalignment of thrust and drag (when accelerating, the moment around the CofG also matters.) A modest tendency to pitch down when the power is reduced is preferable to the opposite. The issue MCAS was supposed to solve was due to the AofA-dependent effect. That's why it has AofA as an input, and not any measurement or proxy for thrust. [1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stability-part-2/ https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil... [2] https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stability-part-3/ https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stabil...
  - lutorm 7y ago
    Isn't that exactly what I said? Edit: not quite, good point about center of drag in steady state conditions.
mimixco 7y ago
The Boeing whistleblower in the recent 60 Minutes Australia exposé said that's exactly why it was designed. Not only was a new type rating unacceptable, engineers were not permitted to make any design changes which would require simulator training.
- deleted 7y ago
  [deleted]
Animats 7y ago
Fly by wire systems are classified as safety critical and require much more redundancy. The Boeing 777 has three flight control computers, four sets of low-level controllers, and a fault-tolerant air data sensor system. Bad air data (airspeed, altitude, angle of attack) has been a big problem in the past. Each of those involves a sensor that sticks out through the skin, and can potentially be damaged or plugged. Remember Air France 447, where the airspeed sensors iced up. Those things need redundancy.
- inferiorhuman 7y ago
  The Boeing 777 has three flight control computers And still only two alpha vanes.
rconti 7y ago
>The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has been added to help with this - to help the crew push the nose down in such situation by using the stabilizer trim. And the pilots canot push the nose down in such a situation by actuating the stabilizers? I've never flown an airplane, but as far as I know, MCAS doesn't have unique access to controls that the pilot does not. (eg, it's different from ABS in a car where the driver does NOT have 4 individual brake levers). It seems to me if the pilots are trained and certified on the Max, they'll do a better job flying it than a poorly designed system. OTOH, the system is also fixable, but it is becoming a matter of trust.
amluto 7y ago
> That's BS. By that logic no Airbus or any fly-by-wire plane could ever be certified - they actually require the computer augmentations to fly. I would imagine that, to be certified, the Airbus planes had to demonstrate that: a) The computer augmentation was unlikely to fail, b) when failed, the plane was still flyable enough, and c) the pilots were trained to fly the plane when the augmentation failed. (I believe this is called “direct law”, and I would be utterly shocked if pilots are not trained to fly the plane in direct law mode.)
- cmurf 7y ago
  They are so trained, and the airplane is still considered airworthy in that mode, including in particular as it related to this conversation, the FAR 25 positive static stability and stall behavior requirements.
  - amluto 7y ago
    Is the 737 MAX’s proposed behavior with one AoA sensor broken also compliant?
cmurf 7y ago
This is completely wrong. An Airbus is not made conformant with FAR 25 airworthiness standard by software protections. It conforms to all of FAR 25 requirements without any of the safeguards in place, whereby stick inputs are directly translated to control surfaces. The airworthiness standards are predicated on aerodynamics, not software abstraction of an otherwise unairworthy design. The idea there are switches in the airplane I can use to make the airplane unairworthy is absurd. And it is equally absurd that the aircraft flight manual and an emergency airworthiness directive would so direct a pilot to make the plane less airworthy than it already is, let alone unairworthy, as a corrective action. It's just absurd. You're welcome to provide a citation, however. I submit FAR 25. I've read the entire thing and there's nothing in there that permits software to paper over an otherwise unairthworthy aircraft design.

by janoc 7y ago

VBprogrammer 7y ago

> The engines on the MAX are forward of the CoG It doesn't make much difference, most of what you've said is accurate to the best of my knowledge, however the thrust can be as far forwards of the CoG as it likes, that doesn't increase the pitch up moment. Having them lower may increase that moment and them having more power too. But the main issue on the Max is airodynamic lift generated by the nacelles at high AoA.

acqq 7y ago
> the main issue on the Max is airodynamic lift generated by the nacelles at high AoA. This simplifies the physics so much that it's wrong. It is true that there are texts attributing everything just to the "nacelles", but the engineering doesn't work the way you argue: First, you can't put the engines on the plane without some casing for them (you have to have nacelles). Second, if you'd try to modify the casing of the engines (the said nacelles) to have other aerodynamic properties, you'd interfere with other design goals (e.g. you can increase the fuel consumption which was the most important design goal for the MAX). Third, the position of the engines definitely does matter for the behavior like "nose up" as long as the rest of the plane has some already determined form, which in this case is what has to be kept to have the plane being 737 and not something completely different. In short: the 737 MAX is Boeing's equivalent of VW's diesel cars. Its behavior was determined by the prescribed design goals (by those who sold the plane): Boeing simply directly lied, selling something that can't be achieved given the "sold" design characteristics. Just like the diesel cars can reduce nitrogen oxides production, but then they are less "efficient" than as they were sold (they need more refills, or use more fuel, i.e. cost more than advertised). Likewise, the 737 MAX-like plane could have been safer, but then Boeing could have not sold it as a plane for which there's "no need for pilot's re-certification and retraining." At the end, Feynman conclusion of his Shuttle investigation report holds: "For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." Note also: https://www.theguardian.com/business/2019/apr/15/former-head-of-volkswagen-could-face-10-years-in-prison https://www.theguardian.com/business/2019/apr/15/former-head... ("Former head of Volkswagen could face 10 years in prison -- Martin Winterkorn and other executives charged with fraud over emissions test cheating") I'm quite sure Boeing's "heads" won't even be charged. And no, the "nacelles" aren't the main issue. It's the fraudulent selling of the planes which can't deliver what they were sold for. Fraud. (P.S. It's "aerodynamic")
- JudgeWapner 7y ago
  I wonder if thrust vectoring could be of use here? I know it's primarily a military technology now, but maybe the MAX can be saved by having engines that slightly divert their thrust in very specific situations to offset this undesirable effect?
  - zaroth 7y ago
    For this to be stable it would seem very elegant if you could design this in such a way that it occurred in equilibrium with any undesirable lift generated by the nascelles. As supposed to an active measure controlled by electronics based on a separate AoA sensor, if somehow the lift caused the engine the thrust vector in opposition to the lift as a matter of physics. I mean, in theory there’s nothing to say inherently such a design would be more reliable than triple redundant sensors and a bunch of code, but that would be pretty neat.
zepearl 7y ago
>But the main issue on the Max is airodynamic lift generated by the nacelles at high AoA. I agree. I did not find the original article I read about it but e.g. this article ( https://moneymaven.io/mishtalk/economics/boeing-737-max-unsafe-to-fly-new-scathing-report-by-pilot-and-software-designer-ed5jwi2s8kCuGTAgFXR0GA/ https://moneymaven.io/mishtalk/economics/boeing-737-max-unsa... ) contains a statement similar to what I read - a bit more aggressive and I did not actually read the article and I do not know the website (so yeah, it might be a bad and unconfirmed source). Anyway, just focusing only on the nacelles and AoA here is the statement: >>because the engine nacelles were so far in front of the wing and so large, a power increase will cause them to actually produce lift, particularly at high angles of attack. So the nacelles make a bad problem worse. >>I’ll say it again: In the 737 Max, the engine nacelles themselves can, at high angles of attack, work as a wing and produce lift. And the lift they produce is well ahead of the wing’s center of lift, meaning the nacelles will cause the 737 Max at a high angle of attack to go to a higher angle of attack. Summarized, I understand this as a looping chain reaction: the higher the AoA the higher the lift produced by the nacelles, which in turn (without auto-trim by MCAS or active pilot correction) will increase the AoA which in turn will increase the lift, etc... .
- zepearl 7y ago
  I admit of not being able to understand this: >>>because the engine nacelles were so far in front of the wing and so large, a power increase will cause them to actually produce lift, I thought that it would make the plane "tilt" upwards, but the quote mentions "lift". Does this happen because the engines, being in front of the wing and as well very near to it are "sucking" air that would otherwise flow as well >>over<< the wing, therefore creating a lower-than-usual pressure above the wing, therefore making the airplane climb/generate lift?
  - mannykannot 7y ago
    It has nothing to do with power. Even cylindrical bodies produce some lift when their axis is not aligned with the airflow, and all engine nacelles do this. If they are forward of the CofG, this produces a pitch-up moment that increases with increasing angle of attack, and therefore reduces stability. The further forward they are, the more the leverage and the stronger the effect. With rear mounted engines, the effect produces a nose-down moment, which contributes to the longitudinal stability. See the links in my other post: https://news.ycombinator.com/item?id=19904685 https://news.ycombinator.com/item?id=19904685

lutorm 7y ago

The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has been added to help with this - to help the crew push the nose down in such situation by using the stabilizer trim. The forward-backward position of the engines have nothing to do with their resultand pitch torque. What matters is that they are below the CG. The engines being forward matters because it shifts the center of pressure forward, giving an inherent pitch-up tendency that becomes more pronounced at high angles of attack. And like you say, all planes with underslung engines have this issue to some degree. Maybe the issue with the MAX is severe enough to cause certification problems but the stall recovery procedures for the NG apparently already call for reducing pitch angle before going to full power or you may not be able to get it down.

mannykannot 7y ago
The fore/aft position has a lot to do with the AofA-dependent effect, as it results from the approximately cylindrical engine nacelles producing some lift when their axis is at a positive angle of attack (fuselages have a similar effect [1].) On airplanes with rear-mounted engines, this effect on their nacelles increases the longitudinal stability [2]. The high/low position matters for the power-dependent effect, and furthermore, what matters in steady-state is the pitch moment created by misalignment of thrust and drag (when accelerating, the moment around the CofG also matters.) A modest tendency to pitch down when the power is reduced is preferable to the opposite. The issue MCAS was supposed to solve was due to the AofA-dependent effect. That's why it has AofA as an input, and not any measurement or proxy for thrust. [1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stability-part-2/ https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil... [2] https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stability-part-3/ https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stabil...
- lutorm 7y ago
  Isn't that exactly what I said? Edit: not quite, good point about center of drag in steady state conditions.

mimixco 7y ago

The Boeing whistleblower in the recent 60 Minutes Australia exposé said that's exactly why it was designed. Not only was a new type rating unacceptable, engineers were not permitted to make any design changes which would require simulator training.

deleted 7y ago
[deleted]

Animats 7y ago

Fly by wire systems are classified as safety critical and require much more redundancy. The Boeing 777 has three flight control computers, four sets of low-level controllers, and a fault-tolerant air data sensor system. Bad air data (airspeed, altitude, angle of attack) has been a big problem in the past. Each of those involves a sensor that sticks out through the skin, and can potentially be damaged or plugged. Remember Air France 447, where the airspeed sensors iced up. Those things need redundancy.

inferiorhuman 7y ago
The Boeing 777 has three flight control computers And still only two alpha vanes.

rconti 7y ago

>The engines on the MAX are forward of the CoG, so they are pushing the nose up as you add thrust - exactly what you do not want when approach a stall. MCAS has been added to help with this - to help the crew push the nose down in such situation by using the stabilizer trim. And the pilots canot push the nose down in such a situation by actuating the stabilizers? I've never flown an airplane, but as far as I know, MCAS doesn't have unique access to controls that the pilot does not. (eg, it's different from ABS in a car where the driver does NOT have 4 individual brake levers). It seems to me if the pilots are trained and certified on the Max, they'll do a better job flying it than a poorly designed system. OTOH, the system is also fixable, but it is becoming a matter of trust.

amluto 7y ago

> That's BS. By that logic no Airbus or any fly-by-wire plane could ever be certified - they actually require the computer augmentations to fly. I would imagine that, to be certified, the Airbus planes had to demonstrate that: a) The computer augmentation was unlikely to fail, b) when failed, the plane was still flyable enough, and c) the pilots were trained to fly the plane when the augmentation failed. (I believe this is called “direct law”, and I would be utterly shocked if pilots are not trained to fly the plane in direct law mode.)

cmurf 7y ago
They are so trained, and the airplane is still considered airworthy in that mode, including in particular as it related to this conversation, the FAR 25 positive static stability and stall behavior requirements.
- amluto 7y ago
  Is the 737 MAX’s proposed behavior with one AoA sensor broken also compliant?

cmurf 7y ago

This is completely wrong. An Airbus is not made conformant with FAR 25 airworthiness standard by software protections. It conforms to all of FAR 25 requirements without any of the safeguards in place, whereby stick inputs are directly translated to control surfaces. The airworthiness standards are predicated on aerodynamics, not software abstraction of an otherwise unairworthy design. The idea there are switches in the airplane I can use to make the airplane unairworthy is absurd. And it is equally absurd that the aircraft flight manual and an emergency airworthiness directive would so direct a pilot to make the plane less airworthy than it already is, let alone unairworthy, as a corrective action. It's just absurd. You're welcome to provide a citation, however. I submit FAR 25. I've read the entire thing and there's nothing in there that permits software to paper over an otherwise unairthworthy aircraft design.