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Why is MCAS needed? If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? The most relevant question is “is the MAX aer

Why is MCAS needed? If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? The most relevant question is “is the MAX aerodynamically stable?” If it is, why do we need the MCAS? That needs to be investigated. If plane at a low altitude at full power is stalling, just lower the nose. That is pilot training 101. Why need a system like MCAS to help. The pilot should be able to disengage the autopilot and take control of the airplane. This should be simple. My suspicion is that there is an inherent, more fundamental problem with the MAX. Before Boeing rushes a software fix, idependendent entities “(remember we can no longer assume the FAA is independent based on what transpired over the last few days) need to investigate and makes sure the plane is safe. I for one will not fly the MAX regardless of what the FAA and Boeing say. Their reputation for prudence and safety is gone.

rosege 8y ago
I agree! After Lion Air I would have been quite hesitant to fly on a Max. Now with all I'm reading - no way would I.
sokoloff 8y ago
The Max has more powerful engines, mounted further forward on the wings (out of geometric necessity), and as a result has an aerodynamic center of lift further forward. From the addition of MCAS, I gather (but don’t know with certainty) that they couldn’t make some certification requirement without MCAS.
VBprogrammer 8y ago
> Why is MCAS needed? If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? The most relevant question is “is the MAX aerodynamically stable?” If it is, why do we need the MCAS? That needs to be investigated. This isn't even a question. The 737 Max is completely aerodynamically stable. It does however exhibit control behaviours which are undesirable. A fair analogy for this is probably a car which oversteers, in general a normal family car is designed to understeer, because for your average driver that is safer. In the 737 Max the controls get lighter close to the stall because of lift generated by the engine nacelle. The certification requirements require that the controls don't get lighter. Boeing applied what now appears to be a poorly thought out fix. What might surprise you is that there are plenty of certified aircraft which are actually aerodynamically unstable, at least along the longitudinal axis. For example the 757 has dual yaw dampers and at least one of them needs to be serviceable before flight. The consensus is that at cruise altitude it would depart controlled flight without one of them working.
- mikejb 8y ago
  I think a fairer car comparison would be this: a new power train is fitted to the car, but it sometimes causes torque steer to the right. To overcome that, the steering wheel gets trimmed to the left when torque steer is detected until torque steer is neutralized. The system detecting the torque steer sometimes has false positive.
- robdachshund 8y ago
  How can a plane be inherently stable while exhibiting wild behaviors that cause the plane to fall out of the sky? Yes, all designs have their quirks and limitations, but I'd say if you need an active system that compensates for the thing not being able to fly without one, it's not a stable design. The airframe is, with the original engines. Not the bigger ones that extend forward and move the center of thrust.
- ajnin 8y ago
  > The certification requirements require Eh, let's not spin it as "the requirements made them do it", they chose to make the new model stick with the existing 737 certification because building a substantially different new plane would require pilots to be retrained, and they didn't want that as Airbus was ahead of them in the development of the A320neo and they needed that commercial advantage to remain competitive.
  - VBprogrammer 8y ago
    I'm not sure where you draw the line though? None of this is new, making modifications to existing designs is the aviation equivalent of developing a new feature. Hanging new engines from an existing fuselage, replacing the avionics and lengthening a fuselage have all been done before with reasonable results. Conversely, clean sheet designs have had terrible safety records initially. Asking a manufacturer to make a clean sheet design every time they make a change is probably going to result in more accidents than it fixes (see the bathtub curve).
- dirkg 8y ago
  > The certification requirements The 737 is self certified by Boeing, which is really a joke. Basically a Boeing engineer can sign off and say LGTM with no oversight or independent audit as I understand.
robdachshund 8y ago
Bigger engines, mounted further forward. Different center of thrust. Apparently it's not stable enough to fly without some fly by wire style compensation. Honestly, while Boeing is at fault for this not working out, they never wanted to re-engine the plane in the first place. They wanted a clean sheet design, but Airbus forced their hand by offering a re-engine of the A320 to American Airlines. That leads me to wonder if the A320neo might have similar problems we haven't seen yet.
philjohn 8y ago
"If plane at a low altitude at full power is stalling, just lower the nose." Except as can be seen time and again basic instinct can kick in "damn, plane is falling, I need to be higher" and pilots have been known to pull back on the stick to get height.
- zaroth 8y ago
  A pilot that deals with a stall by pulling up has failed their training. As I recall, we practiced low speed flight and recovering from a low speed stall either the 2nd or 3rd time I ever went up in a Cessna. Power on stalls were a few days later, they are quite different. I never did get my license, mainly because I experienced moderate nausea / motion sickness which I thought would abate after a dozen flights or so, but never really got over it. The problem isn’t that pilots can’t or shouldn’t be relied upon to detect and recover from stalls or near-stalls by increasing throttle and decreasing pitch. The problem appears to be that a new system, added for the purpose of making a new plane with different handling / characteristics behave the same as an older one for training purposes, is malfunctioning. The plane could be perfectly safe without MCAS but pilots would have had to be recertified.
  - pdonis 8y ago
    > The plane could be perfectly safe without MCAS It depends on what you mean by "perfectly safe". Many people believe that having the yoke effort decrease at higher angles of attack instead of increase is not very safe. That's why the FAA certification requirements force manufacturers to do whatever is necessary to make sure the yoke force increases with increasing angle of attack, so the pilot has to exert more effort to pull up at higher angles of attack. Without MCAS, the 737 MAX as designed would not meet this requirement.
mannykannot 8y ago
The short answer to your first sentence is that MCAS was needed to give acceptable handling characteristics at high angles of attack. the MAX was deemed similar enough to the NG, such that additional training was not thought necessary, only because of the difference MCAS made. More details here: https://leehamnews.com/2019/02/15/bjorns-corner-pitch-stability-part-10-wrap-up/ https://leehamnews.com/2019/02/15/bjorns-corner-pitch-stabil... Stability is a more complicated issue than it might seem, for reasons such as aerodynamic stability being insufficiently damped, and the interaction between roll and yaw. An airplane that is statically stable could still be dynamically unstable, and go into a divergent series of oscillations if not corrected. It is very common for an airplane to be statically stable around all three axes, yet be prone to falling into a spiral dive if not corrected (this is probably what happened to JFK jr.) If you try to increase the stability to fix that, you get an airplane that is susceptible to an oscillation called dutch roll. Most swept-wing aircraft, including airliners, have gyroscopic yaw dampers that operate the rudder to counteract this. Then there's helicopters... All I know about them is that its complicated.
- VBprogrammer 8y ago
  > The short answer to your first sentence is that MCAS was needed in order to make the MAX so similar to the NG versions that it could be claimed that no additional training was needed. I think that places the emphasis in the wrong place in a tabloid headline kind of way. The reason for MCAS is so that the 737 Max passes the certification requirement that the pitch controls cannot get lighter on the approach to a stall. It's correct in that clearly the previous 737 were certified as meeting this standard, but it's wrong in that even if the pervious 737 didn't exist this would still be a requirement of certification.
  - mannykannot 8y ago
    Yes, I had realized my mistake and corrected that, apparently while you were composing this reply. I think the current version covers your objection. Bjorn's Corner in Leeham's has a lot of information on the topic.
mcv 8y ago
> If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? That's the problem. The MAX isn't similar enough to older 737s, so MCAS is necessary to change the handling characteristics to be more like older 737s. It's a fix to cover up a lie. > If plane at a low altitude at full power is stalling, just lower the nose. That is pilot training 101. Why need a system like MCAS to help. They know this, but different planes still do that in a different way. MCAS was supposed to cover up the fact that the MAX did this differently. > The pilot should be able to disengage the autopilot and take control of the airplane. But MCAS is not the autopilot, it's something that tried to make the plane behave like a regular 737 during manual control. So turning off the autopilot does nothing; it's already off. The problem is clearly that Boeing wanted to pretend the MAX flies just like an older 737. It doesn't. If they'd just admitted that and given pilots extra training, all this mess wouldn't be necessary.
- pdonis 8y ago
  > If they'd just admitted that and given pilots extra training, all this mess wouldn't be necessary. But if they'd done that they probably wouldn't have been able to sell very many of them. Airlines don't want to have to retrain pilots.
  - mcv 8y ago
    And that greed is ultimately what caused this situation. Had they been more honest, these crashes wouldn't have happened, but they would also have sold less planes.
BuildTheRobots 8y ago
> Their reputation for prudence and safety is gone. A lot of people felt this went a while ago, see the rudder issues that plagued them during the 1990's and the way they subsequently tried to deny responsibility: https://news.ycombinator.com/item?id=19389983 https://news.ycombinator.com/item?id=19389983

rosege 8y ago

I agree! After Lion Air I would have been quite hesitant to fly on a Max. Now with all I'm reading - no way would I.

sokoloff 8y ago

The Max has more powerful engines, mounted further forward on the wings (out of geometric necessity), and as a result has an aerodynamic center of lift further forward. From the addition of MCAS, I gather (but don’t know with certainty) that they couldn’t make some certification requirement without MCAS.

VBprogrammer 8y ago

> Why is MCAS needed? If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? The most relevant question is “is the MAX aerodynamically stable?” If it is, why do we need the MCAS? That needs to be investigated. This isn't even a question. The 737 Max is completely aerodynamically stable. It does however exhibit control behaviours which are undesirable. A fair analogy for this is probably a car which oversteers, in general a normal family car is designed to understeer, because for your average driver that is safer. In the 737 Max the controls get lighter close to the stall because of lift generated by the engine nacelle. The certification requirements require that the controls don't get lighter. Boeing applied what now appears to be a poorly thought out fix. What might surprise you is that there are plenty of certified aircraft which are actually aerodynamically unstable, at least along the longitudinal axis. For example the 757 has dual yaw dampers and at least one of them needs to be serviceable before flight. The consensus is that at cruise altitude it would depart controlled flight without one of them working.

mikejb 8y ago
I think a fairer car comparison would be this: a new power train is fitted to the car, but it sometimes causes torque steer to the right. To overcome that, the steering wheel gets trimmed to the left when torque steer is detected until torque steer is neutralized. The system detecting the torque steer sometimes has false positive.
robdachshund 8y ago
How can a plane be inherently stable while exhibiting wild behaviors that cause the plane to fall out of the sky? Yes, all designs have their quirks and limitations, but I'd say if you need an active system that compensates for the thing not being able to fly without one, it's not a stable design. The airframe is, with the original engines. Not the bigger ones that extend forward and move the center of thrust.
ajnin 8y ago
> The certification requirements require Eh, let's not spin it as "the requirements made them do it", they chose to make the new model stick with the existing 737 certification because building a substantially different new plane would require pilots to be retrained, and they didn't want that as Airbus was ahead of them in the development of the A320neo and they needed that commercial advantage to remain competitive.
- VBprogrammer 8y ago
  I'm not sure where you draw the line though? None of this is new, making modifications to existing designs is the aviation equivalent of developing a new feature. Hanging new engines from an existing fuselage, replacing the avionics and lengthening a fuselage have all been done before with reasonable results. Conversely, clean sheet designs have had terrible safety records initially. Asking a manufacturer to make a clean sheet design every time they make a change is probably going to result in more accidents than it fixes (see the bathtub curve).
dirkg 8y ago
> The certification requirements The 737 is self certified by Boeing, which is really a joke. Basically a Boeing engineer can sign off and say LGTM with no oversight or independent audit as I understand.

robdachshund 8y ago

Bigger engines, mounted further forward. Different center of thrust. Apparently it's not stable enough to fly without some fly by wire style compensation. Honestly, while Boeing is at fault for this not working out, they never wanted to re-engine the plane in the first place. They wanted a clean sheet design, but Airbus forced their hand by offering a re-engine of the A320 to American Airlines. That leads me to wonder if the A320neo might have similar problems we haven't seen yet.

philjohn 8y ago

"If plane at a low altitude at full power is stalling, just lower the nose." Except as can be seen time and again basic instinct can kick in "damn, plane is falling, I need to be higher" and pilots have been known to pull back on the stick to get height.

zaroth 8y ago
A pilot that deals with a stall by pulling up has failed their training. As I recall, we practiced low speed flight and recovering from a low speed stall either the 2nd or 3rd time I ever went up in a Cessna. Power on stalls were a few days later, they are quite different. I never did get my license, mainly because I experienced moderate nausea / motion sickness which I thought would abate after a dozen flights or so, but never really got over it. The problem isn’t that pilots can’t or shouldn’t be relied upon to detect and recover from stalls or near-stalls by increasing throttle and decreasing pitch. The problem appears to be that a new system, added for the purpose of making a new plane with different handling / characteristics behave the same as an older one for training purposes, is malfunctioning. The plane could be perfectly safe without MCAS but pilots would have had to be recertified.
- pdonis 8y ago
  > The plane could be perfectly safe without MCAS It depends on what you mean by "perfectly safe". Many people believe that having the yoke effort decrease at higher angles of attack instead of increase is not very safe. That's why the FAA certification requirements force manufacturers to do whatever is necessary to make sure the yoke force increases with increasing angle of attack, so the pilot has to exert more effort to pull up at higher angles of attack. Without MCAS, the 737 MAX as designed would not meet this requirement.

mannykannot 8y ago

The short answer to your first sentence is that MCAS was needed to give acceptable handling characteristics at high angles of attack. the MAX was deemed similar enough to the NG, such that additional training was not thought necessary, only because of the difference MCAS made. More details here: https://leehamnews.com/2019/02/15/bjorns-corner-pitch-stability-part-10-wrap-up/ https://leehamnews.com/2019/02/15/bjorns-corner-pitch-stabil... Stability is a more complicated issue than it might seem, for reasons such as aerodynamic stability being insufficiently damped, and the interaction between roll and yaw. An airplane that is statically stable could still be dynamically unstable, and go into a divergent series of oscillations if not corrected. It is very common for an airplane to be statically stable around all three axes, yet be prone to falling into a spiral dive if not corrected (this is probably what happened to JFK jr.) If you try to increase the stability to fix that, you get an airplane that is susceptible to an oscillation called dutch roll. Most swept-wing aircraft, including airliners, have gyroscopic yaw dampers that operate the rudder to counteract this. Then there's helicopters... All I know about them is that its complicated.

VBprogrammer 8y ago
> The short answer to your first sentence is that MCAS was needed in order to make the MAX so similar to the NG versions that it could be claimed that no additional training was needed. I think that places the emphasis in the wrong place in a tabloid headline kind of way. The reason for MCAS is so that the 737 Max passes the certification requirement that the pitch controls cannot get lighter on the approach to a stall. It's correct in that clearly the previous 737 were certified as meeting this standard, but it's wrong in that even if the pervious 737 didn't exist this would still be a requirement of certification.
- mannykannot 8y ago
  Yes, I had realized my mistake and corrected that, apparently while you were composing this reply. I think the current version covers your objection. Bjorn's Corner in Leeham's has a lot of information on the topic.

mcv 8y ago

> If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? That's the problem. The MAX isn't similar enough to older 737s, so MCAS is necessary to change the handling characteristics to be more like older 737s. It's a fix to cover up a lie. > If plane at a low altitude at full power is stalling, just lower the nose. That is pilot training 101. Why need a system like MCAS to help. They know this, but different planes still do that in a different way. MCAS was supposed to cover up the fact that the MAX did this differently. > The pilot should be able to disengage the autopilot and take control of the airplane. But MCAS is not the autopilot, it's something that tried to make the plane behave like a regular 737 during manual control. So turning off the autopilot does nothing; it's already off. The problem is clearly that Boeing wanted to pretend the MAX flies just like an older 737. It doesn't. If they'd just admitted that and given pilots extra training, all this mess wouldn't be necessary.

pdonis 8y ago
> If they'd just admitted that and given pilots extra training, all this mess wouldn't be necessary. But if they'd done that they probably wouldn't have been able to sell very many of them. Airlines don't want to have to retrain pilots.
- mcv 8y ago
  And that greed is ultimately what caused this situation. Had they been more honest, these crashes wouldn't have happened, but they would also have sold less planes.

BuildTheRobots 8y ago

> Their reputation for prudence and safety is gone. A lot of people felt this went a while ago, see the rudder issues that plagued them during the 1990's and the way they subsequently tried to deny responsibility: https://news.ycombinator.com/item?id=19389983 https://news.ycombinator.com/item?id=19389983