5 ms·

edit: I was labouring under an illusion. Comment left up as a monument to my hubris *** >The airframe with the engines mounted differently did not have adequa

by adamjb 6y ago

edit: I was labouring under an illusion. Comment left up as a monument to my hubris *** >The airframe with the engines mounted differently did not have adequately stable handling at high AoA to be certifiable. Boeing decided to create the MCAS system to electronically correct for the aircraft's handling deficiencies. While accurate, it omits that part of the certification Boeing were chasing was a common type rating with previous 737s on the basis of it having similar handling characteristics. MCAS did not exist because the 737 Max was unsafe, just different. The reason they were chasing fleet commonality is because it would massively reduce adoption costs for airlines by making expensive training for existing 737 pilots unnecessary.

dataflow 6y ago
> MCAS did not exist because the 737 Max was unsafe, just different. You're saying the plane is "safe, just different" without MCAS? But Wikipedia says: "The MCAS flight control law was implemented on the 737 MAX to mitigate the aircraft's tendency to pitch up [...]" I'm a layman but it sure doesn't sound like safe or normal behavior for a passenger airplane to be fundamentally designed so that it "tends to pitch up"... is it? Are there other airliners that do such a thing?
- l33tman 6y ago
  The placements of the engines affected tendencies at some pretty extreme parts of the flight envelope as far as I understood, and MCAS helped make these tendencies similar to the previous 737. I guess an analogy could be that depending on the weight distribution and drive train in your car, it will have tendencies to oversteer or understeer when you go too fast into a turn in bad weather, or something. All cars have different behaviours and tendencies in these extreme circumstances. So the 737Max would not "pitch up" in most normal flying, this is under high AoA situations, like a tight overspeed banking turn I think.
- deleted 6y ago
  [deleted]
- roelschroeven 6y ago
  All jet airliners have a tendency to pitch up under thrust, because of the low position of the engines. The 737 Max has a whole other issue separate from that, caused by the aerodynamic effects of the engines placed far forward of the wings. Airplanes are not allowed to pitch up more unless the pilot applies progressively more and more backward stick force. The 737 Max has a tendency in certain conditions (high angle of attack) to pitch up even when the pilot keeps applying a constant force. Some people argue that that behavior, without measures to prevent it, would disqualify the plane from certification completely. Others say that those measures are only needed to keep the Max under the same type rating as previous 737 generations. I'm inclined to the former, but I'm no expert so that's certainly not definitive.
- choffa 6y ago
  This tendency being different from previous versions. > The MCAS is intended to compensate in such cases [where the aircraft pitches up], modeling the pitching behavior of previous models, and meet a certain certification requirement, in order to enhance handling characteristics and thus minimizing the need for significant pilot retraining. With proper training/re-certification pilots would be trained in the _actual_ characteristics of the plane, thus being able to correct for this behaviour.
  - design-material 6y ago
    Except Boeing also aggressively pitched that pilots didn't need to be extensively re-trained for the 737
shakow 6y ago
> MCAS did not exist because the 737 Max was unsafe, just different. That's arguable: https://www.forbes.com/sites/petercohan/2019/04/02/mit-expert-highlights-divergent-condition-caused-by-737-max-engine-placement/ https://www.forbes.com/sites/petercohan/2019/04/02/mit-exper... https://www.eetimes.com/software-wont-fix-boeings-faulty-airframe/ https://www.eetimes.com/software-wont-fix-boeings-faulty-air... The placement of the MAX's engines due to ground clearance requirements lead to a flight behavior at low speeds which would be, if maybe not outright dangerous, at least very finicky if not for the MCAS.
- linuxftw 6y ago
  Fortunately, Boeing's fix will just disable MCAS in several new situations and pilots will be left to figure out how to fly the finicky planes on short notice.
  - kayfox 6y ago
    The scenarios where it is disabled without a sensor failure are also outside the flight regimes where MCAS should be activating. If there is a sensor failure, the aircraft legally should not be dispatched (not meeting MEL), so its on the airline to make sure they don't push pilots into bad situations.
    - linuxftw 6y ago
      If the sensors are more than 5.5 degrees in disagreement, MCAS is disabled. This could obviously happen after takeoff.
      - kayfox 6y ago
        If the sensors are that far in disagreement, its a sensor failure or something worse.
Gwypaas 6y ago
While accurate, it omits that part of the certification Boeing were chasing was a common type rating with previous 737s on the basis of it having similar handling characteristics. MCAS did not exist because the 737 Max was unsafe, just different. This is a common miss conception, planes can behave quite differently, they already do depending on loading. Then you have common type ratings, e.g. A330, A340 and A350 or 777 and 787 which only requires a short difference training to move between types. The real reason is that the aircraft does not comply with stick force regulations when approaching stalls. The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up. http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/mcas.htm
- deleted 6y ago
  [deleted]
- atoav 6y ago
  This is the aerospace equivalent of trying to correct boomy room acoustics with an equalizer: Yeah you can do that, but like painting over a cracked wall it wont fix the underlying issue. And like the painted cracked wall, it might be okay in 99 out of 100 cases, but that one case where it isn't might kill a hundred people.
  - foldr 6y ago
    There are lots of commercial aircraft that don’t meet certification requirements without some form of artificial stability enhancement (yaw dampers being the classic example). There’s nothing inherently wrong with this kind of thing - you just have to get it right.

by adamjb 6y ago

dataflow 6y ago

> MCAS did not exist because the 737 Max was unsafe, just different. You're saying the plane is "safe, just different" without MCAS? But Wikipedia says: "The MCAS flight control law was implemented on the 737 MAX to mitigate the aircraft's tendency to pitch up [...]" I'm a layman but it sure doesn't sound like safe or normal behavior for a passenger airplane to be fundamentally designed so that it "tends to pitch up"... is it? Are there other airliners that do such a thing?

l33tman 6y ago
The placements of the engines affected tendencies at some pretty extreme parts of the flight envelope as far as I understood, and MCAS helped make these tendencies similar to the previous 737. I guess an analogy could be that depending on the weight distribution and drive train in your car, it will have tendencies to oversteer or understeer when you go too fast into a turn in bad weather, or something. All cars have different behaviours and tendencies in these extreme circumstances. So the 737Max would not "pitch up" in most normal flying, this is under high AoA situations, like a tight overspeed banking turn I think.
deleted 6y ago
[deleted]
roelschroeven 6y ago
All jet airliners have a tendency to pitch up under thrust, because of the low position of the engines. The 737 Max has a whole other issue separate from that, caused by the aerodynamic effects of the engines placed far forward of the wings. Airplanes are not allowed to pitch up more unless the pilot applies progressively more and more backward stick force. The 737 Max has a tendency in certain conditions (high angle of attack) to pitch up even when the pilot keeps applying a constant force. Some people argue that that behavior, without measures to prevent it, would disqualify the plane from certification completely. Others say that those measures are only needed to keep the Max under the same type rating as previous 737 generations. I'm inclined to the former, but I'm no expert so that's certainly not definitive.
choffa 6y ago
This tendency being different from previous versions. > The MCAS is intended to compensate in such cases [where the aircraft pitches up], modeling the pitching behavior of previous models, and meet a certain certification requirement, in order to enhance handling characteristics and thus minimizing the need for significant pilot retraining. With proper training/re-certification pilots would be trained in the _actual_ characteristics of the plane, thus being able to correct for this behaviour.
- design-material 6y ago
  Except Boeing also aggressively pitched that pilots didn't need to be extensively re-trained for the 737

shakow 6y ago

> MCAS did not exist because the 737 Max was unsafe, just different. That's arguable: https://www.forbes.com/sites/petercohan/2019/04/02/mit-expert-highlights-divergent-condition-caused-by-737-max-engine-placement/ https://www.forbes.com/sites/petercohan/2019/04/02/mit-exper... https://www.eetimes.com/software-wont-fix-boeings-faulty-airframe/ https://www.eetimes.com/software-wont-fix-boeings-faulty-air... The placement of the MAX's engines due to ground clearance requirements lead to a flight behavior at low speeds which would be, if maybe not outright dangerous, at least very finicky if not for the MCAS.

linuxftw 6y ago
Fortunately, Boeing's fix will just disable MCAS in several new situations and pilots will be left to figure out how to fly the finicky planes on short notice.
- kayfox 6y ago
  The scenarios where it is disabled without a sensor failure are also outside the flight regimes where MCAS should be activating. If there is a sensor failure, the aircraft legally should not be dispatched (not meeting MEL), so its on the airline to make sure they don't push pilots into bad situations.
  - linuxftw 6y ago
    If the sensors are more than 5.5 degrees in disagreement, MCAS is disabled. This could obviously happen after takeoff.
    - kayfox 6y ago
      If the sensors are that far in disagreement, its a sensor failure or something worse.

Gwypaas 6y ago

While accurate, it omits that part of the certification Boeing were chasing was a common type rating with previous 737s on the basis of it having similar handling characteristics. MCAS did not exist because the 737 Max was unsafe, just different. This is a common miss conception, planes can behave quite differently, they already do depending on loading. Then you have common type ratings, e.g. A330, A340 and A350 or 777 and 787 which only requires a short difference training to move between types. The real reason is that the aircraft does not comply with stick force regulations when approaching stalls. The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up. http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/mcas.htm

deleted 6y ago
[deleted]
atoav 6y ago
This is the aerospace equivalent of trying to correct boomy room acoustics with an equalizer: Yeah you can do that, but like painting over a cracked wall it wont fix the underlying issue. And like the painted cracked wall, it might be okay in 99 out of 100 cases, but that one case where it isn't might kill a hundred people.
- foldr 6y ago
  There are lots of commercial aircraft that don’t meet certification requirements without some form of artificial stability enhancement (yaw dampers being the classic example). There’s nothing inherently wrong with this kind of thing - you just have to get it right.