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MCAS has absolutely nothing to do with maintaining commonality. An aircraft with decreasing stick force on approach to stall in its normal mode of operation wo

MCAS has absolutely nothing to do with maintaining commonality. An aircraft with decreasing stick force on approach to stall in its normal mode of operation would've never been certificated.

manigandham 7y ago
I don't follow. Flight stability systems are not new, even Airbus has them and even had similar issues before with their planes. There's a difference between software stability controls and specifically emulating behavior.
- hirowan 7y ago
  I don't know which one you imagine MCAS to be but see [1]; specifically: MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation. ... This new location [of the engines] 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 [centre of gravity], 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. > Airbus Airbuses operate in C* law (they are not positively stable) [2] and the stick is just that, a spring-loaded.... stick; it meets regulations by default. [1] http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/mcas.htm [2] https://www.skybrary.aero/index.php/Fly-By-Wire https://www.skybrary.aero/index.php/Fly-By-Wire
  - manigandham 7y ago
    I think we're talking past each other. MCAS is stability control which a lot of planes have and is allowable to pass certifications. However MCAS also makes the MAX behave like the older design which specifically led to the gap in training and controls over the system. If they just had stability control but without the emulation of behavior then there would be new training and certification, which is something they wanted to avoid but would've prevented these accidents.
    - hirowan 7y ago
      There is no other modern airliner with a software bodge for static stability. No, Airbuses are not comparable (Airbus FBW control laws are _not_ for stability control) and they are (to the best of my knowledge) positively stable throughout the flight envelope in direct law. Emulation is incidental. Redesigning the 737 with FBW is not something that Boeing would've considered, even for a moment.
      - manigandham 7y ago
        It was certified though so it seems we agree that the stability control system was not robust enough and the training for it was not sufficient enough? Is the cause for that just oversight then? And further exacerbated by similar behavior leading to even less training requirements?
- Gibbon1 7y ago
  it's not an anti stall system it's to satisfy an aerodynamic requirement that the force needed to force the nose up increase progressively with angle of attack. At least that was my understanding. I don't think there is anything fundamentally wrong with that. But when you have active systems for that rather than passive ones you're in a different design domain. Boeing was trying to pretend they weren't.
blendo 7y ago
As I understand it, previous 737s did maintain stick force on approach to stall. The MAX, due to different engines, has decreasing stick forces when the lightly loaded, and/or with an aft center of gravity. My simple solution? Disable MCAS, keep the stick trim switches as on previous 737s, and require MAXs to only fly with a forward CG. 2000 kg of forward ballast might work. Even better, reduce aft weight by increasing economy seat pitch (20 rows rather than 30 or so), thereby reducing aft CG :-)
- aerodude 7y ago
  Wouldn't both of those solutions undermine the advantage of the airframe, which is the fuel cost savings of the larger engines? That said, I think seating pitch and spacing is definitely a conversation that needs to be had. I'm an average-sized guy, and I have a hard time with today's airline seats. I would hate to see what it's like for someone taller. There definitely needs to be more stringent regulation of airline seating to mitigate ever-vanishing personal space.

MCAS has absolutely nothing to do with maintaining commonality. An aircraft with decreasing stick force on approach to stall in its normal mode of operation would've never been certificated.

manigandham 7y ago

I don't follow. Flight stability systems are not new, even Airbus has them and even had similar issues before with their planes. There's a difference between software stability controls and specifically emulating behavior.

hirowan 7y ago
I don't know which one you imagine MCAS to be but see [1]; specifically: MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation. ... This new location [of the engines] 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 [centre of gravity], 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. > Airbus Airbuses operate in C* law (they are not positively stable) [2] and the stick is just that, a spring-loaded.... stick; it meets regulations by default. [1] http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/mcas.htm [2] https://www.skybrary.aero/index.php/Fly-By-Wire https://www.skybrary.aero/index.php/Fly-By-Wire
- manigandham 7y ago
  I think we're talking past each other. MCAS is stability control which a lot of planes have and is allowable to pass certifications. However MCAS also makes the MAX behave like the older design which specifically led to the gap in training and controls over the system. If they just had stability control but without the emulation of behavior then there would be new training and certification, which is something they wanted to avoid but would've prevented these accidents.
  - hirowan 7y ago
    There is no other modern airliner with a software bodge for static stability. No, Airbuses are not comparable (Airbus FBW control laws are _not_ for stability control) and they are (to the best of my knowledge) positively stable throughout the flight envelope in direct law. Emulation is incidental. Redesigning the 737 with FBW is not something that Boeing would've considered, even for a moment.
    - manigandham 7y ago
      It was certified though so it seems we agree that the stability control system was not robust enough and the training for it was not sufficient enough? Is the cause for that just oversight then? And further exacerbated by similar behavior leading to even less training requirements?
Gibbon1 7y ago
it's not an anti stall system it's to satisfy an aerodynamic requirement that the force needed to force the nose up increase progressively with angle of attack. At least that was my understanding. I don't think there is anything fundamentally wrong with that. But when you have active systems for that rather than passive ones you're in a different design domain. Boeing was trying to pretend they weren't.

blendo 7y ago

As I understand it, previous 737s did maintain stick force on approach to stall. The MAX, due to different engines, has decreasing stick forces when the lightly loaded, and/or with an aft center of gravity. My simple solution? Disable MCAS, keep the stick trim switches as on previous 737s, and require MAXs to only fly with a forward CG. 2000 kg of forward ballast might work. Even better, reduce aft weight by increasing economy seat pitch (20 rows rather than 30 or so), thereby reducing aft CG :-)

aerodude 7y ago
Wouldn't both of those solutions undermine the advantage of the airframe, which is the fuel cost savings of the larger engines? That said, I think seating pitch and spacing is definitely a conversation that needs to be had. I'm an average-sized guy, and I have a hard time with today's airline seats. I would hate to see what it's like for someone taller. There definitely needs to be more stringent regulation of airline seating to mitigate ever-vanishing personal space.