4 ms·

No, it is not unstable without MCAS. MCAS is only designed to change the trim if the airplane is hand flown in flaps-retracted, low-speed, nose-up flight. This

• acqq 8y ago
  The way you describe it the faulty sensor would have not caused two crashes. If the code would check all these preconditions it would still ignore the AoA sensor. It’s obvious that the code was executed during the regular flight conditions which means it has to be applied even then. The motors simply push the plane nose up too much compared to the previous models, threating the plane to enter the stall. Once in the stall the plane is just not controllable. MCAS was there to hide that. And now that the problem is known to the world either will Boeing provide the proper solution, no matter the cost, or there will be a third crash and that will be too much. Boring still tries to present all that as “business as usual.” It’s wrong.
• phkahler 8y ago
  >> This is something almost everyone is missing: A normal flight should not encounter this. Nobody said there was anything abnormal about the two flights that crashed. It really doesn't matter since the root problem is the plane doing things without telling anyone and then doing it wrongly. Without MCAS these accidents would not have happened.
  • MockObject 8y ago
    > Without MCAS these accidents would not have happened. Without MCAS, wouldn't there be accidents caused by the nonlinear pitch though?
    • salawat 8y ago
      No, because the air frame would not have been certified for airworthyness.
      • linuxftw 8y ago
        That's my understanding. So a simple 'oh we'll just disable it if the readings disagree' seems like a faulty fix. MCAS is required for safe flight. While it may be possible to operate a Max 800 successfully without it, that's not how it was designed to be flown.
• 7952 8y ago
  I agree that normal flight should not encounter these conditions. Although, does it actually check for air speed before activating MCAS?
  • ams6110 8y ago
    As I understand it, no. But airspeed, actually Mach, is an input to the calculation of how much nose down to apply. I.e. the higher the speed, the less trim change is needed to achieve the desired effect.
• petilon 8y ago
  >> A normal flight should not encounter this. What is your source? This is not what I have been reading. What I have been reading is that the MAX has a bigger engine and this engine has been positioned forward on the wing in order to preserve ground clearance. As a result of this unusual engine position, the MAX has a tendency to pitch up during acceleration. Nowhere did I read that this doesn't happen during normal flights.
  • ams6110 8y ago
    MCAS only operates at high angles of attack, higher than normal flight would encounter. The reason it was even a factor in these crashes was the bad AoA sensor readings. If the AoA data had been correct, MCAS would not have been changing the trim.
    • petilon 8y ago
      When an aircraft pitches up, it increases the angle of attack, right? (If not MCAS would not be a fix for the pitching up problem.) And the pitching up can happen during normal flights, correct?
      • jcoby 8y ago
        > When an aircraft pitches up, it increases the angle of attack, right? Yes, but not what you may be thinking. The AoA is the angle of the wing vs relative wind (the angle the wing is attacking the air). You can have high relative pitch (attitude) to the horizon but a low AoA, such as during climb. That same attitude is a full stall at slower speeds/power. The wing doesn't care where the nose points. All it cares about is its relative angle to the wind. Once it diverges past a critical angle the wing stalls. What appears to have happened with these MCAS issues is that the MCAS senses that the AoA is too high when it's completely normal and safe, so it auto-trims down.
      • jboles 8y ago
        They can be related but not necessarily. Pitch is where the nose is pointed, while angle of attack is the angle at which the passing air hits the wings. The nose can be pointed up with a low angle of attack (e.g. a climb during cruise) and conversely the aircraft can be pitched down with a high angle of attack (e.g. descent approach with flaps extended)
• pdonis 8y ago
  > A normal flight should not encounter this. Plenty of normal flights fly manually for a significant portion of the climb-out after takeoff, which is exactly the portion of the flight when MCAS issues happened for the Lion Air and Ethiopian Air flights. Also, you will have higher AoA in a bank, which can be encountered in manual flight on normal flights.
• sergiosgc 8y ago
  As far as I understand, the forward positioning of the engines makes it so that the plane tends to pitch up when it is already pitching up, and tends to pitch down when it is already pitching down. This is the definition of an unstable system. MCAS compensates this by trimming elevators. Without MCAS, it is up to the pilot to handle the unstable system.
• cjbprime 8y ago
  > low-speed Not sure this is right. General aviation pilots often think of wing stalls as a consequence of low speed (their planes don't have AoA indicators!) but they can happen at any airspeed: they simply happen whenever the critical AoA is reached. I think the idea is that the extra lift generated by the nacelles due to the engine position causes the plane to reach high AoA at e.g. full engine power, not just when flying slowly. I think it's also the case that the plane does not actually become aerodynamically unstable, it just starts to handle differently (yoke pressure-wise) in a way that fails airworthiness requirements. So it would only cause a stall if the pilots continued to pull back on the yoke into the stall while it's not fighting them as much as they're expecting it to.