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Mentour Pilot did a video on the second MCAS crash where they basically crashed due to being unable to pull up. MCAS changed the trim to a level where the pilot

• WalterBright 1y ago
  The thumb switches would have pulled the nose up. But the pilots didn't read the instructions, and turned off the stab trim system with the airplane nose down. They also had the engines at full throttle, and the increased speed made it impossible to hand crank the stabilizer. The instructions were: 1. restore normal trim with the thumb switches 2. turn off the trim system
  • throwaway5752 1y ago
    Surely you know this, but MCAS was a badly designed system that failed in a fatal manner in the event of malfunctioning non-redundant sensor (10s of failure events per year). Boeing knew this, but allowed it kill everyone aboard two flights. Documentation in the instructions or not it had no business being on a commercial aircraft, ever.
    • signatoremo 1y ago
      It still exists today after modification that requires input from both AOA sensors. Why do you think it’s badly designed? The implementation was badly done for sure.
      • filleduchaos 1y ago
        Design and implementation are not completely distinct things, though. If e.g. a software company's backups were implemented solely as daily dumps to the same SSD the database was running on, and then they implemented a more sane backup solution after the practically-inevitable data loss, would we just go "well the implementation was badly done" or would people not call it out as bad systems design? Some designs are so poorly thought through (and/or conceived for the wrong reasons to begin with, like MCAS was) that they cannot help but yield poor implementations. (Also, input from one or two sensors was far from the only problem with MCAS' implementation. The fact that it could activate multiple times and that it could trim far harder and faster than human pilots could manually trim were equally contributors to making the saga a tragedy).
        • WalterBright 1y ago
          > conceived for the wrong reasons The reasons were correct. Making the MAX behave, as far as the pilots were concerned, like other models of the 737 makes the airplane safer. Many crashes have resulted from pilots getting confused about which airplane they are flying. The design particulars, however, were incorrect. That is Boeing's fault. > trim far harder and faster The 737 has only one electric trim rate - on and off. It does not trim harder or faster than the autopilot trim or the thumb switch trim. The signal all boils down to a wire that has signals "trim on / trim off". The thumb switches sit on the path of that wire, so when they activate they override the autopilot and MCAS. The cutoff switch is downstream of that. Part of the reason the manual trim wheel could not be moved is the airplane was overspeeding due to being (incorrectly) at full throttle. Overspeeding greatly increases the force on the stabilizer, making it much harder to move. In the EA flight, the overspeed warning horn could be heard on the voice recorder.
          • filleduchaos 1y ago
            The only reason why "making it behave like other 737s" was even a goal to begin with was so that it wouldn't need to undergo more rigorous certification and pilots wouldn't have to undergo comprehensive training to get rated for the 737 MAX. This was very explicitly because pilots _didn't_ need to do that to get rated for the competing A320neo family, not because Airbus did absurd shenanigans to make it handle the same as the A320ceos, but because they weren't desperately holding on to a 1960s design that was amongst other things so low to the ground that the new engines being fitted to it couldn't actually fit. That has nothing to do with actual safety and everything to do with profit. > The 737 has only one electric trim rate - on and off. It does not trim harder or faster than the autopilot trim or the thumb switch trim Is there a reason you cut off the sentence you're quoting before the part that says "than human pilots could MANUALLY trim" and started talking about the electric trim rate instead? Because that part of the sentence was written for a reason - the overwhelmingly more relevant reason why the manual trim wheel would not have saved them is that they would have had to crank it 15 times to trim just one degree (and they were much more than one degree out of trim, due to MCAS activating for five full seconds each time). Even if they weren't already overspeeding, it is highly doubtful that they would have won the battle to keep the aircraft in the air given their proximity to the ground + struggle to keep the nose above the horizon with two pilots versus one. Also mind you by the time they entered the overspeed condition, they had already had the stick shaker going (due to the faulty AoA sensor, with no IAS disagree alert) and had had 2 MCAS activations to deal with, with the autothrottle failing to reduce the thrust to the climb detent because the very same faulty AoA sensor.
      • anonymars 1y ago
        Isn't the fact that it was redesigned (to not rely on a single AoA sensor, to only activate once per AoA-event, to limit the magnitude of intervention) a strong indication that it was badly designed?
      • wat10000 1y ago
        It was implemented exactly as designed.
    • WalterBright 1y ago
      I know this. I also know that there were 3 MCAS incidents. On the first one, the crew restored normal trim with the thumb switches, then turned off the stab trim system. They had no idea that MCAS existed, they simply followed standard procedure for runaway trim. After the first crash, Boeing sent around an Emergency Airworthiness Directive to all MAX pilots with the two steps I showed. There's no excuse for MAX pilots not reading, understanding, and remembering Emergency Airworthiness Directives. It's not just me. I've talked to two 737 pilots who said the same thing.
      • throwaway5752 1y ago
        I see in a sibling comment that you do not think the MCAS system on the MAX was well designed, and the reliance on a single AoA sensor was not a good decision. I agree. The aircraft should have never been declared airworthy. There should have been no need for an EAD in the first place. There is no excuse for putting a control system that flies the craft into the ground when there is a failure in a nonredundant sensor on a commercial aircraft. The high standards required for pilots, like Amelia Earhart, do not obviate the even higher standards for quality and safety from manufacturers.
        • WalterBright 1y ago
          I suspect that Boeing didn't think it would be a problem because the runaway stab trim procedure is enough to stop it and recover. Who would have thought that trained pilots would not remember this procedure? It's supposed to be a "memory item", meaning consulting the checklist was not required. The stab trim cutoff switch is in a prominent position on the central console (not overhead or behind the crew). In the first Lion Air crash, the airplane dipped and recovered (via the thumb switches) 25 times over a period of 11 minutes. They never thought to turn off the stab trim system. It boggles the mind. Would you want to fly with a crew that didn't know what the switches on the console were for? Not me. Both Boeing and the crew are at fault. Some percentage also for whoever made the faulty sensor and the failure to see that it worked correctly after it was installed.
          • 1y ago
  • filleduchaos 1y ago
    > But the pilots didn't read the instructions Actually watching the linked video (or barring that, actually reading the final report) would dispense with this widely spread inaccuracy in fairly short order and make it quite obvious why they turned ON the stab trim system, which is what ultimately killed them. But of course, "the pilots were simply stupid" is a psychologically easier conclusion to walk away with.
    • WalterBright 1y ago
      As I wrote, the column switches override MCAS. It's also how the crew in the first MCAS incident recovered and continued the flight and landed safely.
      • jajko 1y ago
        Didn't investigation confirmed they followed official Boeing training?
        • consumer451 1y ago
          One of the airlines that suffered an MCAS crash had asked Boeing for specific training on MAX and was denied, as it would have set a bad precedent for sales.
      • kashunstva 1y ago
        > the column switches override MCAS. Temporarily, right? Using the pitch trim switches on the yoke suppresses MCAS, but in order to remain at even net zero stabilizer position change, the duration of trim input would have to be of equivalent duration to the reciprocal MCAS-driven change. This is part of what made the problem so insidious - running trim for that long is rarely needed and rarely done. I haven’t flown the MAX but I can’t think of any type I have flown in which continual pitch trim input of 5+ seconds at a time would be required in flight.
        • WalterBright 1y ago
  • cameldrv 1y ago
    This is not true. In the condition of a failed AoA vane like on the two accident flights, there is no way to enable the trim thumb switches and disable MCAS. You either get no electric trim and no MCAS or you get both. If you happened to have known that MCAS pushes the trim down for 9.3 seconds and then pauses for 5 seconds, you might have been able to time it and turn on the electric stab trim at just the right time when MCAS wasn’t trying to kill you, but unfortunately the very existence of MCAS was hidden from pilots. If you flip the stab trim cutout switches, you can turn the wheel manually, but if there have already been a couple of cycles of MCAS, the plane may be so out of trim that you need too much force to move the trim wheel manually. In that case there is one thing that could save you which would be to do the so called rollercoaster procedure, where you push the yoke forward to unload the trim and then crank the trim up while you hurtle towards the ground. This was a documented procedure in the 737-300 manual, but was later deleted. The real crime was that Boeing deliberately removed all mention of MCAS from the flight manual. The problem was that if there were procedures that pilots needed to know in order to recover from an MCAS failure, they would have been required to practice these procedures in a simulator, and Boeing’s contract with Southwest had financial incentives if the MAX didn’t require any additional sim time for pilots to transition from the NG to the MAX. We know that the information about MCAS was deleted from the manual because the only occurrence of the term MCAS in the MAX flight manual is in the glossary where it is a defined term. The tech writers obviously put it in there because it was described in an earlier version of the manual, but later deleted, but the glossary entry wasn’t deleted when the description was.
    • sokoloff 1y ago
      The electric trim switches override MCAS (and indeed the crew of ET302 almost saved the aircraft by turning it back on and commanding aircraft-nose-down (AND) with the electric trim). What is inexplicable is that they stopped doing that while leaving the trim system enabled after correctly disabling it via the memory item checklist. Boeing’s not blameless here, nor should the crew of ET302 bear the primary fault, but they seem to have understood the issue (and it’s not possible they didn’t know about MCAS) and were frustratingly close to saving the flight.
    • WalterBright 1y ago
  • cornholio 1y ago
    This is an insane take. The plane was fitted with an undocumented automatic system that would erroneously change the pitch about ten times faster than what the pilot switches allowed, and that would automatically re-engage repeteadly as soon as the pilot released their switch, negating all progress made by the pilot. It would continue this adjustment blindly despite copious information available to the flight computers that a crash with terrain is imminent and that the pilot is having trouble leveling the plane. So the electronic control route was fried, any normal person in that situation and physical and mental overload would conclude that trim controls are not functional. The manual route, which they tried, involved at that speed an undocumented "roller coaster maneuver", pitching forward (!!) to release pressure on the trim mechanism and allow manual adjustment. It goes without question it's impossible the pilots could have come up with that solution in the mere seconds they had at their disposal, let alone the high risk it entailed. Reducing the throttle early would have saved the plane, but it would have also resulted in immediate pitch down due to the torque the engines create against the body of the plane, as they are mounted off axis. The autothrottle was left to near 95% for the entire ~250s duration of the flight (the setting necessary for take off at the particular altitude of the airport) and once the cascading failures consumed all their situational awareness it stands to reason why they failed to adjust it and then hesitated to perform an action that would have (in the short run) worsened the problem in a near crash situation. The MCAS as designed was a ticking timebomb that killed two plane loads of people in mere months, and Boeing accepted full responsibility for that, even if they are still denying criminal charges.
    • coredog64 1y ago
      The 737 has had the issue of runaway trim forever. The giant "wagon wheels" used for trimming early models were very loud when activated, and part of that was to give the pilots a heads up that something unexpected was happening.
    • WalterBright 1y ago
      > about ten times faster than what the pilot switches allowed This is utterly false. The trim rate has two speeds - on and off. > would automatically re-engage repeteadly This is called "runaway trim". The procedure to counter it is to turn it off. The same procedure for every airplane. > any normal person Pilots are required to read and understand and remember Emergency Airworthiness Directives. Don't forget the first MCAS incident, where normal trim was restored with the column trim switches and then the trim was turned off and the airplane landed safely. Or the second where normal trim was restored 25 times, but never turned off. I would expect a MAX pilot to have paid attention to that crash, and the followup EAD which instructs them to restore normal trim with the column switches and then turn it off.
  • glitchc 1y ago
    Not being trained on distinct and aberrant behaviour of a specific aircraft is not the same as ignorant or stupid.
    • WalterBright 1y ago
      All MAX pilots received an Emergency Airworthiness Directive with the instructions on how to recover.
    • IAmBroom 1y ago
      If the training is available, it really is. This isn't your friend's car you're borrowing to pick your kid up from school. A tad more care should be involved before loading it with human beings and flying over cities.
      • glitchc 1y ago
        Tell that to the airline. It's not up to the pilot. The airline has to agree to pay for it. The 737 Max was offered because it was a lot cheaper for airlines to add marginal additional hours of training, instead of the minimum 25000 hrs of recertification that would be necessary for a brand new platform. In short, the 737 Max is exactly what the airlines were clamoring for and why it sold so well. This was discussed extensively on HN when the incident occurred (and subsequent articles). You may want to go read those comments and enlighten yourself.
  • arunabha 1y ago
    If what you are saying is true, and it was the pilots fault, I'm wondering why did a corporation with the clout and resources of Boeing lose in court?
    • rtkwe 1y ago
      The wrinkle is that yes they could have saved the plane if they had information they didn't have because of Boeing's self serving negligence.
• snozolli 1y ago
  For others reading through these interesting comments, here's a Wikipedia excerpt: The Maneuvering Characteristics Augmentation System (MCAS) is a flight control law built into the Boeing 737 MAX's flight control computer, designed to help the aircraft emulate the handling characteristics of the earlier Boeing 737 Next Generation.