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What is the Boeing 737 Max Manoeuvring Characteristics Augmentation System?

• djsumdog 8y ago
  Going back a few generations, the earlier 737s were plagued by crashes as well, relating to rudder issues: https://en.wikipedia.org/wiki/Boeing_737_rudder_issues https://en.wikipedia.org/wiki/Boeing_737_rudder_issues
  • catchmeifyoucan 8y ago
    Wow... looks like these investigations take forever. Hope Boeing patches soon.
  • BorRagnarok 8y ago
    From that article: "leading the manufacturer to suspect and insist that the pilot had responded incorrectly" After a few years of course it was concluded that it wasn't the pilots fault. I find it appalling that Boeing is always and every time trying its damndest to blame the crew, even before the investigation has concluded. This is a pattern, and imho should be penalized by law. Its disgusting to blame people who are dead and cannot defend themselves against the allegations, when there's no conclusive evidence. I wonder if they apologize to the crews family when they do that and are later found out the be in the wrong. I suspect they couldn't care less though.
    • nisa 8y ago
      Yes, their reaction to crashes are terrible. Just look at what happened to Lauda Air Flight 004 that came up in a thread about this yesterday: https://en.wikipedia.org/wiki/Lauda_Air_Flight_004#Investigation https://en.wikipedia.org/wiki/Lauda_Air_Flight_004#Investiga... "Lauda attempted the flight in the simulator 15 times, and in every instance he was unable to recover. He asked Boeing to issue a statement, but the legal department said it could not be issued because it would take three months to adjust the wording. Lauda asked for a press conference the following day, and told Boeing that if it was possible to recover, he would be willing to fly on a 767 with two pilots and have the thrust reverser deploy in air. Boeing told Lauda that it was not possible, so he asked Boeing to issue a statement saying that it would not be survivable, and Boeing issued it. Lauda then added, "this was the first time in eight months that it had been made clear that the manufacturer [Boeing] was at fault and not the operator of the aeroplane [or Pratt and Whitney]."[18]"
• toufka 8y ago
  Here's a pilot describing the MCAS system on the 737 Max: https://youtu.be/zfQW0upkVus?t=220 https://youtu.be/zfQW0upkVus?t=220
  • lostsock 8y ago
    Super interesting, he's really great at explaining things in laymens terms. Thanks for sharing
    • halfmatthalfcat 8y ago
      I follow Mentour's channel pretty regularly. For anyone with any flying anxiety, it's great to see how seriously they take their jobs, how regimented they are and how really safe you are flying.
• cperciva 8y ago
  I'm struck by the connection between this and the Air France Airbus 332 which crashed en route from South America to France about a decade ago. It seems that the 737MAX (at least, the one last year -- we don't know about the latest one yet) crashed because a faulty sensor caused the computer to automatically point the nose down in order to avoid a perceived stall risk and the pilots weren't able (or didn't know how) to disable that. In contrast, the 332 crashed because a faulty sensor misled the flight crew into pointing the nose up and getting into a stall. In both cases, crashes resulted from reliance on a faulty sensor -- but in one case it was flight crew and in the other case it was software. Solving this isn't going to be as simple as saying "the flight software should defer to the pilot" or vice versa.
  • xeromal 8y ago
    Really, flying by instrumentation is what you're saying is the problem and that's the truth. There's no way around that though. I did read that the sensor for the MAX is on both sides but is only activated on the side of who is flying, the pilot or the co-pilot. If the sensor fails, the plane fails. They could at least have both sensors active as a failsafe in case one is acting up.
    • cperciva 8y ago
      I don't think it's quite that simple. In the case of the Lion Air flight, one of the AoA sensors had been recently replaced due to problems on an earlier flight; in the case of the Air France flight, there were icing conditions which were known to affect the airspeed sensors. It's possible that both of those flights could have been saved given better awareness of how much those sensors could be trusted at the time -- for example, if Lion Air's engineers, upon fiddling with AoA sensors, had been able to tell the avionics systems "the AoA sensors might be unreliable; don't do anything automatically based on those sensors which might crash the plane".
      • laydn 8y ago
        It is difficult to understand the design decisions made on these complex software systems without any real domain knowledge, but one can't help but wonder whether this whole MCAS system is pushed into the field prematurely. For example, we know that the flight control computer has access to altitude information. Why would it let the MCAS push the nose down, when it knows that you have very little altitude margin? (I am talking about the Lion Air flight).
• samcheng 8y ago
  It's ironic that it's a Boeing with bad software (or sensors) interfering between yoke and plane. After all, historically, Boeing has had the philosophy of trusting the pilot with direct control of the plane. This is in comparison with Airbus, where controls are by stick instead of yoke, and pilots are taught to trust the flight computer and move the controls less. (Of course, these days both manufacturers build fly-by-wire planes.) [1] The addition of a 'maneuvering characteristics augmentation system' seems very unlike Boeing. I guess it became necessary after adding too-big engines without otherwise adjusting the airframe... [2] This "we'll fix it with software later" attitude works well with CPU errata and maybe Teslas, but it doesn't look like we're there yet for airplanes, unfortunately. [1] https://www.pprune.org/tech-log/10321-stick-vs-yoke-airbus-v.. https://www.pprune.org/tech-log/10321-stick-vs-yoke-airbus-v.... [2] https://theaircurrent.com/aviation-safety/what-is-the-boeing.. https://theaircurrent.com/aviation-safety/what-is-the-boeing....
  • goodcanadian 8y ago
    This "we'll fix it with software later" attitude works well with CPU errata and maybe Teslas, but it doesn't look like we're there yet for airplanes, unfortunately. I'm sure I'm preaching to the choir here, but I'm absolutely against the "fix it with software" attitude. I'm working with embedded systems, now, and while we can often make it work by tweaking the software, it always results in a less than optimal solution. Fix the hardware; get it right, preferably before manufacturing a few thousand units.
    • iSnow 8y ago
      Doesn't work in the current speed of development any more. Time to market is more critical than maturity, sadly. That's why every system can be fixed in software now, and that's also why every system can be pwned now.
• oasisbob 8y ago
  Another few good technical articles on MCAS, and other preexisting automatic stabilizer trim adjustments on the 737: http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/mcas.htm http://www.b737.org.uk/flightcontrols.htm#Stab_Trim http://www.b737.org.uk/flightcontrols.htm#Stab_Trim
• raverbashing 8y ago
  To me, the biggest contradiction (as it seems on the design of the system) is that it seems to rely on very fickle measurements to command big movements on the aircraft. So, in the case of Lion Air, you have 2 contradictory inputs and you're basing an automated decision on it? Also, it seems aviation hasn't caught up with 20th century fluid measuring techniques. Speed of fluids in pipes can be measured non-invasively. With the biggest caveat that doing things inside a pipe on the ground is easier it seems we're still using 19th century techniques to measure fluid speeds and direction. Which didn't matter too much when their noisy/unreliable inputs were simply ignored or compensated by pilots and mechanical devices. No real world scenario (at least not a surviveable one) can produce the sensor readings that happened in AF447 (sudden drop in airspeed) and in the Lion Air case. But this is being taken at "face value" (there is some filtering) and acted upon it (with no contradictory input from other systems - for example an accelerometers - and yes it's not so obvious how to put everything together).
  • dreamcompiler 8y ago
    The AoA sensor doesn't measure fluid pressure in a pipe; it measures fluid direction with a small mechanical wing that rotates to align itself with airflow [0]. Thus it has to move, unlike a pitot tube. If it can't rotate for some reason, it gives faulty data. [0] https://aviation.stackexchange.com/questions/2317/how-does-an-alpha-aoa-vane-work https://aviation.stackexchange.com/questions/2317/how-does-a...
    • raverbashing 8y ago
      Yes, I'm aware of the differences, but you could measure air speed in two axis then calculate AoA from there, that's what I mean.
      • burfog 8y ago
        It's done. Here you go, only a few hundred dollars: https://www.dynonavionics.com/aoa-pitot-probes.php https://www.dynonavionics.com/aoa-pitot-probes.php
• avar 8y ago
  On the subject of why Boeing didn't think to tell pilots about the MCAS: > "Since it operates in situations where the aircraft is under relatively high g load and near stall, a pilot should never see the operation of MCAS" This seems to me to indicate a much deeper issue with the aviation industry than the 737 MAX issue. Aren't pilots as part of their simulation time testing the limits of the flight envelope of the aircraft they're flying? If so surely they'd be expected to encounter the MCAS system.
  • plt11 8y ago
    I think Boeing claimed that no retraining of existing 737 pilots was necessary for the new model. This attitude has led to crashes in the past as well.
    • alkonaut 8y ago
      If airlines would need to invest in a lot of training for existing 737-non-MAX pilots, that would probably have made the plane a lot less attractive. Being "a regular 737" is part of the appeal of the 737 MAX.
• andy_ppp 8y ago
  I mean just to give an anecdote that is slightly worrying; I was about to land in Grenoble and it was quite rocky the turbulence and incredibly windy, you could tell we were coming in too fast and the pilot decided to back out of the landing and take off again with the wheels nearly on the runway. Suffice to say the angle of attack was extreme and the power produced by the engines quite impressive/terrifying. I would say anything that decided to change the express wishes of the pilot in that situation would have been hugely dangerous. Can someone explain why ever overriding the pilot rather than warning them would be a good thing?
  • porki 8y ago
    You say that «the angle of attack was extreme», but are you sure about that? Bear in mind that AoA is not equal to pitch. As the engines were producing a lot of thrust, the AoA might as well have been relatively low once positive climb was established.
    • andy_ppp 8y ago
      From the diagrams I see online [1] it's difficult to know either way because the wind direction plays a part in the AoA. It could have been more or could have been less than I felt it was, but for certain the pitch was much higher than normal flying. [1] https://en.wikipedia.org/wiki/Angle_of_attack https://en.wikipedia.org/wiki/Angle_of_attack
  • sean-duffy 8y ago
    I can't find a source right now but I remember reading that there had been a few cases where there had been an emergency situation and pilots had ignored stall warnings, resulting in the plane stalling and crashing. This was why Boeing had elected to make the system on the 737 Max 8 override the pilots inputs. I think having an emergency system that overrides the pilot is fine, but it is crucial that the pilot can easily and quickly disable such a system when they know the plane is doing the wrong thing.
    • tarabanga 8y ago
• bronco21016 8y ago
  Lots of great technical discussion about MCAS and potential improvements in here. I think the bigger issue though that needs to be addressed is the certification and training process. What factors, other than the obvious engineering factors, were driving the design process? Why were those factors allowed to also influence the training administered to pilots? After the Lion Air crash the response from US pilots unions seemed to speak volumes. Pilots were not informed of this system on their new aircraft and as such were treating the aircraft the same as all other aircraft falling under the 737NG type rating. Airlines and Boeing worked together to minimize training costs to help the aircraft sell easier and as a result critical systems knowledge was left out. Knowledge that likely could have saved Lion Air.
• spookyuser 8y ago
  Interesting that this website prevents you from copying text
  • doodlebugging 8y ago
    You can use the option in your browser to view the page source and grab it from there.
• ChicagoBoy11 8y ago
  The big issue here is how much of the existence of this system, the desire to NOT make OTHER changes to the plane that would inhibit the pitch-up tendencies, and the lack of necessity for another type rating were all driven by simply an attempt to skirt existing regulations and save Boeing and Southwest a ton of money. The reality is that this was the whole deal with the Max. All of these decisions were made in a context were avoiding the requirement for a new certification or type rating for pilots was a must. We can't avoid the reality that had the necessity to skirt regs not been there, Boeing would have definitely come up with a different solution from an engineering perspective.
  • rpiguy 8y ago
    To add to your hypothesis - making a change to the fly-by-wire system was probably much cheaper than making airframe changes to make the aircraft more neutral in flight (airframe changes which would likely also negatively affect fuel economy). So Boeing took the cheap rout and decided to have the computer handle the pitch-up tendencies of the aircraft... This kind of fix is quite common on fighter aircraft which are purposely designed to not be neutral/stable in flight for better handling characteristics (among many other reasons). I have no idea how common this kind of fix is on a passenger aircraft.
    • PaulHoule 8y ago
      It is a violation of the brand expectations pilots have for Boeing. Boeing introduced fly by wire after Airbus and it did it in such a way to preserve the feel of flying a plane, in particular if you have a fight with the system you could pull harder and win. Some pilots have strong feelings about this and are loathe to switch from Boeing to Airbus or vice versa.
      • ChicagoBoy11 8y ago
        As a pilot (sadly not a Boeing, or anything close to that side), I think this is a non-trivial point. Every second professional airline pilot you will talk to will talk about that philosophy difference between Airbus and Boeing, and 99.999% of them will take Boeing's side. Sadly, pilots really don't have much of a say in those discussions. They'll fly what's put in front of them, but I can assure you that all feel safer being given a plane not trying to actively fight them.
• coryfklein 8y ago
  Two planes full of passengers destroyed because the pilot couldn't get the nose to point upward and we find a secret computer system that overrides manual controls and automatically points the nose downwards? Yeah, sounds like a real possibility to me.