Ethiopian 737-8 MAX down. No survivors. | Page 15 | FerrariChat

Ethiopian 737-8 MAX down. No survivors.

Discussion in 'Aviation Chat' started by RWatters, Mar 10, 2019.

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  1. SVCalifornia

    SVCalifornia Formula 3
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    Zowie. I think I would have. A hundred questions a bout this!

    Any system that would blindly drive itself into the ground isn’t worthy of using!

    I’m a computer design engineer, manager, director and I can’t imagine how this system would be empowered to do what it did!

    However, I’m not trying to bash Boeing. These kinds of systems are difficult. But human factors engineering dictate trying to help, not hinder and to be transparent, which this is glaringly, not.

    Can someone enlighten me on why an AOA sensor needs to be located outside the airframe?? There are not just a bird strike to worry about but flocks of them, no?

    SV


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  2. BMW.SauberF1Team

    BMW.SauberF1Team F1 World Champ

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    AOA is relative to airflow so it needs to be outside. Pitot tube is also dependent on condition external to the planes. Someone please correct me if I'm wrong.
     
  3. Bob Parks

    Bob Parks F1 Veteran
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    The AOA vanes operate from the airstream attacking the aircraft and measures the angle between the horizontal reference line of the airframe and the airstream. You have a good point, however, maybe an internally mounted gyro would be a better choice. I'm no instrument or cyber wonkie but it might be something to look at.
     
  4. tazandjan

    tazandjan Three Time F1 World Champ
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    Bob- Internal gyros or INS/GPS are too slow reacting to use as anything except a back-up to the AOA sensor. Ones that we put on fighters are a lot tougher than those vane sensors on airliners. They are basically a pretty solid piece of aluminum or steel with small slots/holes. Would take a pretty good bird strike to damage one of those since they have to survive Mach 2.5 and 2000+ psf Q loads. We had a yaw sensor that looked just like the AOA sensors. You can see the difference here. We started out with one AOA sensor and no yaw sensor, but one of each was added as we learned we needed them.

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  5. Bob Parks

    Bob Parks F1 Veteran
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    Thanks for the input, Taz.
     
  6. solofast

    solofast Formula 3

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    furmano, MCAS was employed to "enhance" the handling characteristics not as a safety related system. It was employed to make the Max handle similar to the existing 737. Because of the engine location on the Max, there is an additional nose up pitching moment under certain conditions. The MCAS is supposed to adjust trim so that the Max feels the same as the existing 737 under those conditions. This allows type rated pilots to transition to the Max with a minimum of sim time and training. This was not considered to be critical, assuming that if it failed, the aircraft would not handle the same only under the specific flight conditions (low speed, nose up) but it would still be manageable. What Boeing missed is that erroneous inputs to the MCAS software would cause a runaway trim condition and that this could become unmanageable as it did in these two cases. This goes back to my post earlier where the FAA doesn't understand that the failure of a sensor doesn't just make a computer go dead and not function. What happens in the real world is that failure of a sensor can cause the computer to do things that are not desired. If the system stopped functioning that would have in these cases been fine. But what it did was cause the system to repeatedly invoke a nose down trim command that proved to be deadly. The reality of computers in aircraft are that the computers themselves are dual redundant, but these are very reliable anyway and seldom are an issue. The reality is that sensors are the weak link and they are notoriously unreliable. Also typically, both flight control computers share sensors. This means that both computers get the same information, do the same processing with identical software, and then they both make the same bad decision. This is why you need a higher level of redundancy than is currently mandated by the FAA reg's. The FAA mandates at least 2 inputs for flight critical systems. But if one sensor fails and returns a bad input how do you determine which sensor is correct? Airbus uses 3 airspeed inputs and this assumes that more than one can't be bad. Which worked until it didn't and an airplane crashed. In the Airbus case a sensor was declared "bad", and then not utilized again even though the error wasn't real and sensors input was only an erroneous for a few seconds. The bottom line is that the levels of redundancy that the FAA requires is not consistent with what modern systems look like and how they work in the real world.
     
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  7. furmano

    furmano Three Time F1 World Champ
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    Because it wasn't considered a critical system, it didn't require the added redundancy, I get that.

    But if Boeing decided during the design phase to add redundancy to MCAS, just to go above and beyond, what would be the challenges to doing that? More lines of code? More development time? I'm just trying to figure out the technical challenges to adding redundancy, even if it wasn't required. Like if someone on the design team said, "Hey, I know we aren't required to add a secondary sensor input, but I say we should add a secondary sensor input." what would be the arguments against that? Besides just, it's not required.

    Secondly, wouldn't the design team think through all the situations that might occur and see the possibility of MCAS taking over beyond the normal circumstances of augmentation? I mean, wouldn't someone do the thought experiment and recognize, hey, if MCAS gets a faulty input, and the pilots don't disengage the MCAS (stab trim cutout) quickly, it's possible MCAS could create a runaway trim situation that would be unrecoverable? Isn't thinking through ALL possibilities part of the design/test process?

    -F
     
  8. mike01606

    mike01606 Formula Junior

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    Speaking with 20:20 hindsight a simple FMEA on the system should have identified the risk of a sensor failure.
    I would imagine that process is being poured over and that’s where Boeing could hit rock. If it was badly deficient it becomes a systemic issue and the return to flight requirements should be far more than updating MCAS and pilot training.
     
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  9. solofast

    solofast Formula 3

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    You're right, it isn't required, and they should have gone further. If they had the experience we had they would have but they didn't. The question is, where do you stop??? You can literally spend hundreds of man years making things perfect. OTOH, the FAA requirements are woefully inadequate. Going back to my experience in a similar software case, the designer of the software was happy when we figured out that the failure was actually two levels deep. She was like "hey, we did our job, it worked until it failed at 2 levels deep".... So she was all smiles until I told her that was not going to cut it and she had screwed up. But in this instance we had a pilot (me) making that decision, and not a software designer. Just because it met the FAA requirement I decided that the levels of redundancy were not adequate and I made it safe. In our case the first level of failure was a erroneous pressure signal that the ECU got often when starting, caused by low batter voltage. On startup the sensor would be declared as "bad" and the software didn't heal itself even though the sensor was giving a bad signal only for a moment. So do you go back an look at he sensor again later and if it's good do you use it again? We had a case where the sensor was failing frequently, so it really only took one other failure to have the entire system fail. It met the FAA requirements, but the system was by no means adequate because most of the time it was flying around without any redundency. Boeing is going through all of this right now and is, I am sure, looking at all of the potential failure modes and what can happen give failures or partial failures of sensors and hardware. Things like the AoA sensor are supposed to have a MTBF in the tens of thousands of hours. But the reality is that they appear not to be that reliable given that two failed in the field a couple of months apart. If you have a frequent failure you really don't have 2 levels of redundancy, you only have one. If you did a mag check at run up and failed it, you'd just go back to the hangar and park it. You'd never take off with one bad magneto. So are you willing to ground an airliner if one sensor is giving a bad signal? If your aircraft is designed to FAA criteria you couldn't launch with even one bad sensor. Obviously given sensor reliability a higher level of system redundancy is required, and a more robust system that can withstand multiple failures of less reliable components is necessary. As Boeing peels back the onion and sees this they are most likely looking at all of their FC computer systems and this could well be why they are taking longer to fix this than just implementing a quick fix to get them flying again. If it was just the MCAS issue they'd be back in the air by now. I'm guessing that they've found a lot of other things that met the FAA requirements but weren't safe and they're addressing them as quickly as possible.
     
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  10. Rifledriver

    Rifledriver Three Time F1 World Champ

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    Just a question from an observer. The aircraft industry for much of its history was loath to use electronics in place of mechanical systems for reliability reasons. We progressed to a point mechanical systems (controls, instrumentation etc) were no longer capable of doing what was needed or desired and are now in this spot. Doesn't t make sense to have rudimentary mechanical back up to give pilots a reality check to confirm what the gadgetry is doing or saying? You know, needle, ball and airspeed that can be counted on?


    Having to deal with these garbage systems in automobiles daily long ago convinced me that any car I'll ever own has already been built. I am pretty philosophical about air travel but it is at the point I'd rather be on a 707 than an Airbus or Max8.
     
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  11. furmano

    furmano Three Time F1 World Champ
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    Exactly. Good FMEA (just sitting around a table and thinking through all the ways a system can fail and where that leads) would have revealed this scenario. Or I guess, maybe not, I guess it's hard to say. But the FMEA sessions I've been involved with have been very illuminating. It just required some solid logical processing along with a little thinking outside the box.

    -F
     
  12. solofast

    solofast Formula 3

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    Yes, exactly. I think that for a long time Boeing was of pretty much the same opinion. In the old days you had some "assist", but it didn't fly the airplane. The military went to computerized control in the F16 and they learned a lot of painful lessons, but they spent a ton of our money to do it and they pretty much did it right. Airbus broke the mold and went to computerized control for commercial aircraft and had a number of crashes and I've always said I avoid their aircraft because I don't believe that is a safe approach. This is really Boeing's first foray into computerized "assistance" and obviously it didn't go well. The reality is that there needs to be a more robust level of redundancy for computer systems than has been assumed and if it isn't there the level of safety that we've become accustomed to isn't there either.

    As to whether or not an more rigorous FMEA would have caught it is hard to say. I am surprised that they didn't, as part of the certification require failing sensors in the sim and see what happens, but again, you have to fail it in the same way that it failed in the real world to get the right answer. I think that most likely the FMEA didn't go deep enough because it wasn't considered "flight critical", even though it actually was, and that's what led to the incomplete analysis. Boeing is going to pay for this one big time.

    We can armchair quarterback this to death, and 20/20 hindsight is always perfect, but clearly the FAA and Boeing missed it.
     
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  13. Rifledriver

    Rifledriver Three Time F1 World Champ

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    Well we could require a big red switch to turn it all off when it runs amok but then we would need real live pilots at the helm and it seems to me one of the ultimate reasons for the entire exercise is to rid ourselves of that need.
     
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  14. F1tommy

    F1tommy F1 World Champ
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  15. BMW.SauberF1Team

    BMW.SauberF1Team F1 World Champ

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    Parking lot? Surely it was made to support the weight of those birds...
     
  16. max930

    max930 Two Time F1 World Champ
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    I think Boeing is going bankrupt over this...……………
     
  17. Veedub00

    Veedub00 F1 Rookie
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    There are over 600,000 parts on a new 737. I'm sure they did not do a full blown system level FMEA with the new parts. Even if they did, I am sure they used a very high failure rate for the AOA and thus the likelihood of occurrence was too low to notice.
     
  18. F1tommy

    F1tommy F1 World Champ
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    No Boeing will recover and this will make them a better company as they will learn from their mistakes. An A321 has an uncontrolled movement on takeoff yesterday and the A320 series of aircraft has had alot of problems overt the years and they are still selling like crazy. I do hope Boeing pushes ahead with the composite mid range narrow body sooner now. That would make them jump ahead of Airbus again but this time with narrow bodies.
     
  19. Jeff Kennedy

    Jeff Kennedy F1 Veteran
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  20. Fave

    Fave F1 Rookie

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    Even if they did I'm sure uncle Sam will "invest" à la Chrysler and GM a few years back
     
  21. furmano

    furmano Three Time F1 World Champ
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    Not to come off as blasé but, Boeing will be fine. They just need to get through the update and they will have to compensate the families. But once they get this resolved, the flying public will forget this ever happened.

    Remember the growing pains the 787 went through? Yeah, you do, I do, the flying public doesn't.

    -F
     
  22. jcurry

    jcurry Two Time F1 World Champ
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  23. Bisonte

    Bisonte F1 Veteran
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    787s had some issues, but two of them didn't drill holes in the ground within a six-month period, killing 300+ people. That's a major difference.
     
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  24. Fave

    Fave F1 Rookie

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  25. Bob Parks

    Bob Parks F1 Veteran
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    I hate to say it but you're correct, it's not the same company. I'm sure that most have heard the old saying that," SxxxT runs down hill " There has to be some radical changes at the top of the company to address the philosophy and sense of purpose. That doesn't include the imbalance of the "bottom line people" and marketeers and whizzbang production procedures to "cut costs." Firm disciplines from mahogany row down to the tool room!
     

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