How much longer to fix the 737 Max

Discussion in 'Flight Following' started by brien23, Apr 13, 2019.

  1. Bell206

    Bell206 En-Route

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    No. The original MCAS version also had an accelerometer input, in addition to the AoA input, as the MCAS was only needed when the aircraft went into "ballistic" mode. The AoA option was for a 3rd AoA as all aircraft have 2 as stated above.
    FYI: the air carriers dictate the market not the other way around. The Airbus A320 NEO was serving that market and the MAX was Boeing's response. As to a "cheaper version," if that was the case then there would only be Airbus aircraft flying as everything built by Airbus is always cheaper dollar wise when compared to like aircraft. They make their money on the support side.
    Technically yes. However, if it were to say a single AoA vane could lead to a catastrophic failure risk then it would be closer to being correct.
    To pick up from the answer above and without getting into the complexity of risk analysis, the pilot's interaction to a system failure is one of the first items addressed in determining it's failure risk. There is actually FAA approve guidance on how the aircrew reacts right down to the length of time it takes to react. If I recall during the original risk assessment of the MCAS it was determined the aircrew would react with the 3 second limit, which along with the rest of the original assessment requirements, kept an MCAS failure below the catastrophic risk and the major risk categories. So there's more than a simple "assumption" if the crew can intervene, it's actually part of the certification guidance. Hence the ongoing regulatory review of cockpit data saturation levels and pilot training to see if the 3 second rule and other guidance is still valid.
    Another cherry-picked item. The forces were calculated back when the manual trim system was certified years ago. There was zero requirement to recalculate for MCAS install. In the Ethiopia case, it's my understanding there are other issues involved outside the MCAS involvement.
    And if you had to build a web server under an equal level of regulatory oversight it would be interesting to see if that backup would still be there or simply offered as an option especially if your customer picked the price point it would buy at.;)
     
    Last edited: Feb 17, 2020
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  2. denverpilot

    denverpilot Tied Down

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    Thanks for clearing that up. It was an honest question.

    In this case, I was thinking they don’t get to choose mandatory safety systems.

    Wasn’t intending to cherry pick. But your answer is worse than I suspected. Add a system that can move the entire stab and because there’s “no requirement”, nobody checks to see if you just made the trim impossible for the human to override it?

    It’s similar to other problems in corporate engineering. “Full stack” aerospace engineer, or project manager ticket closers?

    Have to watch the big picture.

    I’m sure it would have it. And I’m sure customers would have no choice on the price.

    Just like my Garmin has to be certified so it cost close to $10,000 for $1000 worth of hardware and the real cost is in software and mandatory safety hoops.

    And yet... there’s a mistake fixed in that software roughly annually since release. Which is a whole different kettle of fish about how software is managed today.

    Look it flies! It met the requirement. :)

    [​IMG]
     
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  3. Bell206

    Bell206 En-Route

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    Curious. What "mandatory" safety systems were missing on the MAX?
    You mean just like the autopilot or trim switches do? What would be different if the MCAS moved the stab from a run-away stab emergency procedure point of view? Regardless, as I mentioned earlier, if you are referencing the Ethiopia flight, there were several other issues involved unrelated to the MCAS failure that contributed to crew unable to re-trim the aircraft via the manual trim wheel. But as you said, in the big picture, if keeping the aircraft in trim was so difficult how did 2 previous pilots on separate flights manage to handle errant MCAS activation and land the plane?
     
  4. denverpilot

    denverpilot Tied Down

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    I was under the impression the missing piece was a second data input for MCAS to cross check against and failsafe if they disagreed. Was that inaccurate info?

    Were these “other issues” in the Ethiopia flight things that were far out of the norm or unpredictable?

    Honest question. You seem to know.
     
  5. Larry in TN

    Larry in TN Cleared for Takeoff

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    The design was that if there was an MCAS runaway the pilots would use primary trim to keep the airplane in-trim while they accomplish the existing runaway stabilizer procedure to disable electric trim. That plan worked after the first unscheduled MCAS event but failed on the next two. That's when it was decided that they couldn't rely on the pilots to fix it and would have to introduce a more complex system logic to reduce the probability of unscheduled MCAS events.

    Inadequate re-trimming with primary trim
    Failure to disengage the auto-throttle
    Failure to reduce thrust from the climb power setting
    Allowing the airspeed to eventually reach over 390 KIAS (340 KIAS is redline, 210-250 kts would have been the expected range at that point)
    Re-retracting the flaps, after having re-extending them and stopping MCAS, in response to a (flap) overspeed warning
    Failing to disable the electric trim system before the stabilizer reach a near-full nose-down position
    Failing to work together to turn the manual trim wheels once the electric trim was disabled
    Re-enabling the electric trim system but, instead of using the primary trim switches, repeatedly attempting the engage the autopilot (which won't engage unless the airplane is in-trim)

    I could probably think of a few more but that should be enough.
     
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  6. Bell206

    Bell206 En-Route

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    The only existing "missing pieces" I'm aware of that were not installed on the Lion or Ethiopian MAXs were the optional AoA Disagree Light and AoA data display on the PFD. Neither of which are required/mandatory by any CAA. And it's my understanding one of these options will become standard equipment as part of the mitigation process on the MAX.
    Most of the info I have is out in the public domain with some forwarded from semi-private sites. But all readily available to those who look for it. As to the Ethiopian MAX can't answer the later part of your question as the final report is not out, but the discussions center around 2 main points: continuous TO thrust throughout the 6 min flight even after they turned toward the airport and the crew's multiple attempts at corrective measures only to reverse those actions or repeat incorrect measures. There are other items but do not have the support of aircraft data at the moment which should come out in the final report.
     
  7. Tantalum

    Tantalum Final Approach

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    A plane that first flew in 1987 vs one that first flew in 1967.. and was never envisioned for an A320 competitive role.. hence the short gear and stout body. The 737 was a short haul plane to serve small bases that may not have jetways.. it was never meant to be in the same league or to compete with the same market as the A320. The 757 was supposed to serve the markets that the A321 and 737 are trying to fill the gaps in, problem is, Boeing killed the 757 and the Airbus 320 is actually a modern flexible platform with a competent flight control system vs one that has had so many different additions and add ons done it looks like one of those poorly architected hodge podge houses you see, something that started as a 700 SF bungalow in 1946 and is now a 2,800 ft 2 story peculiarity and blight on the neighborhood

    How did the pilots screw up THAT badly..? Is honestly none of this on Boeing, for designing (and failing to train the crews) in a system that could, apparently quite easily, full nose down trim a plane without the pilot's input?


    **Either way we slice it, 350 people died in relatively close sequence, both tied (in some capacity) to Boeing's cheapest-possible-fix solution to a plane that is something the original engineers never meant it to be. There can get be an analysis paralysis or find way to justify why it was built the way it was.. but at the end of the day Boeing is not new to this rodeo, they've been building planes longer than Airbus, but seem to have completely lost their way after they rolled out the 777
     
  8. denverpilot

    denverpilot Tied Down

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    Yeah I haven’t read heavily on it. Thus why I’m asking. You appear to have.

    And the AoA Disagree warning is what I was thinking of. Thanks.

    That it wasn’t required really doesn’t interest me as an engineer who has to catch all sorts of things that aren’t required by anybody, but will bite people square in the ass.

    They pay me to stop those things.

    Seems stupid to make that light optional.

    Which seems to be accurate if it’s now going to be required.

    A good friend flies them. He started off thinking the whole thing was just poor training.

    After he read up on it, he decided his own company’s plan to just send everybody a PDF with the changes would have been very inadequate. He also wasn’t happy none of the sims could accurately simulate MCAS problems.

    Of course for job protection reasons he’s not here discussing it. LOL.
     
  9. Bell206

    Bell206 En-Route

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    And what if the FAA certification requirements are lacking vs Boeing selecting a "cheap" version? The DOT Special Committee report on the MAX's certification process was released last month which found all current processes were followed in the certification of the MAX. They even went as far to question if the MAX would have gone through a complete TC certification vs an ATC certification would the plane have come out different. The consensus no. The review did make recommendations to change things especially on the flight crew/aircraft interface, etc. So I'm sure those will be implemented. The RCAs that are out there on the MAX accidents point to a number of holes lining up at a number of different stakeholders. So while it's easy to point a finger strictly at Boeing if there was ever a case of an entire system failure this will be it. And once the final investigative report is out will probably bear even more details and issues.
     
  10. Bell206

    Bell206 En-Route

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    In hindsight maybe. But, to me, it would seem even more stupid for the operator not to buy all those options especially since they planned to use low-time/experience co-pilots and at least give their crews all the advantages possible.
     
  11. denverpilot

    denverpilot Tied Down

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    True. But I see no good reason to make it optional. Guessing whether a system has failed isn’t helpful.

    Shades of AF447 and even all the way back to Eastern 401. Questionable / misunderstood indications.

    We already know pilots need unambiguous clear indications during system failures. Well studied and documented.

    Fatigue, confusion, overload, whatever... has turned warning system design flaws deadly ... forever.

    I can’t really fault most traditionally trained pilots from keeping thrust high when they want to climb. Any airplane that won’t go up at takeoff thrust is reversing a lot of training and “normal” airplane behavior in people’s heads.

    If that isn’t going to work, it seems important to train that one. Not sure how anyone could do that with sims that were unable to. Might have even noticed the lack of an indication confused people in the box when deploying that.

    Oh well. Hopefully they fight through their culture issues and think harder about it. Low experience crews aren’t going away worldwide anytime soon.
     
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  12. Spring Ford

    Spring Ford Pre-takeoff checklist

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    My understanding is that the design was that the AoA disagree light was not optional but that an implementation error meant that the light only worked when the customer ordered the AoA indicator (an icon on the main display panel).

    According to this Reuters report Boeing noticed this in 2017 and scheduled the software fix (the "light" is another icon on the panel) for 2020.

    https://www.reuters.com/article/us-...t-for-three-years-u-s-lawmakers-idUSKCN1T8284

    https://boeing.mediaroom.com/news-releases-statements?item=130431
    "Boeing Statement on AOA Disagree Alert"
    "The Boeing design requirements for the 737 MAX included the AOA Disagree alert as a standard, standalone feature"
     
    Last edited: Feb 17, 2020
  13. Tantalum

    Tantalum Final Approach

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    It's a fascinating and tantalizing case study for sure. Even if the FAA process was lacking I sort of expect that (government sucks and all that), but to see it from Boeing.. sucks
     
  14. Spring Ford

    Spring Ford Pre-takeoff checklist

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    Also - there was that stick shaker thing telling the crew to get the nose down to avert a stall. Reducing power when there is a strong pre-stall indication might be a tough ask?
     
  15. denverpilot

    denverpilot Tied Down

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    Didn’t know about that. That’s ugly. Why was it activated at over 300 knots? The same AoA vane failure?
     
  16. Sluggo63

    Sluggo63 Line Up and Wait

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    Explain to me how this was a problem...

    We should count ourselves lucky that this didn't happen in the US. If it did, I'd say there was a 50/50 chance that it would have had the same outcome as Lion Air and Ethiopian. Then we wouldn't have foreign pilots to blame...
     
  17. Spring Ford

    Spring Ford Pre-takeoff checklist

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    Yes high AoA = stick shaker.

    The first symptoms were simultaneously -
    Captain's (Pilot Flying) Stick shaker - which is very loud I have read
    Airspeed disagree flag - airspeed and alt are corrected for AoA.
    Altitude disagree flag

    In the LionAir case it happened at Weight off wheels
    In the Ethiopian case at a low altitude (say <100ft?, it's in the preliminary report).
    http://www.ecaa.gov.et/Home/wp-content/uploads/2019/07/Preliminary-Report-B737-800MAX-ET-AVJ.pdf

    Then when the flaps were retracted (1000ft?) MCAS kicked in and started the nose down trim thing. MCAS is inhibited with flaps down.

    Maybe the pilots were not the next Chuck Yeager but almost no one is now blaming the pilots in any published material. It seems to me that they had a lot on their plates.
     
    Last edited: Feb 17, 2020
  18. Greg Bockelman

    Greg Bockelman Administrator Management Council Member

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    Yes, same alpha vane. Failure caused a high angle of attack indication that caused MCAS activation. It wasn’t a failure of MCAS because it did exactly what it was designed to do.
     
  19. denverpilot

    denverpilot Tied Down

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    Ugh. I don’t like hearing airliners only have one sensor. Or one active and no indication the offline one doesn’t agree. Or whatever.

    Too much at risk.

    One in a spamcan is fine... I guess.
     
  20. Spring Ford

    Spring Ford Pre-takeoff checklist

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    Hmmm ...

    upload_2020-2-18_0-34-11.png
     
  21. Bell206

    Bell206 En-Route

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    FYI: it was a failure of a AoA vane. Just as Lion. The stick shaker and the MCAS did exactly as they were programmed to do based on the bad AoA data.
     
  22. denverpilot

    denverpilot Tied Down

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    What Greg was saying is as a module/black box, it did what it was designed to do — bad inputs caused it to do that.

    Which I’m fine with as an engineer.

    The overall outcome was bad, but the MCAS box itself performed what it was programmed to.

    The failure is in knowing you can never trust a single analog sensor to feed anything critical.

    Kinda like when the bosses bought data center generators that only had one oil pressure sensor...

    But we had oodles of time to figure that out, because the batteries were running the place.

    Aircrews suffer time dilation even when they have oodles of time, especially at low altitude, not able to climb. So they’re going to revert back to primacy... power means up.

    Well except for Colgan. And AF447. And...

    If we are talking lightbulbs, Eastern 401 because one was burnt out.
     
  23. kgruber

    kgruber En-Route

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    And now to top it off............The 800MAX wiring is hosed.
     
  24. Greg Bockelman

    Greg Bockelman Administrator Management Council Member

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    Yeah, but...

    The FAA is just as complicit in that as Boeing is.
     
  25. Spring Ford

    Spring Ford Pre-takeoff checklist

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    I guess I don't understand why that is interesting. I don't feel the need to write that the windows operated correctly for example, the aircraft had millions of components that operated correctly. The MCAS system certainly failed. Saying that the MCAS code was to specification doesn't seem to help much.

    The MCAS system includes, sensors, inputs, hardware, algorithms, code, outputs, actuators, a human interface (although version one didn't seem to have one, I bet version 2 will be in the manual), ...

    In any case the MCAS code is now being redesigned and re-implemented as I understand it, although this seems to be causing Boeing some difficulty.

    Hopefully they will be on track soon.
     
  26. denverpilot

    denverpilot Tied Down

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    That sensor is shared across multiple critical and non-critical systems. What was missed was the team that built the MCAS was probably told to trust that input. And nobody thought about it.

    Silos.
     
  27. Larry in TN

    Larry in TN Cleared for Takeoff

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    There is no AoA Disagree light. It is an AoA Disagree message which appears near the lower-right corner of each pilot's primary flight display. As has been pointed out, the AoA Disagree message was not an option. They found that a software bug prevented it from working if the AoA Indication (also on each pilot's primary flight display) was not enabled.

    The AoA display is typically only ordered when the airline is also ordering the HUD system as the HUD also has an AoA display. The HUD is used for hand-flown CAT II and CAT III landings. Other airlines use autoland for approaches below CAT I so don't need to buy the HUD nor AoA display.

    Every airplane with powered trim can have a failure which runs the trim to full nose-down without the pilot's input. That would include every transport jet as well as many G.A. airplanes. Pilot's of those airplanes need to know how to disable the system.

    MCAS is an additional software function of the Speed Trim System. It did exactly what it was programmed to do. The problem is that the required risk analysis relied on the pilots to disable the electric trim system in the event of an unscheduled MCAS activation and two out of the three crews who experienced the failure failed to do so.
     
  28. PPC1052

    PPC1052 En-Route

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    I have never flown a large transport category aircraft, and I know you have. But my understanding is that the failure to keep the airspeed down resulted in strong forces on the control surfaces, which made it nearly impossible to manually re-trim the nose down pitch once the electric trim was disabled.
     
  29. Bell206

    Bell206 En-Route

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    It was my understanding the AoA Disagree message could not be enabled unless the optional AoA Indicator was also installed/enabled and was a carry over option from the NG? I believe some operators (American, flyDubai, etc) had these AoA systems installed/enabled in their NG/MAXs? Was the software bug a different problem?
     
  30. Tantalum

    Tantalum Final Approach

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    Yet the issue manifested itself on just the MAX. Certainly there is something defective with the design and implementation of it on this jet, which you touched on below, that they relied on the pilot to be the fail safe, yet seemingly didn't train the pilots accordingly

    Absolutely, but at least from what the media reported, many pilots weren't even aware of what MCAS is or how it's meant to operate, receiving an absolute bare minimum of training. There was an assumption they'd recognize an uncommanded electric trim to full nose down and catch it in time before it was too late. If Piper started to quietly install software on Meridians that have a propensity to fail and might put the nose full down at critical phases of flight, but don't tell the pilots, I could see that being an issue for those planes too. Probably not deadly, as atleast for all but the physically feeble most GA planes can be hand flown with a trim way out of whack. Every envelope protection GA plane I've flown has had a lot of documentation on its operation, and at least give you a PFD alert when it's active. For the MCAS to just go off and not tell the pilots at all what it's doing in an obvious manner is pretty egregious. For what its worth at least the Garmin systems I've flown if there's a disagreement in the AHRS data you get a CAS that you've lost the envelope protection functions

    That's a pretty big hole.. in the ultra-redundant world of commercial jet aviation I'm honestly surprised that a system designed to save lives could fail so easily, and not tell the pilots it has failed with either a disagree light, or an alert to let the pilots know it was on
     
  31. Larry in TN

    Larry in TN Cleared for Takeoff

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    It isn't impossible but it is significantly more difficult.

    Each trim wheel has a fold-out handle. The two wheels are positioned so that the two handles are 90deg of rotation apart. This is so that, throughout the full rotation, at least one pilot always has his handle in a position that offers good leverage.

    When the airplane is close to in-trim, the pilot-flying can manual trim the airplane with his inboard hand while flying the airplane with his outboard hand.

    As the out-of-trim condition increases, the pilot-flying calls for "Trim up", "Stop trim", etc. while the pilot-monitoring trims.

    As the out-of-trim condition continues to increase, the two pilots work together to trim with both wheels simultaneously.

    If that isn't enough due to being excessively out of trim at very high airspeeds, you "unload" the tail by allowing the nose to slowly drop as both pilots trim together.

    This system was used in the Boeing 707, 720, 727, and 737 aircraft. I did all four steps in the sim during my initial training and, again, in recurrent last month. With full nose-down trim, and airspeed well into the 300kt range, we were able to bring the aircraft back into trim without significant altitude loss during the "unloading" process but neither one of us would have been able to do it alone.

    I've seen no evidence to indicate that either of the accident crews progressed beyond a single pilot trying to turn the wheel on his own.

    No. It was supposed to work on all airplanes. A software bug prevented it from working on airplanes that did not have the AoA display enabled. That bug wasn't discovered until the accident investigation.

    It's really a red herring, though. The "AoA Disagree" message would not have helped. The important piece of information that both accident crews missed, or were slow to realise, was that they had a runaway stabilizer. That was the primary threat and identifying and correct it was the key to a successful outcome. Wasting time considering, or even actioning, an "AoA Disagree" message would have even further delayed them reaching the needed procedure.
     
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  32. Sluggo63

    Sluggo63 Line Up and Wait

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    My point was (and I understand there are a lot of interrelated aerodynamic forces at play) that the faster they went, the more "in-trim" they got.

    With the stabilizer trimmed nose-down (for a higher airspeed) you are out of trim if you are slower than that speed. The slower you are from your trimmed speed, the higher resultant stick forces.

    In the KC-135, we would practice this in the actual aircraft during flight. We would accelerate the aircraft to 250-300 knots, trim it out, then slow to 200 knots without retrimming. The result is that the aircraft was out-of-trim in the nose-down direction (similar to, but obviously not as drastic as the MAX MCAS situation). We would have the student hold the aircraft level and feel how heavy the yoke was. Often times the pilot flying had to wrap the yoke in a bear hug just to be able to hold the aircraft in level flight. The other pilot would then try to use the manual trim wheel to retrim the aircraft, finding it impossible unless the PF bunted/unloaded the jet to allow the manual wheel to turn. Then we would accelerate the aircraft and show that as you accelerated back towards trimmed speed the stick forces got lighter and the manual trim wheel became usable without unloading. It's my understanding that Boeing had the unload/bunt/roller-coaster maneuver in early 737 flight manuals but removed it and airline crews were no longer trained on it due to the fact that modern aircraft have a trim brake that would prevent the plane getting out-of-trim far enough to warrant having to perform that maneuver. The KC-135 didn't have a trim brake until relatively recently so Runaway Stab Trim was an often practiced maneuver in the aircraft and in the simulator and we were all taught the unloading maneuver to retrim the aircraft.*

    Like I mentioned above, when the aircraft is severely out-of-trim, one pilot probably has to use both hands to just hold the aircraft level and would be unable to help with the wheel. By going faster and faster, the aircraft would have been going from being severely untrimmed to being closer and closer to being bank in-trim. If they could have matched their airspeed with what the speed that the stabilizer was actually trimmed for, they would have had normal stick forces and would have had no issue keeping the nose up because they would have been on-speed. I don't know what speed full nose-down trim in the 737-MAX8 would have been, but it was probably faster than they were going when they crashed.







    *Extraneous KC-135 info: There are only two flight control surfaces in the KC-135 that are hydraulically powered. The rudder and the spoilers/speedbrakes. Besides normal electric trim, the autopilot trim system and the manual trim wheel, there is another, ingenious, way to trim the KC-135. Use the speedbrakes. The KC-135 has two sets of spoilers on each wing, inboard and outboard. No one really knows if it was a thought out design feature, or a fortunate accident but it just so happens that where the spoilers sit on the wing, the inboard spoilers are forward of the center of lift and the outboard spoilers are aft of the center of lift. With toggle switches located under the glareshield, we were able to cut-out hydraulic power to either the inboard or outboard spoilers rendering them inoperative when the speedbrake handle was deployed. The result would be that if only one set of spoilers was used it would force the nose up of down depending on which spoilers were used and which were cut-out. We called the switches colloquially the "Nose-Up Switch" (inboard spoiler cut-out) and the "Nose Down Switch" (outboard spoiler cut-out). For example, if we cut-out the inboard spoilers and deployed the speedbrake handle, only the outboard spoilers would raise. This would kill lift aft of the wing's center of lift and the nose would raise. The opposite effect would happen with the outboard spoilers cut-out. When we did our stabilizer trim training that I mentioned above, we would also show the effectiveness of using split spoilers to get the aircraft back into a trimmed feel. On top of that, we would also fly a "jammed stab" demo in the airplane where we would simulate the stabilizer would not be able to move by using any means, to include the manual trim wheel. We would have the student fly the aircraft using only split spoilers as the method of trimming for an entire traffic pattern to include doing a touch-and-go without the use of trim and only spoilers. Several years back the Air Force decided that this particular demonstration was too risky to continue to perform in the aircraft and relegated it to "simulator only."
     
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  33. Piperonca

    Piperonca Pre-takeoff checklist

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    Clearly the pilots in both accidents failed in some of their actions. Justifiable or not, those are the facts. However, Boeing and FAA are culpable as well. Also seems beyond dispute.

    My questions are these:
    1. Is the manner in which the electric trim system is disabled on the Max changed from previous 737 models?
    2. Does pulling back on the yoke to disable a runaway stabilizer (or anything else) change on the Max vs. previous models?
    3. If either (1) or (2) is true, were there instructions regarding same in the training or operating documents for the Max?
     
  34. kayoh190

    kayoh190 Super Moderator Management Council Member PoA Supporter

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    1. There's a minor difference. In the NG the two cutout switches were labeled STAB TRIM MAIN ELECT and STAB TRIM AUTOPILOT. As you can guess from the labels, one disconnected autopilot trim inputs to the motor and one disconnected inputs from the main electric trim. On the MAX, the same two switches are labeled STAB TRIM PRI and STAB TRIM B/U. In this case, if either switch is placed in CUTOUT, both autopilot and main electric trim inputs are disconnected from the trim motor.

    2. Not that I'm aware of (I'm reluctant to say 'no' because I no longer have the trust in my manual that I used to).

    3. The differences in the cutout switches were noted in our systems manual, but from a practical standpoint nothing changed, as the method to handle a trim runaway (MCAS related or not) is identical between the NG and MAX. Both switches get placed to CUTOFF.

    Remember, as @Larry in TN has mentioned numerous times in various threads, MCAS is merely an additional piece of software in the Speed Trim System (STS). The airplane providing automatic trim inputs to the stabilizer when the plane is being hand flown isn't something new to the MAX. Here's a little blurb on it for those that are curious (mention of MCAS is still missing as of the latest version of our manual):

     
    Last edited: Feb 18, 2020
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  35. PPC1052

    PPC1052 En-Route

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    Thanks. That's pretty informative. I had been thinking about excess airspeed causing too great of a force on the elevator trim, which in turn made it difficult to manually turn the trim wheel. I was not really thinking about the airspeed being too slow for the trim position and the resulting back force on the yoke causing the resistance to the trim wheel. It's kind of obvious in retrospect, given that I was aware of the unloading procedure you described. Obviously, if it is trimmed for a greater airspeed, it's going to require a heavy back force on the yoke to keep the nose up. But if the airspeed is too slow for the trim position at 390-400kias, is that going to result in excess force on the trim tab such that it's going to be extremely difficult to change the trim setting? Or, perhaps the better question is, if the airspeed were to remain below 250 where it would be if set properly per the unreliable airspeed checklist, would the trim wheel be less difficult to turn to proper trim than it would be at 400? Which contributes more to the resistance to turn the trim wheel, the excess airspeed of the plane or the excess amount of nose down trim? Assuming the plane reached full nose down trim, would it be easier or harder to turn the trim wheel as the plane accelerates? On the one hand, it may be less out of trim as the speed comes up. But on the other, the airspeed would be greater, potentially creating greater air pressure on the control surfaces.
     
    Last edited: Feb 18, 2020
  36. Sluggo63

    Sluggo63 Line Up and Wait

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    I'm not a 737 guy, and never have been. But what I understand of the system the major difference between the two is that the stabilizer trim brake, which would be operational for a normal runaway stab trim is disabled during MCAS operation. I think this is huge. In a normal, run-of-the-mill runaway stab trim, the brake would stop the stop the trim moving while the aircraft was maybe 0.5 to 1.0 units out of trim. This buys you scads of time to get the trim switches to cut-off without even having to fight the airplane. The MCAS doesn't allow that.
     
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  37. kayoh190

    kayoh190 Super Moderator Management Council Member PoA Supporter

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    Our manual doesn't make a distinction between the MAX and NG with regard to the trim brake. That said it still doesn't mention MCAS either. ;) I agree with you though - it's a problem (among others) if it's true.
     
  38. denverpilot

    denverpilot Tied Down

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  39. Cap'n Jack

    Cap'n Jack Final Approach

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  40. denverpilot

    denverpilot Tied Down

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