Indonesian aircraft missing off Jakarta
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Pretty simple to program if AoA = disagree then MCAS = off. That may be simplistic but may well be something like what Boeing is considering? Whether this or something else they come up with meets certification requirements is yet to be seen.
Exclusive: Boeing eyes Lion Air crash software upgrade in 6-8 weeks
https://www.reuters.com/article/us-i...-idUSKCN1NZ00S
Last edited by climber314; 5th Dec 2018 at 19:25.
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It's two subtly but significantly different statements.
that attributed to Boeing - "pilots weren’t ever likely to encounter" sounds like the kind of wording used in failures assessment guidance. (I suspect if I dug deep enough i could find it verbatim in guidance somewhere) On the other hand, "it'll never happen" is not the kind of statement any certification engineer would ever be capable of or comfortable making, and I'm guessing it's a paraphrasing (with a dollop of simplification) of the former statement.
that attributed to Boeing - "pilots weren’t ever likely to encounter" sounds like the kind of wording used in failures assessment guidance. (I suspect if I dug deep enough i could find it verbatim in guidance somewhere) On the other hand, "it'll never happen" is not the kind of statement any certification engineer would ever be capable of or comfortable making, and I'm guessing it's a paraphrasing (with a dollop of simplification) of the former statement.
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What would be interesting to know is how a Brazilian Max operator, GOL, implemented the type B training and checking for the MCAS system and what their pilots were told about the system.
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I rather believe that faced with a similar failure scenario - low speed limited authority "uncommanded motion" of the stab (it's commanded, but not in the circumstances truly intended) - with the pre-existing ability to disable the system and a pre-existing procedure which would address the runaway case (if somehow it repeated) then that would probably be classified as no more than a MAJOR hazard. In other words, I strongly suspect (and this may be an unpopular view) that the system as has been presented in public information is perfectly certifiable, and I suspect analogies could be found on other aircraft.
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Even assuming you did that - and I am sure they did do the corresponding analysis, we can argue about "properly" - you'd still be left with determining the hazard classification.
I rather believe that faced with a similar failure scenario - low speed limited authority "uncommanded motion" of the stab (it's commanded, but not in the circumstances truly intended) - with the pre-existing ability to disable the system and a pre-existing procedure which would address the runaway case (if somehow it repeated) then that would probably be classified as no more than a MAJOR hazard. In other words, I strongly suspect (and this may be an unpopular view) that the system as has been presented in public information is perfectly certifiable, and I suspect analogies could be found on other aircraft.
I rather believe that faced with a similar failure scenario - low speed limited authority "uncommanded motion" of the stab (it's commanded, but not in the circumstances truly intended) - with the pre-existing ability to disable the system and a pre-existing procedure which would address the runaway case (if somehow it repeated) then that would probably be classified as no more than a MAJOR hazard. In other words, I strongly suspect (and this may be an unpopular view) that the system as has been presented in public information is perfectly certifiable, and I suspect analogies could be found on other aircraft.
Similiar system on 737, STS has its own warning lamp to properly indicate failure to flight crew, per FAR25.672a clause
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I have a copy of the QRH Runaway Stabilizer NNC (my bold):
From the FDR traces, the failure mode does not appear at face value to meet the NNC condition statement. The adverse trim will stop for 5 seconds after pilot manual electric trim. This is an intermittent uncommanded trim mode, not continuous (as per NNC condition statement). Was this taken into account with the failure mode analysis?
In the failure mode analysis, would this condition statement be true for the single point failure (AoA signal) to be classified only as a major hazard, given that it could also generate an intermittent trim movement under some conditions (flaps up), continuous stick shaker AND UAS simultaneously.
In other words, with the design of this MCAS system, a single point failure has created a very complex scenario. Did Boeing actually consider the combined interactions of each of these outcomes in totality or individually when classifying the hazard? I would view the resultant accident as an indication of failure of the failure mode analysis. Why was the analysis flawed, that is the big question.
While you are contemplating this, try putting up with what this crew had to endure while they were trying to figure out what was going on.
Condition: Uncommanded stabilizer trim movement occurs continuously
In the failure mode analysis, would this condition statement be true for the single point failure (AoA signal) to be classified only as a major hazard, given that it could also generate an intermittent trim movement under some conditions (flaps up), continuous stick shaker AND UAS simultaneously.
In other words, with the design of this MCAS system, a single point failure has created a very complex scenario. Did Boeing actually consider the combined interactions of each of these outcomes in totality or individually when classifying the hazard? I would view the resultant accident as an indication of failure of the failure mode analysis. Why was the analysis flawed, that is the big question.
While you are contemplating this, try putting up with what this crew had to endure while they were trying to figure out what was going on.
CT, Good points about certification process. How might EASA have evaluated this; rubber stamp, or take an active interest in the specific changes ?
See the discussion re drills and training here :-
Indonesian aircraft missing off Jakarta
See the discussion re drills and training here :-
Indonesian aircraft missing off Jakarta
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And I'm still wondering why only Brazil? That information had to come from Boeing. Why not world-wide disto?
1. Neither the FAA nor Boeing were particularly forthcoming in their initial correspondence to operators on this matter. Neither the FAA Emergency AD or the Boeing FCOM Bulletin mentioned MCAS. Boeing's bulletin simply stated that
'In the event of erroneous AOA data, the pitch trim system can trim the stabiliser nose down in increments lasting up to 10 seconds.'
2. Having had at least another three days to come up with a clear description of MCAS the best that Boeing could manage was still somewhat ambiguous. What the subsequent Boeing correspondence fails to make clear is that:
a. In the absence of crew trim inputs, MCAS will only make one nose down trim adjustment of up to 2.5 units.
b. MCAS is apparently not interested in overcoming the attitude condition that triggered it - having made its initial nose down trim adjustment the trigger condition AOA can persist and (so long as there is no crew trim input) MCAS will do nothing further.
c. MCAS also apparently makes a subsequent nose up trim adjustment to return the stabiliser to the previously trimmed position.
3. Boeing fails to make it clear that a trigger for MCAS to continue trimming in more nose down trim is a countermand from the crew. Frankly, I think that is astounding. The system will ignore the persistence of the trigger condition after it makes its initial nose down trim adjustment but it will make a further nose down trim adjustment if the crew countermands it! Moreover, it will do that each and every time the crew countermands it. That is a very clear cut case of an automated system that is designed to override a specific crew command. To borrow an image from Gums, when the crew looks to wind off some of the automatically commanded nose down trim, HAL says, 'I'm afraid I can't allow that, Dave.'
Last edited by MickG0105; 5th Dec 2018 at 23:45. Reason: Formatting
A top executive at a 737 MAX customer agreed pilots didn’t need to know the system’s details. “They’re not engineers and their job is to fly the aircraft,” this executive said."
Psychophysiological entity
Mr Cheese.
It's just that one letter that makes the difference. It suggests ". . . limited to 2.5 degrees each and are . . ." might be an improved wording.
I see lots of folks assuming MCAS trims 2.5ND each time - but that's not what I get from the description of MCAS, which provides: Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers.
So - depending on speed and AoA, MCAS will trim up to 2.5ND - correct? Or - is there something else that confirms MCAS trimmed JT610 2.5ND repeatedly, and then continuously?
So - depending on speed and AoA, MCAS will trim up to 2.5ND - correct? Or - is there something else that confirms MCAS trimmed JT610 2.5ND repeatedly, and then continuously?
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The old system required you to roll into a 45 degree bank in the simulator and raise the nose the degree or two to maintain level flight, increasing the load factor, and either holding pressure or trimming it off while getting your instrument crosscheck warmed up. The new aerodynamics of the Max, if replicated in whatever simulator might have them, might allow you to roll into 45 degree bank and as you pulled , you might not have to pull anymore or pull less because the engines aero effect started pulling for you. What's wrong with that I don't know. If you have a good crosscheck you fly the attitude and performance instruments and do what ever it requires with the stick. If there is any other flight envelope where the old system would be deficient for the MAX and you needed the MCAS we don't know and Boeing isn't telling us but previous threads have hinted that a normal flight would never see it. This is all supposition by me wondering why steep (or step) turns were mentioned in the one of the Boeing blurbs.
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Why did Boeing not fit a stick nudger as was done for the 747 on the British register?
If the 737 MAX would be a clean sheet design, the horizontal stabilizer and elevator would for sure look differently.
Trim has always been a killer item requiring close attention and it will always be, even if now a box of chips and a bunch of sensors pays the attention. Having new aircraft designs without a trim wheel is unbelieveable for me, but it becomes the standard. Trim becomes a computer assist item.
During each pre-flight, admittedly in a different airframe, each pilot would test the stick shaker and I would expect from that, they would be quite used to the feel on their column and sound, is it the same on the B737 MAX ?
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While it does not present itself as continuously running run-away automatic stabilizer motion, with the airplane flying at a relatively steady flight condition repeated instances of the system moving the airplane away from the trim condition that the pilot has established manually should be recognized as improper automatic stabilizer operation and disabled via the cutout switches.
Last edited by FCeng84; 6th Dec 2018 at 05:22. Reason: typo
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Volume - MCAS was not introduced to make up for deficient elevator nose down control authority. MCAS is there to improve handling characteristics as proscribed by the FARs at elevated AOA. The pilot has plenty of control power to lower the nose via the column. That is not the issue.
Last edited by FCeng84; 6th Dec 2018 at 10:24. Reason: typo
Any of you who are interested in flight test and the required handling qualitiies of aircraft, particularly at the stall, will probably enjoy these podcasts and find them most informative. At 1hr 14mins into the second podcast he describes the 707 and in the third he talks about the 747 and later the T-tail aircraft. Much of what he has to say is relevant to this 737 accident.
D P Davies interviews on certificating aircraft
D P Davies interviews on certificating aircraft
Last edited by Bergerie1; 6th Dec 2018 at 10:58.
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Boeing fails to make it clear that a trigger for MCAS to continue trimming in more nose down trim is a countermand from the crew. Frankly, I think that is astounding. The system will ignore the persistence of the trigger condition after it makes its initial nose down trim adjustment but it will make a further nose down trim adjustment if the crew countermands it! Moreover, it will do that each and every time the crew countermands it. That is a very clear cut case of an automated system that is designed to override a specific crew command. To borrow an image from Gums, when the crew looks to wind off some of the automatically commanded nose down trim, HAL says, 'I'm afraid I can't allow that, Dave.