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Boeing admits flaw in 737 Max flight simulator

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Boeing admits flaw in 737 Max flight simulator

Old 20th May 2019, 05:20
  #41 (permalink)  
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Originally Posted by Water pilot View Post
And the hits keep coming. Did nobody test this thing before releasing it to the public?

This is rather concerning, because the simulator designers are going to be using the same engineering calculations used to inform the designers of the hardware. If the maximum forces involved in this system are much larger than calculated then that calls into question all of the engineering that went into it, from bolt sizes to the number of strands in the cable. This is at least the second indication that the engineering projections related to relocating the engines did not match reality.
As other posters have stated before, Boeing’s supply binaries to the TDMs for integration. The TDMs are no longer involved in design.
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Old 20th May 2019, 06:17
  #42 (permalink)  
 
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Originally Posted by gums View Post
My examination of the FAR requirements do not reference stall prevention, just a requirement to have increased back stick force or actual elevator force as AoA increases. They could have done the trim kludge at a much lower AoA.
Gums,

to accept MCAS is there only to increase back stick force, we need to accept that Boeing engineers decided it was a good idea to repeatedly move the stab, probably one of the most critical control surfaces, just in oder to simulate increased stick force.

We need to accept that noone asked if this might by any chance be a bad idea. Like, you know, trimming all the way nose down in close proximity to the mother Earth. Noone remembered AF447 where pilot error resulted in full up trim, making stall unrecoverable - so nobody said "uhm guys, if something goes wrong and this system fails, it could mean trouble".

Again - all with the goal to produce some back stick force? I mean, for sure there must be better solutions for that alone?

What if there is another explanation? Perhaps the nacelles are so far out that it's a serious problem? Perhaps they act to some degree as canards when approaching stall? Imagine the wing stalls but the nacelles still produce lift... you'd be in big, big trouble.

Now this big big trouble would be something to justify MCAS. But due to competition and regulation requirements Boeing could not be open about it, or it would have meant entierly new certification. That's why it's half baked, hidden and relies on a single AoA.

Test flight data should show if this is indeed the case. Requirement for 2.5 units trim every 5s surely does seem like a smoking gun. Testimony from test pilot would be interesting. I'm sure we will hear about it in the coming years.

Then another question is, why were the cutout switches changed to prevent manual electric trim along with MCAS. There must be a reason. Without a reason it makes no sense. Did Boeing by any chance estimated that keeping the electric trim on without MCAS could easily put the airplane in the high AoA situation as described above? That could be another smoking gun. If that is not the reason to change the switches, then what was it? Any reasonably ideas?
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Old 20th May 2019, 13:00
  #43 (permalink)  
 
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Gums et al, Boeing admits flaw in 737 Max flight simulator
Re ‘like other 737’, stability ‘enhancement’, or anti stall.

First point, this discussion is probably in the wrong thread. Second, there is very little definitive information on these aspects (see Boeing refs), thus supposition reigns. Third, using what is known about an aircraft to build a simulator is not the same as taking a simulator and inferring what the actual aircraft characteristics are, but there ominous indications in this instance.

Like other 737’, is more of a marketing or commercial argument; as such it could be managed by other means (even if not saving $1M per a/c).
Stability vs stall’; the critical points are when and where. Any problems with stall ident - the point of stall, or characteristics at that point, or recovery capability, requires quick and forceful action, loud and positive, thus stick push or similar.
Anti stall’, not a good term, is more associated with stability, usually prior to stall warning - stick shake; awareness, but not always with action.

I conclude that MCAS is a requirement of stability certification, quiet and subtle background enhancement, ‘dressed-up’ to placate customers. Note the use of AoA vs speed input, flaps up only, and Mach input to manage what appears to be a complex problem in a small part of the flight envelope - wt, cg.

The trim input was intended to be small, incremental, and unobtrusive. The design theory appears to be OK, the engineering implementation, checking, and certification a disaster.
MCAS should not be compared with STS, or even Mach trim; nor its implementation as FBY.
I agree with gums # 36 …

Stability involves ‘feel’, measured as stick force in certification.
As an explanatory description only: consider trimmed flight at constant speed, constant thrust. Enter a turn without changing trim position or thrust (aircraft will descend). The ‘feel’ increases as a pull force; this can be assessed at various bank angles - different AoA.
Repeating these tests for a range of speeds (different trim speed) there should be a consistent relationship between bank angle and pull force. Inconsistent force - reduced pull force with bank angle, or with speed decrease at constant bank angle, could add workload in manual flight.
In extreme, slowing down in a turn or level flight at constant trim should always require more pull force, if not the aircraft feel is an unnatural ‘push to turn’. In some cases reducing forces could tend to oscillation or pitchup. There could be similar interactions with thrust - altitude.

MCAS “will not be a significant issue after modification”, nor require hands-on training except if the handling in the failed state is significantly degraded - cruise flight only. [opinion]

The important issues arising from the simulator mistake are in the processes of checking and certification of training simulators, and the possibly of a shortfall in knowledge about the aircraft, particularly the effect of trim failure.

Gums, [engineering or aerodynamic fix] Instead of lengthening the undercarriage, Boeing should have put the engines on top of the wing [/ fix]


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Old 20th May 2019, 14:38
  #44 (permalink)  
 
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Salute!
Thanks PEI and Derjo. Two good points.
Way back I mentioned the "canard" effect of moving the motors forward. I even propose using vanes or vortex doofers on the motors to keep the pitch moments close to the older model planes. A far superior solution than a half-assed sftwe thing embedded in an existing black box. End of rant
.
Secondly, I like having the discussion here, as it seems civilized and less visited by newbies that have not kept up with all the posts. I still have trouble with the control force versus the AoA as a criteria. This is from cert of the Airbus 320 and subsequent. My 'bus friends here will gladly help and confirm that there is zero feedback to those independent sticks. I am concerned here because the 737 and other Boeing planes have had artifical "feel" for decades. Looks like when all the "help" quits, the 737 still had a cable for the elevator and the little wheel for trim. Attaboy for the folks in the "just fly the plane" camp.

However, and this is important, those days are going fast, and I loved them as a wannabe astronaut. But we can't go back, and shouldn't, except to remind us of our roots.

We may never get to the xBox or Nintendo or Playstation world except the folks at Creech AFB. That's because in real aerospace vehicles you can sense gees and body rates. And so far, the carbon life-form creatures have done well in a few military jets and in the Airbus with zero feedback from the control surfaces. Besides, if you get high enuf there ain't no aero loads on the surfaces unless you are going the speed of stink ( ask my Blackbird friends).

Gums sends...

Last edited by gums; 20th May 2019 at 22:49.
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Old 20th May 2019, 14:53
  #45 (permalink)  
 
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Well, the side stick is spring loaded and therefore provides exactly what is needed. Feedback increases linear with angukar displacement which is G command.
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Old 20th May 2019, 15:40
  #46 (permalink)  
 
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Simulator stuff...

The crux of this discussion revolves around the simulator accurately representing the aircraft.

How does any simulator do that?

Well, firstly it starts with DATA. Any simulator is essentially constrained to operate within the 'box' defined by the data provided by the airframe manufacturer as part of the data package.
(There is one caveat surrounding upset and recovery training, as recently introduced, which typically is based on engineering data or similar).

However, there is also the issue as to whether the simulator manufacturer implemented the data correctly. Aside from engineering simulators within Boeing (of which I know nothing), there are five Level D 737MAX simulators that have been certified - four manufactured by TRU Simulation + Training and one by CAE which I believe is currently operating with a Level C certification.

Was adequate data provided for the trim wheel loading? My guess is the data for the trim loading at high speed was NOT provided at low altitude at very high speed...

I suspect that is true of all simulators, whether 737MAX. NG or whatever.

- GY
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Old 20th May 2019, 15:58
  #47 (permalink)  
 
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Originally Posted by GarageYears View Post
The crux of this discussion revolves around the simulator accurately representing the aircraft.

How does any simulator do that?

Well, firstly it starts with DATA. Any simulator is essentially constrained to operate within the 'box' defined by the data provided by the airframe manufacturer as part of the data package.
(There is one caveat surrounding upset and recovery training, as recently introduced, which typically is based on engineering data or similar).

However, there is also the issue as to whether the simulator manufacturer implemented the data correctly. Aside from engineering simulators within Boeing (of which I know nothing), there are five Level D 737MAX simulators that have been certified - four manufactured by TRU Simulation + Training and one by CAE which I believe is currently operating with a Level C certification.

Was adequate data provided for the trim wheel loading? My guess is the data for the trim loading at high speed was NOT provided at low altitude at very high speed...

I suspect that is true of all simulators, whether 737MAX. NG or whatever.

- GY
You've said it well but still missed the main point....

The MAX simulator software is developed and delivered in binary form by Boeing.

The sim builders (both of them) just plug it in to the box. It does what Boeing decided it would do. No more, no less. Boeing decided what training was and was not required, what data and code was or was not necessary and what the course content would be.

All in the name of "I. P. protection". Yeah right.

And..... Guess what? The 777x / 777-9 will be done exactly the same way.


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Old 20th May 2019, 21:41
  #48 (permalink)  
 
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Originally Posted by derjodel View Post

Then another question is, why were the cutout switches changed to prevent manual electric trim along with MCAS. There must be a reason. Without a reason it makes no sense. Did Boeing by any chance estimated that keeping the electric trim on without MCAS could easily put the airplane in the high AoA situation as described above? That could be another smoking gun. If that is not the reason to change the switches, then what was it? Any reasonably ideas?


If the switches weren't changed then you could be flying the plane in manual mode with MCAS disabled. Kind of defeats the whole purpose of it. Much easier for Boeing to just fudge that too.
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Old 20th May 2019, 23:04
  #49 (permalink)  
 
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Again, if you disable MCAS with the stab trim switches, you are left with manual trim. No AP option.
Manual stab trim with no AoA input?

How long is the flight?
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Old 21st May 2019, 00:20
  #50 (permalink)  
 
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Originally Posted by Smythe View Post
Again, if you disable MCAS with the stab trim switches, you are left with manual trim. No AP option.
Manual stab trim with no AoA input?

How long is the flight?
This statement makes no sense. I assume you aren't a pilot? AoA and manual stab trim aren't related. You trim the aircraft to relieve control pressures, not because of whatever the AOA indicates.
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Old 21st May 2019, 07:55
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Originally Posted by Byros View Post
If the 737 MAX simulator cannot correlate to real aircraft, then no amount of sim training could have prevented the outcome.

Boeing is in deep trouble.
Byros
there are dozens of scenarios which are explained by instruction rather than actually having to do them in the sim. Ditching for example which Sully did rather well.
. I was trained 30 years ago when sims never really represented the real plane that the STAB was a killer and that if you let it get way out of trim at high speed you have to “yo yo” to get it back in trim. So- the trick was and still is, don’t let it get out of trim. There is rarely a need for total authenticity in replicating many situations since they are usually demonstrated only once during type rating- manual reversion for example.
Happy flying
y
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Old 21st May 2019, 10:42
  #52 (permalink)  
 
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Originally Posted by Gove N.T. View Post

Please remember that the media are describing what the system is designed to correct in certain circumstances in simple language for the poor, ill-educated masses, including some politicians it seems, who don’t have the benefits of your inside knowledge.

Not for 'ill-educated masses', simply for non-pilots
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Old 21st May 2019, 12:16
  #53 (permalink)  
 
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wilbert33 # 58,
Your summary represents the emerging understanding.
The important issue is why these aspects could have been overlooked, unknown, or unaccounted for.

The tail force-to-speed relationship might breakdown at larger angles; the tail could be adversely affected by wing downwash. The measured stick forces during test were probably limited by high forces, thus the extremes were extrapolated (non linear), supported by aerodynamic / structural theory (prediction). This may not have included the elevator, but presumably some aspect was considered in identifying the need for the yo-yo manoeuvre - when; 737-200 vintage extrapolated to new variants.
Longer fuselage, higher thrust engines, increased tail area ?

Similarly the additive effect of nose up recovery elevator which would be an opposing force (couple) on the tail mechanism could have been misjudged - not a normal evaluation of cross-control; possibly no data or extrapolation.

There has also been discussions that with large deviations of the combined tail angle and elevator, the elevator control suffers jack stall, thus is less effective.

The elevator feel adjustment is a well established device used with stick shake (AoA), it would be an unwanted additive to tail trim offset, but not a problem in isolation - AoA fail, no MCAS trim input.

Do we need superman ? No, we require humans in design, evaluation, test, and certification, who can foresee relevant eventualities whilst constrained by management and working practices, and subject to normal human variability due to the working environment. (Reasonable judgement)
The industry requires a robust processes of design - certification, something which will tolerate the variabilities above, so that pilots, with similar human limitations, can manage what is expected of them; manage the residual uncertainty.


yanrair, # 60,
So - the trick was and still is, don’t let it get out of trim.’
Aviation safety is not founded on “tricks”, particularly those which might not be described or repeated.
The are significant differences between what you ‘know’ (know what), and ‘how’ to use what you know; knowhow.
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Old 21st May 2019, 12:50
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Originally Posted by PEI_3721 View Post
wilbert33 # 58,

So - the trick was and still is, don’t let it get out of trim.’

Aviation safety is not founded on “tricks”, particularly those which might not be described or repeated.
The are significant differences between what you ‘know’ (know what), and ‘how’ to use what you know; knowhow.
I think you are misreading what he said. Proper trimming technique for any aircraft is not a "trick," it is a basic skill learned from day 1. Not defending the horrible design of MCAS, but aircraft manufacturers have to make some underlying assumptions regarding the pilots who will be operating the aircraft they build. Believing the pilot will apply normal operating techniques like trimming away undesired control forces is not an unwarranted assumption.
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Old 21st May 2019, 13:22
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Originally Posted by PEI_3721 View Post
wilbert33 # 58,
Your summary represents the emerging understanding.
The important issue is why these aspects could have been overlooked, unknown, or unaccounted for.

The tail force-to-speed relationship might breakdown at larger angles; the tail could be adversely affected by wing downwash. The measured stick forces during test were probably limited by high forces, thus the extremes were extrapolated (non linear), supported by aerodynamic / structural theory (prediction). This may not have included the elevator, but presumably some aspect was considered in identifying the need for the yo-yo manoeuvre - when; 737-200 vintage extrapolated to new variants.
Longer fuselage, higher thrust engines, increased tail area ?

Similarly the additive effect of nose up recovery elevator which would be an opposing force (couple) on the tail mechanism could have been misjudged - not a normal evaluation of cross-control; possibly no data or extrapolation.

There has also been discussions that with large deviations of the combined tail angle and elevator, the elevator control suffers jack stall, thus is less effective.

The elevator feel adjustment is a well established device used with stick shake (AoA), it would be an unwanted additive to tail trim offset, but not a problem in isolation - AoA fail, no MCAS trim input.

Do we need superman ? No, we require humans in design, evaluation, test, and certification, who can foresee relevant eventualities whilst constrained by management and working practices, and subject to normal human variability due to the working environment. (Reasonable judgement)
The industry requires a robust processes of design - certification, something which will tolerate the variabilities above, so that pilots, with similar human limitations, can manage what is expected of them; manage the residual uncertainty.


yanrair, # 60,
So - the trick was and still is, don’t let it get out of trim.’
Aviation safety is not founded on “tricks”, particularly those which might not be described or repeated.
The are significant differences between what you ‘know’ (know what), and ‘how’ to use what you know; knowhow.
Apologies for poor use of English Wilbert- for "trick " read "correct procedure/technique". Since STAB loads increase rapidly with airspeed changes it is not recommended to change speed. Pariculary with Jammed Stabiliser where it won't budge. Stay at same speed and plane stays in trim. Until of course you have to slow down to land but that is a different trick - sorry, procedure!
Cheers and thanks for your post.
Y
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Old 21st May 2019, 13:39
  #56 (permalink)  
 
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Trick or treat?
Actually one of my major concerns about these two crashes and others like them, and the trend in general, is that the "tricks" - techniques, airmanship, generations of wisdom, are not being passed on to the next generation of trainers who in turn train the pilots. My last job involved doing just that - helping to pass on the gene pool of learning that can so easily be forgotten. So, my generation have massive respect if not fear of a runaway stabiliser - not fear because we know how to fix it - let's call it respect. Then earlier on in this thread we hear from a 737 pilot who wasn't even told that there were handles on the stabiliser wheels and what they were for!

I do agree that it is no good having a really good training captain full or "tricks" passing these onto those lucky enough to be with him during training since this only glides over the problem. If a trainer has a good idea that is not out there - tell us all about it. Let it then become airline policy.
Y
Happy flying.
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Old 21st May 2019, 18:10
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Originally Posted by yoko1 View Post
I think you are misreading what he said. Proper trimming technique for any aircraft is not a "trick," it is a basic skill learned from day 1. Not defending the horrible design of MCAS, but aircraft manufacturers have to make some underlying assumptions regarding the pilots who will be operating the aircraft they build. Believing the pilot will apply normal operating techniques like trimming away undesired control forces is not an unwarranted assumption.
And that is a good summation of the problem - one that is highlighted repeatedly here.
Boeing is being castigated for assuming that pilots would do what in the past would be considered "a basic skill learned from day 1". In fact it would have been deemed insulting to not assume it is an innate skill. Yet in multiple postings here and other threads it has been said that trimming to unload the control column is something special that needs to be highlighted in NNCs (what are those Ns - oh yes...). Is special training really required for experienced pilots in _trimming_?? Apparently yes. What would have happened to both accident aircraft if the pilots had kept them in trim? -A clue - that is what the crew of the penultimate Lion Air flight did.

So the real question is what _other_ assumptions based on past expected crew capabilities have been made by A or B? With training both initial and continuation cut to the tick box bone - and real hands-on experience being limited there surely are other assumptions that have been invalidly made.

Which approach will the bean counters favor? Automate out the pilots or (re)train them and increase continuation training? Does automation require a pension/401K?
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Old 21st May 2019, 20:19
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Originally Posted by Takwis View Post
Boeing has already done that trick, back in 1984. When replacing the underslung JT-8Ds with the new CFM-56s, they had to push them out front (canard style), and then, yes, had to put a hefty strake on the side of the engine, to direct airflow where it needed to go. The engines got bigger at the NG stage, and the strake got a little bigger, too. I suspect there is a limit to how much you can do with that same solution...and the MAX (aptly named, for a number of reasons) reached, or passed, that limit. Hence the MCAS. I rather like PEI_3721's, solution, in that, at least it would be better than MCAS!

I still believe that one can still build a 707 type airplane, with cables, and pulleys, and servo tabs and balance bays...clean it up with more modern aerodynamics, new engines (only two of them, alas), stick 'em under the wings for nostalgia's sake (sorry PEI), and have it behave more predictably, more safely, more comfortably, more intuitively than an electric jet. But then, I'm getting close to retirement, and so are my ideas.
Tak, that's NOT why the strake is there, The reason for the strake is engine out approach - basically if the engine is not running, at higher angles of attack all that airflow spilling out of the inlet can cause flow separation on the wing - the strake acts as a big vortex generator to re-energize the flow and keep it attached over the wing. That's why you'll see strakes on the inboard side of the nacelles of most wing mounted big turbofan engines. Not to say they may not help in the way you're suggesting, but that's not why we put them there.

As for the engines being hung below the wings instead of out front - there are two main reasons for that unrelated to ground clearance (granted, the MAX is a pretty extreme case due to ground clearance). One is rotorburst - sticking the engine out front limits the amount of wing (and associated systems) that are exposed to damage due to a rotorburst. The other is that it's aerodynamically better - if the engine is too close to the wing, at cruise speeds and power settings you get interference drag due to the interaction of the fan flow with the free stream airflow. The drag penalty can be substantial - as much as 2% fuel burn. On the 747-8, they couldn't mount the engines as far forward as they wanted due to flutter issues and had to pay a not insignificant drag penalty as a result.

Last edited by tdracer; 22nd May 2019 at 06:44. Reason: Fixed a major typo... Never proof your own writing
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Old 21st May 2019, 20:31
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Originally Posted by Ian W View Post
Boeing is being castigated for assuming that pilots would do what in the past would be considered "a basic skill learned from day 1". In fact it would have been deemed insulting to not assume it is an innate skill. Yet in multiple postings here and other threads it has been said that trimming to unload the control column is something special that needs to be highlighted in NNCs (what are those Ns - oh yes...). Is special training really required for experienced pilots in _trimming_?? Apparently yes. What would have happened to both accident aircraft if the pilots had kept them in trim? -A clue - that is what the crew of the penultimate Lion Air flight did.
Posted about this on one of the countless other MAX threads, but it probably bears repeating.
Not too long ago I was at special event at the Museum of Flight - not only was I seated at a table with a bunch of current and retired Alaska Air pilots, during the cocktail hour I ran into a flight test pilot friend who'd been involved in the MAX development (Alaska is an all 737 operator - classics and NGs). To a man, they all agreed that if the stab tirm started doing something you didn't understand or like, the very first thing they'd do is turn it off and trim it manually. Hence the reason Boeing didn't treat MCAS as a flight critical system. However these were all older, high time experienced pilots
That being said, they also all agreed that no sim training for MCAS (or any of the other MAX differences) was a huge miss...

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Old 21st May 2019, 22:11
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Originally Posted by tdracer View Post
To a man, they all agreed that if the stab tirm started doing something you didn't understand or like, the very first thing they'd do is turn it off and trim it manually. Hence the reason Boeing didn't treat MCAS as a flight critical system. However these were all older, high time experienced pilots
That being said, they also all agreed that no sim training for MCAS (or any of the other MAX differences) was a huge miss...
I suppose no one here will disagree with the above.
The unknown is and -remains -, when an how do you notice that the stab trim is doing something that you don't understand ?
In the real aiplane, with a continuous stickshaker, STS spinning the wheels and while running NNC's, how long does it take to notice the stab trim is acting up ?
One, two, ten, twenty seconds ?
The other unknown is, after the above number of seconds, is the out-of-trim recoverable with the manual wheels ? Or, at what speed and out-of-trim degree does the airplane become manually un-trimmable ?
Does someone really know, or do people just "believe", "suppose", "imagine" ?

Yeah, I know, "they shouldn't have..", "the trick is..".
But in the real world of real airplanes ?

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