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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 16th Mar 2019, 12:40
  #1581 (permalink)  
 
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(pax). In #1633 the two aoa traces track each other with what looks like a fixed offset but, at the end, the top trace moves down to match the other. Why may that be?
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Old 16th Mar 2019, 13:04
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Originally Posted by Mr Optimistic
(pax). In #1633 the two aoa traces track each other with what looks like a fixed offset but, at the end, the top trace moves down to match the other. Why may that be?
higher drag due to increased speed finally moved the AoA vane in the correct position?
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Old 16th Mar 2019, 13:07
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Originally Posted by fdr
§ 25.672 Stability augmentation and automatic and power-operated systems

If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with § 25.671 and the following:

(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.

(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failures of the type specified in § 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.

(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system -

(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;

(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplaneconfigurations) which is described in the Airplane Flight Manual; and

(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.

[Amdt. 25-23, [url=https://www.law.cornell.edu/rio/citation/35_FR_5675]35 FR 5675 Apr. 8, 1970]
A chapter dear to my heart and one that came immediately to mind after the first crash. My initial assumption was that the fleet would be grounded shortly after the first crash given the suspicion that the aircraft did not comply with the section quoted above - effectively removing a major strand of its certification.

A great deal of flight testing is devoted to assuring the requirements above are met. What surprises me is that the system does not appear to be designed to meet these requirements at the outset.
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Old 16th Mar 2019, 13:12
  #1584 (permalink)  
 
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Before the days of MCAS

And now for something completely different.
I believe this may have been mentioned before but it is an interesting read.
It occured in 2007 a refers to a Boeing 737-3Q8 landing at Bournemouth and at a height of 2,500'.
The full report can be downloaded at
https://www.gov.uk/aaib-reports/aar-...september-2007
A brief summary includes:-

The Boeing 737-300 was on approach to Bournemouth Airport following a routine passenger flight from Faro, Portugal. Early in the ILS approach the auto-throttle disengaged with the thrust levers in the idle thrust position. The disengagement was neither commanded nor recognised by the crew and the thrust levers remained at idle throughout the approach.
Because the aircraft was fully configured for landing, the air speed decayed rapidly to a
value below that appropriate for the approach. The commander took control and initiated a go-around. During the go-around the aircraft pitched up excessively; flight crew attempts to reduce the aircraft’s pitch were largely ineffective. The aircraft reached a maximum pitch of 44º nose-up and the indicated airspeed reduced to 82 kt.

Apart from holding the control column fully forward, the flight crew made
no other pitch control actions throughout the 44º nose-up excursion until the
aircraft had stalled and the nose had dropped towards the horizon. At this
stage the thrust was reduced to go-around thrust. This thrust reduction allowed
sufficient control authority to recover the aircraft.
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Old 16th Mar 2019, 13:15
  #1585 (permalink)  
 
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I will freely admit to being a bit of a dinosaur, having started my flying career on an aircraft with no computer assistance, via aircraft that did to finish on another with minimal assistance. Along the way I learnt a healthy distrust of software being written to help my job - most of the time it worked, but sometimes it didn't, with the most common statement seeming to be 'what is it doing now?' or words to that effect.

Now we have a software input which has possibly, maybe even probably caused two major accidents on a newly introduced virtually computer driven ac. Everyone seems to be looking at just the MCAS algorithms, but has anything else in the entire flight control system been added/removed which could give rise to other maybe unexpected but related happenings in the cockpit?

Software engineers have 'improved' incrementally many other non-aviation systems in the last few years - TSB and other banking outages spring to mind, as does Facebook the other day. Who's to say some apparently minor code change has not given rise to something else happening when MCAS trips in.

I ask as an 'interested observer', no longer an occupant of the panoramic window seat.
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Old 16th Mar 2019, 13:40
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Originally Posted by Compton3fox
MCAS is not supposed to operate if the AP is engaged. I would want to look at these events further to be sure they are not related or that they were a result of crew error. If not, there maybe a further complication to the current issue or another issue lurking that we don't know about.
In both cases when the crew took over manually the nose down stopped. It MCAS had been involved then the aircraft would still have had nose down trim in manual control. These cases were activation of an autopilot that for whatever reason was set for a lower target altitude, that is a human factors issue in its own right and needs to be assessed.
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Old 16th Mar 2019, 14:28
  #1587 (permalink)  
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Originally Posted by XN593
And now for something completely different.
I believe this may have been mentioned before but it is an interesting read.
It occured in 2007 a refers to a Boeing 737-3Q8 landing at Bournemouth and at a height of 2,500'.
The full report can be downloaded at
https://www.gov.uk/aaib-reports/aar-...september-2007
A brief summary includes:-

The Boeing 737-300 was on approach to Bournemouth Airport following a routine passenger flight from Faro, Portugal. Early in the ILS approach the auto-throttle disengaged with the thrust levers in the idle thrust position. The disengagement was neither commanded nor recognised by the crew and the thrust levers remained at idle throughout the approach.
Because the aircraft was fully configured for landing, the air speed decayed rapidly to a
value below that appropriate for the approach. The commander took control and initiated a go-around. During the go-around the aircraft pitched up excessively; flight crew attempts to reduce the aircraft’s pitch were largely ineffective. The aircraft reached a maximum pitch of 44º nose-up and the indicated airspeed reduced to 82 kt.

Apart from holding the control column fully forward, the flight crew made
no other pitch control actions throughout the 44º nose-up excursion until the
aircraft had stalled and the nose had dropped towards the horizon. At this
stage the thrust was reduced to go-around thrust. This thrust reduction allowed
sufficient control authority to recover the aircraft.
Are you saying then that the recovery of a MAX pitching up the same as seen at Bournemouth could then be recovered in the same way without MCAS features added on?
In which case why have MCAS at all?
So as long as pilots know that at slow speeds with a high thrust setting applied will lead to a high AoA - thus avoidance of this scenario surely is what pilots are trained
for?
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Old 16th Mar 2019, 14:37
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Originally Posted by GlobalNav
Agreed. This failure mode, whatever it turns out to be, has demonstrated catastrophic consequences, possibly in two cases The system safety of the type design requires improvement. Training is not an acceptable alternative to safe design. I would be curious to know how a software "enhancement" answers the mail, and I wonder what the design assurance level of the software is.
Design Assurance Level assures the software is working as designed, whatever level was used, I strongly suspect that the MCAS software is working exactly as designed.

In my opinion, the design (of MCAS) is broken, but I am not sure the blame sits with whoever designed MCAS but rather with whoever designed the handling of the plane so that MCAS was required (and possibly then failed to spot the issue until very late in development). I suspect whoever designed MCAS was backed into a (coffin) corner constrained by schedule and to use only what hardware was already on the aircraft.
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Old 16th Mar 2019, 14:55
  #1589 (permalink)  
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Originally Posted by rog747
Are you saying then that the recovery of a MAX pitching up the same as seen at Bournemouth could then be recovered in the same way without MCAS features added on?
In which case why have MCAS at all?
So as long as pilots know that at slow speeds with a high thrust setting applied will lead to a high AoA - thus avoidance of this scenario surely is what pilots are trained
for?
No, it's been explained before but MCAS is required as stick force requirement reduces at high alpha on the MAX and that is a certification failure. Nothing to do with the pitch power couple.
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Old 16th Mar 2019, 15:07
  #1590 (permalink)  
 
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Originally Posted by DaveReidUK
Thanks for that.
Can you clarify - are you using the value for hPa/ft at SL, or the value at 8000' ?
Hi Dave. It depends.
My answer to your first question was:
Originally Posted by Luc Lion
Originally Posted by DaveReidUK
Let's put it another way - what pressure difference at Bole's elevation (i.e. QFE) would you expect to result from a difference at SL between 1029 mb and ISA?
∆p = (1029 – 1013.25) hPa * (1 - 0.0019812(°K/ft) * 7657 ft/288.15 °K)^5.2561
∆p = 11.85 hPa
So, physically, your question is equivalent to "If a translation is applied to the ISA pressure model so that 0 ft is moved from 1013.25 hPa to 1029 hPa, by how much pressure is translated the point at altitude 7657 ft ? As the whole model is translated everywhere by the amount of feet, 420 ft, it means applying the pressure lapse rate that exists at 7657 ft to the height difference that matches 1029 hPa - 1013.25 hPa = 15.75 hPa at sea level pressure lapse rate.
As the pressure lapse rate at 7657 ft is about 75% of the lapse rate at sea level, I found 15.75 hPa * 0.75 = 11.85 hPa.
So it is pressure lapse rate at 7657 ft for your first question.

Your second question was more straight forward:
Originally Posted by Luc Lion
Originally Posted by DaveReidUK
Let's try a thought experiment: sitting on the runway at Bole in a helicopter on the day in question, if you set the QNH (1029) your altimeter should read the field elevation (7,625'). Now adjust the subscale to 1013.2 - how much lower will the altimeter read, and why ? How much must you climb in order that the altimeter once more reads 7,625' ?
∆h = 1/(0.0375 hPa/ft) * (1029 – 1013.25) hPa
∆h = 420 ft
Again, we know that the whole ISA model is translated everywhere by the amount of feet when the reference isobaric surface is moved.
You move the 0 ft surface from 1013.15 to 1029, so basically these are atmospheric layers very close to the 1013.25 reference surface where the pressure lapse rate is 0.0375 hPa per feet.
And 0.0375 hPa/ft * 15.75 hPa = 420 ft
So it is pressure lapse rate at seal level for your second question.

There is a common misconception, even in the pilot community, about the altitude difference versus pressure difference (aka pressure lapse rate) when adjusting the value in the Kollsman window.
It is well known that pressure lapse rate of the real atmosphere is decreasing, but it is less known that the altitude/pressure ratio for each hPa tuned in the Kollsman window also decrease with decreasing pressure.
It follows an exponential curve with (almost) the same coefficient as the pressure/altitude relation of the ISA model.
When one changes the altimeter setting with a value very close to 1013.25 hPa, each 3.75 hPa moves the altitude by 100 ft.
When one changes the altimeter setting with a value very close to 800 hPa (setting only possible with SAE AS8002A altimeters) only 3.03 hPa is enough to move the altitude by 100 ft.

Last edited by Luc Lion; 16th Mar 2019 at 15:33.
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Old 16th Mar 2019, 15:10
  #1591 (permalink)  
 
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Thanks to fdr for posting the quote from the 'regs'. I'm only SLF but have some expertise in reading British and European Standards and trying to interpret what the people who drafted them intended them to mean. Look at (a) below.
Originally Posted by fdr§ 25.672 Stability augmentation and automatic and power-operated systems

If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with **** and the following:

(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.
My question is: was MCAS intended to operate under 'normal' flight conditions? If it was intended to operate only in 'abnormal' flight conditions, then there is no need to tell the pilot anything, even if it fails. I presume this is how the wording has been interpreted, but surely it should make the pilot aware when it is operating. Compare this to a car with 'anti skid' and 'traction Control' you don't need to know it is there and working, but when the wheels start spinning you get a warning that the system is operating, you also get a warning if it has failed.

I sit down the back and muse over what you guys (and girls) up the front don't know, or more importantly haven't been told/taught.

Having read this thread from page one I'm still concerned, (like others have posted) that there are things lurking that you still don't know about, that might come and bite you in the backside at the wrong moment.

I have an adage when risk assessments go wrong: YOU DON'T KNOW WHAT YOU DON'T KNOW UNTIL YOU FIND OUT YOU DIDN'T KNOW.
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Old 16th Mar 2019, 15:23
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This is really a classic case of the Swiss cheese accident model. A design flaw in the aircraft, a faulty sensor/inadequate maintenance and deficient crew training. The holes have lined up twice in a relatively short period of time. Only one of the holes needs to close to avoid another accident but ideally it should be at least three of them that get welded shut.
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Old 16th Mar 2019, 15:25
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Thumbs up

Originally Posted by FGD135
I can see what Boeing were trying to do. From their point of view, the pilots are already trained (supposedly) to deal with the situation the MCAS may bring up, so why complicate things?

If the pilots had followed the prescribed procedure for inadvertent/inappropriate/runaway stabiliser trim (a procedure which has existed for decades, across all Boeing models) then these crashes would not have occurred and the pilots would be none the wiser about MCAS.

I quite agree


and from DaveReidUK

But what's really needed is an in-depth look at the (changed) relationship between the regulators and the industry (by no means confined to the USA).
The regulator challenge is not so much one of "them-vs-us" relationship between the FAA engineers and Boeing. It is the challenge of actually understanding the system versus the end-user. It's easy for the engineer who designs and the engineer who approves to make assumptions about the human that actually uses the product. But there doesn't seem to be much validation in the certification phase of this knowledge transfer to the pilot.

This is a lot more than just a Boeing problem. It is fundamental to the certification base among all national regulators. Words alone in a design assumption are not enough, there must be validation all the way to the user.
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Old 16th Mar 2019, 15:26
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D6-27370-MAX-TBCNFF

According to the "Flight Crew Operations Manual Bulletin" D6-27370-MAX-TBCNFF:
"It is possible for the stabilizer to reach the nose down limit unless the system inputs are counteracted completely by pilot trim inputs and both STAB TRIM CUTOUT switches are moved to CUTOUT. "
The bulletin also says that an AoA failure could automatically disable the autopilot, which then could and probably would activate MCAS.

So the flight takes off, and at about 1100ft, an AoA fault shuts off autopilot, the stick shaker rattles the left yoke, and MCAS trims nose down, as designed.
Do audible stall warnings sound?
The pilot instinctively pulls back and trims, but MCAS re-engages after 5 seconds, as designed.
The pilot pulls the yoke to adjust pitch (and forgetting that MCAS could be engaged) before calling to circle back.
The first officer may have even suggested flipping both STAB TRIM CUTOUT switches - assuming he was a good FO - and he read the bulletin.
The pilot attempts to steady the craft using yoke only - because he already compensated for potential runaway trim, right? And what does a green FO know.

The only way to save the flight is by flipping both STAB TRIM CUTOUT switches. But the pilot really needs to return the stab to neutral before flipping the switches to avoid the need to find the trim wheel handle, flip it out, and hand crank it back into place without the motors - which would take too much time (minutes) anyway.

For a good pilot, this procedure might seem routine. But even good pilots have bad days. One faulty AoA sensor can disable autopilot, initialize MCAS, override manual pilot trim, and screw the stab to a full down position. This is clearly a design flaw, even if there is a procedure to mitigate it.
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Old 16th Mar 2019, 15:45
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Originally Posted by FGD135
It seems that everybody here wants to bash Boeing for not training pilots at how to deal with the MCAS inadvertent activation. But Boeing have provided training and guidance. The problem is that this guidance wasn't followed by the Lion Air crew - and possibly the Ethiopian crew.

The guidance from Boeing would have related to uncommanded/inappropriate trim activation. I'm not a 737 pilot, and I don't have a 737 checklist handy, but it absolutely certainly would have had a documented procedure for this scenario which would have called for the deactivation, via the stab cutout switches, of the electric stab trim.
The issue isn’t (just) whether Boeing provided guidance. It is whether it is reasonable to expect the pilots to recognize the situation in order to follow that guidance.

Sitting dumb fat and happy with everything going well and suddenly the trim starts moving, sure easy enough to recognize and action. Throw in AoA disagree, unreliable airspeed, stick shaker, etc. then maybe that becomes a more difficult problem. It looks like in both the Lionair accident and the preceding flight, neither crew thought about trim cutout initially. Eventually the preceding flight crew did, and did cut it out, but it took several minutes.

This is where it looks like a poor FMEA analysis was done, treating the need for MCAS cutout as a single fault situation. It seems that a single sensor fault in the AOA can effectively lead to a double fault, and reliable airspeed, and inappropriate MCAS activation. Well rather a single fault causing two significant system failures, getting the pilots a definite challenge.
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Old 16th Mar 2019, 15:56
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Quote:
Originally Posted by FGD135
I can see what Boeing were trying to do. From their point of view, the pilots are already trained (supposedly) to deal with the situation the MCAS may bring up, so why complicate things?

If the pilots had followed the prescribed procedure for inadvertent/inappropriate/runaway stabiliser trim (a procedure which has existed for decades, across all Boeing models) then these crashes would not have occurred and the pilots would be none the wiser about MCAS.
I quite agree
The preceding flight Lion air pilots did (eventually) follow the runaway action items and were in fact "none the wiser about MCAS". This in turn led to an inadequate tech write up which caiused the accident flight to be released with the same fault.


and from DaveReidUK
...
...
. It is the challenge of actually understanding the system versus the end-user. It's easy for the engineer who designs and the engineer who approves to make assumptions about the human that actually uses the product. But there doesn't seem to be much validation in the certification phase of this knowledge transfer to the pilot.

This is a lot more than just a Boeing problem. It is fundamental to the certification base among all national regulators. Words alone in a design assumption are not enough, there must be validation all the way to the user.
Are there any certification requirements to validate "average pilot assumptions" in any way, ideally in high fidelity sim. This would also be usefull to validate training modules, did the 'average' pilot aquire the requisite knowledge for safe operation?
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Old 16th Mar 2019, 16:06
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There is a lot here to read, but the discussion about MCAS sems to be about a sensor disagree.

My understanding for the need for MCAS was due to the much larger engines (without a redesigned wing) that leads to instabilities on DEP angles of attack when at certain altitudes trim adjustments are required. A software patch, MCAS, was developed to compensate for this issue, not a sensor disagree?
Was this not the reason MCAS was needed to cert the ac?
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Old 16th Mar 2019, 16:11
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Originally Posted by CONSO
So Boeng now claims a software fix ready to load in 10 days ? Run a test flight after loading software and put back in service ??

IMHO- this is the equivalent of 1 male plus 1 female = 1 baby in 9 months, therefore 9 men plus 1 woman = 1 baby in 1 month software coding logic

And of course the rest of the world will agree, cuz Boeing said so ??
The change to the initiation of MCAS was being developed as soon as it became apparent that the Lion air crash was due to inability to handle MCAS trimming in error. So that has been since ~Nov 2018. I can envisage simple changes that would not incur your disbelief - for example limit the trimming to a single trim action for MCAS, as there does not appear to be a good reason for multiple nose down trim actions for the claimed raison d'etre, and emable AOA disagree warnings in all Max aircraft. Not all fixes entail significant change to code.
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Old 16th Mar 2019, 16:12
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Originally Posted by FGD135
I can see what Boeing were trying to do. From their point of view, the pilots are already trained (supposedly) to deal with the situation the MCAS may bring up, so why complicate things?

If the pilots had followed the prescribed procedure for inadvertent/inappropriate/runaway stabiliser trim (a procedure which has existed for decades, across all Boeing models) then these crashes would not have occurred and the pilots would be none the wiser about MCAS.
If you take that literally, then yes but we are looking at this a bit post hoc. The question is how do you define the trigger(s) for applying the runaway stabiliser trim drill? STS, MCAS and the AP will (and do) adjust the trim without pilot input, so what exactly are you looking for? Also, this is not on a low workload flight deck level at 20,000’, this is shortly after takeoff in a critical flight phase where all margins are much smaller and spare cognitive bandwidth is much reduced. It needs to be simple logic, not a large branching flowchart.

I guess what I’m trying to say is that suppose the trim starts moving after takeoff: is that a runaway or is it normal operation? I can’t see a quick and easy way to figure it out, so in the interests of safety you’d be better assuming it’s a runaway but that means on virtually every flight you’re disconnecting the trim on the climb-out, not ideal.

Add a UAS event to the mix and I can absolutely see why things go wrong. There is as much, maybe more, to the Human Factors side of these accidents than the technical, IMO.
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Old 16th Mar 2019, 16:21
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Originally Posted by ernst_mulder
A (possibly stupid) question from SLF,

What I am wondering about is why, after a struggle with MCAS, the plane ends up in a nosedown dive. Wouldn't the pilots in principle be able to keep fighting the unwanted trim commands indefinitely? Also earlier in this thread I read that it is possible to fly an airplane even with a full downward trimmed HS. Could another mechanical problem be the cause, i.e. elevator(s) breaking off after too much stress trying to compensate for the full downward trimmed HS?
The faster that the plane is going the greater the force required to oppose it and the more violent the reaction of the plane to the control surface, At low altitude and high speed it would be just about impossible to either overcome the stick force or to recover even if you could do so. The two obvious questions are why didn't the plane gain altitude along the way and why was it going so fast at low altitude if in fact it couldn't climb.
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