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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 12th Apr 2019, 16:07
  #3921 (permalink)  
 
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Originally Posted by SLF3 View Post

This approach is not specific to Boeing or commerical aviation: it's the way the game is played.
Exactly.

Witness the Ethiopian report declaring that their pilots "did everything correctly". By releasing this blatently incorrect statement prior to the publication of the initial report, they controlled the message.

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Old 12th Apr 2019, 17:33
  #3922 (permalink)  
 
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"Children of the Magenta Line".
This phrase was by American Airlines pilot Warren Van der Burgh...

Latest on the issue...seems as though in the ITC complaint against Bombardier, B provided some interesting details on the MAX 8...

Before last month’s crash of a flight that began in Ethiopia, Boeing Co. said in a legal document that large, upgraded 737s “cannot be used at what are referred to as ‘high/hot’ airports."

At an elevation of 7,657 feet -- or more than a mile high -- Addis Ababa’s Bole International Airport falls into that category. High elevations require longer runways and faster speeds for takeoff. The Ethiopian airport’s altitude hasn’t been cited as a factor in the downing of Flight 302 and likely didn’t cause the crash. But it could have exacerbated the situation because an airplane’s performance degrades at higher altitudes, said a 737 pilot who flies into high-elevation airports such as Denver and agreed to speak on background since he’s not authorized to talk with the media.

Boeing cited the performance of the 737 Max 8 in a casebrought before the U.S. International Trade Commission in 2017. Boeing charged that unfair competition from Bombardier -- which beat out Boeing for a large order from Delta Air Lines -- threatened its 737-700 and Max 7, the smallest of its upgraded single-aisle jets. By pointing out the limitations of the Max 8, the planemaker sought to preserve market share for the 700 and Max 7.

Boeing stated in a brief filed in the trade case that the “737 Max 7 has greater performance capabilities at challenging airports. In particular, the 737 Max 7 can serve certain ‘high/hot’ airports and has a greater range operating out of constrained airfields.” The brief then cites a number of such airports -- the names of which are redacted -- that the Max 7 can fly into that “the 8, 9 and 10 cannot."

“Larger 737 variants cannot be used at what are referred to has ‘high/hot’ airports,” the brief stated. Certain U.S. airports are unsuitable for the Max 8 “due to a combination of short runway lengths, elevation, temperature, humidity and other environmental conditions."

Documents in the trade case referred to at least 16 U.S. airports considered “high and hot” and therefore unsuitable for the Max 8, though the names of those facilities weren’t made public. Asked during a trade commission hearing to specify which airports, an expert witness for Boeing replied that “sometimes Denver would qualify as that.” The expert, Jerry Nickelsburg, an adjunct economics professor at UCLA, added that “Mexico City certainly qualifies as that.”


https://www.bloomberg.com/news/artic...rtain-airports

Last edited by Smythe; 12th Apr 2019 at 17:46.
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Old 12th Apr 2019, 18:25
  #3923 (permalink)  
 
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Hot and high has zero to do with both accidents. With two healthy donkís in a twoholer it is no problem since you have lots of additional power. Why does this totally unrelated topic creep in here?
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Old 12th Apr 2019, 18:38
  #3924 (permalink)  
 
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All this talk about rwy lenghths and high hot airfields. Nothing to do with those at all. All to do with tweaking and twiddling with an aeroplane to fit the bill and having to write up some software because it is prone to go tits up and that no one has yet managed to train a tame gorilla to take the pilot`s seat. May be one day that too will be possible, but they better be ready to swap the control cables for ships hawsers.
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Old 12th Apr 2019, 19:12
  #3925 (permalink)  
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That double blip of the thumb switches. The big question seems to be, why didn't he continue trimming? I know of no circuitry which will do this, but what happens to that Stab input if it is already exceeding some load limit? i.e., could there be an electrical current limit that clips the power to the Stab motor/clutches if the motor is taking current commensurate with torque that's too high?
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Old 12th Apr 2019, 19:28
  #3926 (permalink)  
 
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Originally Posted by Loose rivets View Post
That double blip of the thumb switches. The big question seems to be, why didn't he continue trimming? I know of no circuitry which will do this, but what happens to that Stab input if it is already exceeding some load limit? i.e., could there be an electrical current limit that clips the power to the Stab motor/clutches if the motor is taking current commensurate with torque that's too high?
Covered quite well here: https://leehamnews.com/2019/04/05/bj...irst-analysis/
The insufficient trim mystery after re-activation of Electric Trim
After 7 PF commands Electric Trim Nose Up in two short cycles. I asked my selves (as did others) why these short trims? They are fighting to get the nose up to the extent they risk switching in the Electric Trim again. Then why not trim nose up continuously or for at least long cycles once Electric Trim is there? It took me several hours to find an explanation. Here my take:

To understand the blip trims one must have flown fast jets at low altitude. At the speed ET302 is flying, 360kts, itís hypersensitive to trim. The least trim action and the aircraft reacts violently. Any trimming is in short blips.

As PF holds the nose up with a very high stick force, now for a long time, heís sensitivity to release stick with trim is not there (this is what Pilots do when they trim nose up, otherwise the aircraft pitches up fast). He trims therefore in short blips and has difficulty to judge the trim effect he has achieved. His is not flying on feel. He canít, he is severely out of trim, holding on to the Yoke with a strong pull force.

Anyone who has flown a grossly out of trim aircraft at high speeds knows your feel is compromised. The sensors you have to rely on are your eyes, not your hands.

PF has the horizon glued to read the aircraft. The result is the short nose-up trims we see. The nose goes up and the stick force needed is reduced. His judgment is; this is enough for now, it was a powerful response. Any MCAS attack I now trim against, then I correct my trim if I need to.

But the aggressive MCAS, trimming with a speed 50% higher than the pilot and for a full nine seconds, kicks in at 8 with a force they didnít expect. Speed is now at 375kts and MCAS was never designed to trim at these Speed/Altitude combinations. Dynamic pressures, which governs how the aircraft reacts to control surface movements, is now almost double it was when last MCAS trimmed (Dynamic pressure increases with Speed squared).

The Pilots are thrown off their seats, hitting the cockpit roof. Look at the Pitch Attitude Disp trace and the Accel Vert trace. These are on the way to Zero G and we can see how PF loses stick pull in the process (Ctrl Column Pos L). He can barely hold on to the Yoke, let alone pull or trim against.

His reduced pull increases the pitch down further, which increases the speed even more. At 05.45.30 the Pilots have hit the seats again (Accel Vert trace and Ctrl Columns force trace) and can start pulling in a desperate last move. But itís too late. Despite them creating the largest Control Column movement ever, pitch down attitude is only marginally affected.

We have Control Column displacement this time, JT610 was Force. If the elevator reacts to these displacements, at the Dynamic Pressure we have, we should have seen the diving stop. The lack of reaction to the large Control Column displacement of two Pilots pulling makes me think we now have blowback. This is not a design fault, we are well beyond Vmo. But it explains the rapid dive, unhindered by the Pilotsí actions.

Itís easy to say ďWhy didnít they trim then?Ē. Because they are going down at 20 degrees nose down (which is a lot, a normal landing approach is 3į) and at 400kts. Then you just pull for all you have. And the aircraft is not reacting to the largest Control Column displacement since takeoff. This makes them pull even harder, the aircraft is unresponsive and they are fighting for theirs and all the passenger lives.
​​​​​​
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Old 12th Apr 2019, 20:17
  #3927 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
...the suggestion they were very slowly winding the wrong way.
[Not a pilot] FO asked for permission to try "manual," received permission, and gave up, all within an eight second period.

That would seem not related to the gradual change during the (from memory) two and one half minute period.

Last edited by fotoguzzi; 12th Apr 2019 at 20:34. Reason: readability
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Old 12th Apr 2019, 20:41
  #3928 (permalink)  
 
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Originally Posted by fotoguzzi View Post
[Not a pilot] FO asked for permission to try "manual," received permission, and gave up, all within an eight second period.

That would not seem related to the gradual change during the (from memory) two and one half minute period.
That exchange was at/after the electrical trim was re-enabled, I believe the 'manual' refers to electrical trim switches.

The slow change (wrong direction) was while electrical trim was disabled.
The prelim report did not include much (if any) CVR details from that time, does not mean things were not attempted.

LooseRivets:
That double blip of the thumb switches. The big question seems to be, why didn't he continue trimming? I know of no circuitry which will do this, but what happens to that Stab input if it is already exceeding some load limit? i.e., could there be an electrical current limit that clips the power to the Stab motor/clutches if the motor is taking current commensurate with torque that's too high?
Without knowing exactly where the fdr gets its inputs hard to be sure, but since the automatic and pilot trim inputs are shown separately I suspect the 'blips' are commands not motor activity.

One thing that does comes to mind is a different pilot doing the trim, this matches the same ineffective inputs seen in Lion Air aftert the pilot transfers control to FO.
Is it possible that the last pilots in each case were not used to and or afraid of the long inputs that would have been required?

The only "non pilot" thing I can think of is something that made the switches harder to press/confusing/fail while applying extreme pull.

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Old 12th Apr 2019, 20:44
  #3929 (permalink)  
 
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Originally Posted by Loose rivets View Post
I'm swinging between both extremes of scenario. However, that balance weight flopping near the end of ET's flight is hard to reconcile, even with Vic's logic.

That slight change of AoA at 05:41:22 - ish, is also troubling me. Why there? Why so consistent before and then after the change? i.e., if it can change at all, why would it remain steady either side of the change?
The correction applied for the difference between AoA vane and AoA body is dependant on Mach number, Pitch rate, Side slip, Flaps, Gear Position and Ground Effect. It is difficult to tell from the 6 figures shown in Fig 9 what form these computations take. Are they look up tables, computed functions, accumulated correction, or serial processed, or what? Iím guessing that they represent different correction lines for different values of the 6 parameters. In which case the single headed arrow line across the Mach number lines could show correction lines to correct AoA Vane to AoA Body from Mach 0 to 0.99, the double headed arrow on Sideslip could indicate + and Ė sideslip, but there is a single headed arrow for Pitch Rate. Did they not allow for a larger negative Pitch Rate? Could this have led to a negative index in a table? Could some of the computations overflow because of rogue parameters? It could be that one of more of the 6 correction factors used garbage function/table constants to calculate an AoA value of, say 434.5deg, which then ended up as a plausible, but unlikely, 74.5deg on the ARINC 429 bus.

So, the AoA values at the end of ETís flight (and the 10deg blip at 05:41:22) could be due to the change in the Pitch rate that occurred then, which gave the computations a kick, resulting in different garbage values being picked up. If any of the tables/lines make use of the last computed value of AoA, this can lead to lock in to a particularly high (or low) value. The correction algorithm is far more complex than I first thought, once it goes wrong, any resulting behaviour for a value of AoA is possible!

Whatever damping there may be in the AoA sensor it has to be fairly light otherwise there would be a significant delay on the AoA signal and obviously the pilot does not want to be told he is approaching a stall condition after he has got there! More seriously, if the counterweight without a vane can really get from 35.7 deg to 74.5 deg in 0.75 of a second, then there is very little damping. I do not think it is feasible that the counterweight can then stop, nearly instantaneously, onto the 74.5 deg position, without any overshoot at all, or bouncing off of any end stop that is set at 74.5 deg. Naah, you need software to do that sort of screw up.


Originally Posted by GarageYears View Post
Well, how does that align with ALL the information?
At 05:38:44, shortly after liftoff, the left and right recorded AOA values deviated. Left AOA decreased to 11.1į then increased to 35.7į while value of right AOA indicated 14.94į. Then after, the left AOA value reached 74.5į in ĺ seconds while the right AOA reached a maximum value of 15.3į

The First Officer called out Master Caution Anti-Ice on CVR. Four seconds later, the recorded Left AOA Heat parameter changed state.

There is very little in terms of cause and effect that would satisfy the heater parameter change other than losing the vane itself. (Unless you want to believe this is entirely spurious).

- GY
Firstly, the AoA values are all driven by the correction algorithm, as I have theorised above, mainly the one using Pitch Rate as a driver.

Secondly, good point, Iíve no idea why the L AoA Heat changed state. Iím puzzled as to why the state was On at the beginning, when the outside temperature was 16deg C. However, Iím more inclined to believe that it was spurious than I am to believe there was anything other than a single underlying failure which affected all 3 of the flights.

Vic


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Old 12th Apr 2019, 21:35
  #3930 (permalink)  
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Comments re thrust on ET302 seem to suggest that from an arm chair the crew can be criticised for keeping it at high thrust.

The BAC FCTM states to attempt to match the out of trim speed in order to reduce load on the trim system. The aircraft is trimmed to an extremely high warp factor (aoa) and the inability to trim a correction arises due to the attendant forces applied by the off trim of the stab, and the corrective elevator input.

Reducing power as suggested does not achieve the matching of speed necessary to unload the stab, yet the comments suggest that the crew erred by keeping power on.... really? The fastest way to restore control, with the least altitude loss comes in this case by having high thrust applied.

Now, in the first instance of a problem with trim, keeping speed in check sounds like a nice principle, but that presumes that the trim has not run away fully, in which case, the control problem is exacerbated by the speed error between current and trimmed speed (aoa).

These guys had a substantial problem to fix in a time critical event, and the comments critical of their technique show the general malaise and lack of knowledge that exists in this industry. Hard to call it a profession. If criticising their actions at least get the physics right.
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Old 12th Apr 2019, 21:45
  #3931 (permalink)  
 
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Originally Posted by VicMel View Post
...
...
Whatever damping there may be in the AoA sensor it has to be fairly light otherwise there would be a significant delay on the AoA signal and obviously the pilot does not want to be told he is approaching a stall condition after he has got there! More seriously, if the counterweight without a vane can really get from 35.7 deg to 74.5 deg in 0.75 of a second, then there is very little damping. I do not think it is feasible that the counterweight can then stop, nearly instantaneously, onto the 74.5 deg position, without any overshoot at all, or bouncing off of any end stop that is set at 74.5 deg. Naah, you need software to do that sort of screw up.
...
...
Secondly, good point, Iíve no idea why the L AoA Heat changed state. Iím puzzled as to why the state was On at the beginning, when the outside temperature was 16deg C. However, Iím more inclined to believe that it was spurious than I am to believe there was anything other than a single underlying failure which affected all 3 of the flights.

Vic
The damping is set for significantly higher forces than the weight itself which is just to counterbalance the vane.
The end stop could well be a 'soft' stop where things get wedged, the ticks downward from ~.5g spikes suggest just that.

If you compare the reported left AoA position to vertical g at the end it is a close to perfect match.

The AoA vane heat is likely on by default, it does get a bit colder at higher altitudes. Also there was a reason it was included in the prelim report, another 1000+ (I may be off here) parameters were not.
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Old 12th Apr 2019, 22:57
  #3932 (permalink)  
 
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Originally Posted by Cows getting bigger View Post
I think there is another HF issue (already commented on some time back) in that it is utterly counter-intuitive to reduce thrust when the nose is pitching down. It is like leaning to right when you want to turn your bicycle to the left.
Reducing speed would have been also against what Boeing recommends in case of a severe AND mis-trim, which is increasing speed (not above VMO, though) in the hope of reducing elevator loads
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Old 12th Apr 2019, 23:17
  #3933 (permalink)  
 
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Originally Posted by VicMel View Post
Firstly, the AoA values are all driven by the correction algorithm, as I have theorised above, mainly the one using Pitch Rate as a driver.
I don't know if AoA value shown on the graph was value from the vane or value corrected by algorithms. I suppose both are recorded. Anyway, pitch rate correction cannot drive corrected value from +75į to -60į !
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Old 12th Apr 2019, 23:29
  #3934 (permalink)  
 
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Firstly, the AoA values are all driven by the correction algorithm, as I have theorised above, mainly the one using Pitch Rate as a driver.
Thank you! Finally, someone else who understands that the output from the AoA vane is subject to numerous algorithms, depending on conditions. To completely base pending stall conditions, and an automated response based on the AoA vane measurement is foolhardy.

First off, the algorithm corrects between the location of the AoA vane, the CG, and wing location angles of attack. The algorithm then corrects for airspeed and sideslip. What is interesting is the AoA measurement becomes less accurate in a climb, especially in a climbing turn (per Boeing) as the winds around the fuselage are deflected, shielded, or otherwise disrupted.

The best solution would be to mount the AoA vane on the wing at midspan, but the disruption from the engine, and moving leading edge prevents this.

The engines are mounted higher on the wing, and further forward, yet the AoA location remained unchanged. Another indication is the green band, on the 738, it is from 1 to 6, yet on the 737-8, it is 3 to 8.....

As noted, originally, the software and modelling people came up with a 0.6 correction on the trim. In testing, that turned into 2.5 degrees. This tells me that the assumptions that created the algorithm are not correct.
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Old 13th Apr 2019, 00:16
  #3935 (permalink)  
 
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AOA corrects airspeed and then airspeed corrects AOA?
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Old 13th Apr 2019, 00:26
  #3936 (permalink)  
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Thanks for the replies.

The double blips. I begged the question some 1,000 posts ago about the possibility of the blips causing alarm because of sudden g forces, despite the graphs not showing any such detail. Strangely, the higher accelerative fluctuations come later - I imagine distressing belt-tugging forces.

The consensus back then dismissed the possibility. So, looking again for why the trim wasn't continued. The extract above is again thought provoking; giving an insight into the psychology. But I'm still not convinced. The g force noise does go up during that period, but not in any recognisable sustained pattern. The column inputs are very well mirrored in g forces shortly after.

05:41:35-40 -ish shows a sustained average positive g despite coming from a series of no doubt powerful tugs. Since it seems to be real, and showing in a definite improved pitch attitude, one wonders why this could not be sustained.

All in all, I still have great sympathy for the pilots, which is no doubt strongly biased by my short flight with the stick shaker going. My highly competent FO pointed in turn at the six speed and attitude dials, but even then half a lifetime of indoctrination was relentlessly tugging at my mind, and that's with nothing else being wrong.
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Old 13th Apr 2019, 01:02
  #3937 (permalink)  
 
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Originally Posted by ecto1 View Post
AOA corrects airspeed and then airspeed corrects AOA?
No, they do not correct each other.
The stall algorithm uses a varied number of inputs to determine if the ac is getting into a stall, most of which are algorithm based due to latency.
Latency of all of the calculations is another issue.
While the critical angle of attack doesnt change, the various factors involves such as weight, configuration, CG, and winds, change the airspeed at which an ac will stall.

Now you have unreliable airspeed indicators, (since the pitot tubes do not move into the winds, they are shielded on turns, angle of attack and crosswinds) unreliable AoA measurements, and a varying configuration feeding an automated trim correction.
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Old 13th Apr 2019, 03:17
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Originally Posted by Smythe View Post
No, they do not correct each other.
.
I'm quite sure that's not fully correct. There is a correction to same side static pressure based on AOA, which affects both airspeed and altitude on the same side. Airspeed is not used to modify the sensed or displayed AOA as far as I know.
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Old 13th Apr 2019, 05:04
  #3939 (permalink)  
 
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Originally Posted by fdr View Post
Comments re thrust on ET302 seem to suggest that from an arm chair the crew can be criticised for keeping it at high thrust.

The BAC FCTM states to attempt to match the out of trim speed in order to reduce load on the trim system. The aircraft is trimmed to an extremely high warp factor (aoa) and the inability to trim a correction arises due to the attendant forces applied by the off trim of the stab, and the corrective elevator input.

Reducing power as suggested does not achieve the matching of speed necessary to unload the stab, yet the comments suggest that the crew erred by keeping power on.... really? The fastest way to restore control, with the least altitude loss comes in this case by having high thrust applied.

Now, in the first instance of a problem with trim, keeping speed in check sounds like a nice principle, but that presumes that the trim has not run away fully, in which case, the control problem is exacerbated by the speed error between current and trimmed speed (aoa).

These guys had a substantial problem to fix in a time critical event, and the comments critical of their technique show the general malaise and lack of knowledge that exists in this industry. Hard to call it a profession. If criticising their actions at least get the physics right.
The crew did not ďkeepĒ the thrust high. They ignored a control available to them and ignored its effects upon the aircraft. There was no cognitive thought process that said letís go past Vmo because that will be better for us.
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Old 13th Apr 2019, 08:09
  #3940 (permalink)  
 
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VicMel, an interesting analysis #3958. Together with that of Smythe … now confused.

(If) Vane-sensed AoA requires correction; is this correction in the vane unit or elsewhere. Peter Lemme argues that errors originate from the vane unit as AoA output is ‘sent’ and used by many separate boxes.
Alternatively a separate (remote) calculation would be more logical with the need of other factors, e.g. embedded in ADC for Mach; which box (function) is not so important if the corrected AoA is available on a digital bus to all other computational boxes. This would also negate the view of a vane-origin error.

Airspeed pressure-error corrections depend on the corrected AoA; differences between ADC computations are alerted (Speed, Alt, Disagree). This should not change the AoA.

AoA is use to position the low speed awareness symbol on the airspeed scale, but this too uses output AoA, it should not correct - change either AoA or airspeed.

Thus what correction is being discussed; where is the computation done, and how is the output accessed by many other functions requiring AoA - MCAS one amongst many.

Re vane damping; an earlier suggestion was that the vane slipped position on the shaft; fortuitously catching at different times and values in each accident. A slipping vane/shaft, when snagged would then act as a single unit involving aerodynamic and mass damping - but incorrect AoA.
Although unlikely, Occam might like the simple mechanical approach.

Alternatively, the digital views might follow Moore-Murphy, where with increasing complexity and add-ons, there is even greater risk of error, the need crosschecking and multiple sources would be paramount. Somewhat lacking in this system.




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