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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 11th Apr 2019, 11:09
  #3861 (permalink)  
 
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Originally Posted by HundredPercentPlease View Post
I disagree. Immediately (at the end of the rotation) the aircraft was indicating a stall, and so the crew performed a stall recovery (lowered AoA by reducing pitch and increasing speed).



It was three memory checklists:
  1. Stall recovery.
  2. UAS recovery.
  3. Runaway stab (during the UAS recovery and aurally masked by the stick shaker).
The evidence is that 3 is too many - unless possibly you have a third pilot to monitor and assist.

I have a feeling that the Ethiopian captain may have engaged the autopilot as a pre-considered homebrew anti-MCAS strategy. Autopilot in = no MCAS risk.
MCAS would not be a factor until the flaps were up.

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Old 11th Apr 2019, 12:33
  #3862 (permalink)  
 
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Originally Posted by HundredPercentPlease View Post
It was three memory checklists:
  1. Stall recovery.
  2. UAS recovery.
  3. Runaway stab (during the UAS recovery and aurally masked by the stick shaker).
The evidence is that 3 is too many - unless possibly you have a third pilot to monitor and assist.

I have a feeling that the Ethiopian captain may have engaged the autopilot as a pre-considered homebrew anti-MCAS strategy. Autopilot in = no MCAS risk.
Interesting idea, but I wonder why he would have decided that his strategy was to engage the autopilot rather than simply leave flaps extended. If he knew enough about MCAS to know that it doesn't operate when autopilot is on, he would presumably also know it doesn't operate with flaps extended. Also if that it what he was thinking, then that implies he knew that the stall warning was false?
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Old 11th Apr 2019, 12:42
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Originally Posted by ams6110 View Post
Interesting idea, but I wonder why he would have decided that his strategy was to engage the autopilot rather than simply leave flaps extended. If he knew enough about MCAS to know that it doesn't operate when autopilot is on, he would presumably also know it doesn't operate with flaps extended. Also if that it what he was thinking, then that implies he knew that the stall warning was false?
Or.... insufficient confidence in knowing which of the three plus inadequate height agl
So
Power maintained to mitigate stall risk
Autopilot engaged to mitigate against MCAS
UAS less of a risk than either of the above

??
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Old 11th Apr 2019, 13:44
  #3864 (permalink)  
 
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Ethiopian Crash Data Analysis Points To Vane Detachment |

Firewall but but the first paragraph says a lot
now why did the vane come off ? Still doesn’t excuse Boeing of MCAS reliance on one vane.
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Old 11th Apr 2019, 14:33
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Originally Posted by shackson View Post
Is it known if the vane has been recovered or was destroyed?
A bit of a needle in a haystack, but they should sure be looking for it. It should be possible to determine a specific location where it detached and hence a search area.

Understanding why it fell off is important, especially since it is not the same problem as affected Lion Air, despite the resultant similar crashes.

- GY
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Old 11th Apr 2019, 14:40
  #3866 (permalink)  
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Can't get the above link.


Is the vane detachment - the very bizarre coincidence of two different failures - solely based on the different angle split readouts?
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Old 11th Apr 2019, 14:48
  #3867 (permalink)  
 
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Recall Emergencies

Originally Posted by HundredPercentPlease View Post
I disagree. Immediately (at the end of the rotation) the aircraft was indicating a stall, and so the crew performed a stall recovery (lowered AoA by reducing pitch and increasing speed).



It was three memory checklists:
  1. Stall recovery.
  2. UAS recovery.
  3. Runaway stab (during the UAS recovery and aurally masked by the stick shaker).
The evidence is that 3 is too many - unless possibly you have a third pilot to monitor and assist.

I have a feeling that the Ethiopian captain may have engaged the autopilot as a pre-considered homebrew anti-MCAS strategy. Autopilot in = no MCAS risk.
If one rolls down the runway with both engines providing the advertised thrust (94%), the airspeed cross check at 80 indicates no disparity, and the aircraft is rotated normally at Vr with both airspeed indicators working normally then rotated to a climb attitude and the engines continue to turn and burn, then any erroneous stall warning has to be ignored and treated as such. That is why manufacturers provide nominal pitch/power settings to insure a stall free climb or cruise while it gets sorted out. In fact, I had the same stick shaker after take-off many years ago on the 737-200 after lift-off; yes, it gets your attention but if the attitude of the aircraft is ok and the engines are running fine then it cannot be a bona fide stall but an erroneous indication.
If the crew thought it was a stall, then the stall recovery should have been implemented - it was not and in fact they (Ethiopian) tried to engage the autopilot, a definite no-no in a stall.
So even setting aside the MCAS issue later, sadly this all points to training and experience to handle a pretty basic emergency. And that points back to the airline and the CAA who are responsible for that.
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Old 11th Apr 2019, 15:45
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L39 Guy,

“… sadly this all points to …”
… the persistence, the difficulty in avoiding hindsight, prejudging, assuming; -

How would a crew know that stick-shake just after lift off is erroneous. How is the accuracy of AoA established, of attitude, or at that instant, speed, until the accuracy of these is confirmed all that remains is assumption.
Whatever is argued, the correct AoA cannot be determined even with an EFIS display - which one is correct.
The on-side airspeed will be inaccurate because AoA is used in pressure error correction. Similarly the EFIS low speed awareness - based on AoA, perhaps adding to a belief that stick shake is valid.
Add distractions of Speed and Altitude Disagree alerts, and Feel Diff Press, together with the surprise of an unexpected event, or even higher stick force due to Feel Diff, then all available thinking ability is required to manage what exists.
In time, crosschecking speed, attitude, ‘feel’ (experience) might judge otherwise.

Assumptions such as “… it cannot be a bona fide stall but an erroneous indication” can be just as hazardous as an abnormal technical situation.
And thus any conclusion based on such assumption fails because of false reasoning.

Back to the links in Ethiopian airliner down in Africa - background reading.


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Old 11th Apr 2019, 16:19
  #3869 (permalink)  
 
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Originally Posted by Loose rivets View Post
Can't get the above link.


Is the vane detachment - the very bizarre coincidence of two different failures - solely based on the different angle split readouts?
Article summarises the fact that evidence is all in the FDR readout:
- Initial wild gyration of AOA sensor immediately after takeoff.
- Thereafter stable at implausibly high AOA.
- Moments before the final crash, the aircraft entered a negative-g bunt, AOA sensor flipped 180 degrees, stick shaker stopped.
Already hinted before in this thread, but the only physical explanation consistent will all of these data points is vane detachment. The event is clearly different from Lion Air, which had a constant 20 degree offset, but the end result was the same.

Re-posting annotated FDR readout:


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Old 11th Apr 2019, 16:21
  #3870 (permalink)  
 
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Originally Posted by alf5071h View Post
L39 Guy,

“… sadly this all points to …”
… the persistence, the difficulty in avoiding hindsight, prejudging, assuming; -

How would a crew know that stick-shake just after lift off is erroneous. How is the accuracy of AoA established, of attitude, or at that instant, speed, until the accuracy of these is confirmed all that remains is assumption.
Whatever is argued, the correct AoA cannot be determined even with an EFIS display - which one is correct.
The on-side airspeed will be inaccurate because AoA is used in pressure error correction. Similarly the EFIS low speed awareness - based on AoA, perhaps adding to a belief that stick shake is valid.
Add distractions of Speed and Altitude Disagree alerts, and Feel Diff Press, together with the surprise of an unexpected event, or even higher stick force due to Feel Diff, then all available thinking ability is required to manage what exists.
In time, crosschecking speed, attitude, ‘feel’ (experience) might judge otherwise.

Assumptions such as “… it cannot be a bona fide stall but an erroneous indication” can be just as hazardous as an abnormal technical situation.
And thus any conclusion based on such assumption fails because of false reasoning.

Back to the links in Ethiopian airliner down in Africa - background reading.


Well, you can have that problem on all aeroplanes which have an AoA dependent stall warning system. And there are a lot of those certified.
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Old 11th Apr 2019, 16:32
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From GordonR
Article summarises the fact that evidence is all in the FDR readout:
- Initial wild gyration of AOA sensor immediately after takeoff.
- Thereafter stable at implausibly high AOA.
- Moments before the final crash, the aircraft entered a negative-g bunt, AOA sensor flipped 180 degrees, stick shaker stopped.
Already hinted before in this thread, but the only physical explanation consistent will all of these data points is vane detachment. The event is clearly different from Lion Air, which had a constant 20 degree offset, but the end result was the same.
The other item that strongly supports the departing vane theory is the master caution anti icing alarm and AoA heat trace off at the same time.
This would not (likely) not happen if some coupling/gear inside the sensor failed

Last edited by MurphyWasRight; 11th Apr 2019 at 16:35. Reason: typo
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Old 11th Apr 2019, 16:43
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Originally Posted by Loose rivets View Post
Is the vane detachment - the very bizarre coincidence of two different failures - solely based on the different angle split readouts?
Two independent failures would indeed be a bizarre coincidence.
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Old 11th Apr 2019, 17:45
  #3873 (permalink)  
 
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Originally Posted by DaveReidUK View Post
Two independent failures would indeed be a bizarre coincidence.
To understand the prior probability of two independent failures, would require use of Bayes Theorem, and statistical data on each failure mode. It is clear from the (lack of) response to queries about rates of AOA failure, that this data is scarce.

I have stated previously that AOA fail nose-down would give interesting information, but if the vane departs, the failure will always be nose up. Isn't that even more ironic, that the only known failure mode is 100% guaranteed to trigger MCAS!?

The fact that the MAX is a new aircraft may have something to do with the Lion Air fixed offset error, but presumably bird-strike frequency has nothing to do with the model type.
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Old 11th Apr 2019, 17:54
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bill fly, #3910,
The point was not about whether a system can be certificated or not; it is about the human reaction and ability to manage the failure.
The 737 Max has several consequential ‘failures’ associated with AoA inaccuracy; in many other aircraft there is little or no interaction due to different system design or redundancy.
The certification judgement should involve crew workload, conflicting alerts, ambiguity, and distraction; consideration of the wider situation, risk of assumption, and comparison with additional tasks on top of a range of normal procedures. Thus it could be argued that the 737 requires a different approach, this could involve training or from what has been learnt for these accidents, reconsidering the overall system - AoA, speed correction, low speed awareness, alerts …
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Old 11th Apr 2019, 18:19
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Gordon et al.
Would the scenario of a loose vane about the shaft fit all incidents.

Re Lion, the vane rotated on the shaft as far as 22 deg (grub screw); the vane aligns with the airflow, but misreads due to the repositioned shaft under the influence of the balance weight. This also enabled the vane to be checked by maintenance without fault, because the vane was still free to move around the shaft.

With Ethiopian, the vane became completely free to rotate during takeoff, then the balance weight positioned the shaft equivalent to full up ? (down?). The vane remained free in the airflow, but without connection it was irrelevant. The shaft remained free such that the balance weight reflected the effects of ‘g’ at the end of the flight.
Have the ‘errant’ AoA vane units been found; loose vane, or no vane.
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Old 11th Apr 2019, 18:28
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Originally Posted by GordonR_Cape View Post
To understand the prior probability of two independent failures, would require use of Bayes Theorem, and statistical data on each failure mode.
Isn't Bayes' Theorem a way of calculating conditional probability? If two failures are truly independent of each other, you don't need Bayes to work out the probability of both happening together.

Having said that, I have no idea what are the two independent failures the poster is referring to - one is the AoA vane detachment, but what is the other one ?
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Old 11th Apr 2019, 18:50
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Originally Posted by DaveReidUK View Post
...what are the two independent failures ...
https://www.bloomberg.com/news/artic...lure-data-showA review of public databases by Bloomberg News reveals the potential hazards of relying on the devices, which are mounted on the fuselage near the plane’s nose and are vulnerable to damage. There are at least 140 instances since the early 1990s of sensors on U.S. planes being damaged by jetways and other equipment on the ground or hitting birds in flight. In at least 25 cases in the U.S., Canada and Europe, the damage triggered cockpit alerts or emergencies.



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Old 11th Apr 2019, 19:10
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Originally Posted by safetypee View Post
Gordon et al.
Would the scenario of a loose vane about the shaft fit all incidents.

Re Lion, the vane rotated on the shaft as far as 22 deg (grub screw); the vane aligns with the airflow, but misreads due to the repositioned shaft under the influence of the balance weight. This also enabled the vane to be checked by maintenance without fault, because the vane was still free to move around the shaft.

With Ethiopian, the vane became completely free to rotate during takeoff, then the balance weight positioned the shaft equivalent to full up ? (down?). The vane remained free in the airflow, but without connection it was irrelevant. The shaft remained free such that the balance weight reflected the effects of ‘g’ at the end of the flight.
Have the ‘errant’ AoA vane units been found; loose vane, or no vane.
Not sure why a common AoA failure cause is needed, in any case there are very significant differences between the 2.

Lion Air :Incorrect (offset) values present for entire (2!) flights, the offset shows as soon as speed is sufficient.
Not sure of aerodynamics but doubt that a damaged vane would have a consistent offset.
Since this was a refurbished unit it is possible that it was mis-assembled or damaged on installation/test, only other possibility is a wiring issue, which might fit with prior unit being intermittently bad. One can be sure the investigators will be looking at this in depth.

Ethiopian: Values ok at first then a sudden and dramatic shift to full over and at the same time the heater circuit opens.
This can be explained by bird strike or prior (ramp rash) damage.

Recovery of either unit intact enough for analysis is unlikely due to high energy of crashes and position of sensors.
Finding the missing vane is also unlikely due to small size and other factors, an all out search might find it but would at most just confirm one of the 2 causes, prior damage or bird strike.

Last edited by MurphyWasRight; 11th Apr 2019 at 19:16. Reason: Added note on finding vane.
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Old 11th Apr 2019, 19:34
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Originally Posted by MurphyWasRight View Post
Not sure why a common AoA failure cause is needed, in any case there are very significant differences between the 2.

Lion Air :Incorrect (offset) values present for entire (2!) flights, the offset shows as soon as speed is sufficient.
Not sure of aerodynamics but doubt that a damaged vane would have a consistent offset.
Since this was a refurbished unit it is possible that it was mis-assembled or damaged on installation/test, only other possibility is a wiring issue, which might fit with prior unit being intermittently bad. One can be sure the investigators will be looking at this in depth.

Ethiopian: Values ok at first then a sudden and dramatic shift to full over and at the same time the heater circuit opens.
This can be explained by bird strike or prior (ramp rash) damage.

Recovery of either unit intact enough for analysis is unlikely due to high energy of crashes and position of sensors.
Finding the missing vane is also unlikely due to small size and other factors, an all out search might find it but would at most just confirm one of the 2 causes, prior damage or bird strike.
If the rwy was closed after the accident and if someone then bothered to examine it for FOD after the event, it is unlikely any feathers will turn up along with the missing AoA vane.
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Old 11th Apr 2019, 20:07
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I had a look again at the data recorder readout in the context of the AoA being detached and have doubts about the coincidence of AoA going rogue exactly at rotate.
Perhaps the stall warning was genuine.
The RTOW was 72400 and actual TOW was 71896.
Possibly more baggage than standard weights with a lot of long haul pax transferring.
The rotate induced 1.5 G and peaked at 20 deg pitch swiftly reduced to less than 10 deg and 0.8G.
Somehow this kicked off the erroneous AoA disparity and subsequent activation of MCAS.
This was a take-off at max weight and high altitude that demanded careful handling.
Apologies if this has been covered.
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