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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 4th Apr 2019, 23:03
  #3201 (permalink)  
 
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When all is said and done, the one common indicator is the AoA reading on the Cap's side. Debate everything else till your heart's content.

Why did it flip to ~75deg at 150kn? More interestingly, why did it flip back to "normal" for a moment just before end of flight before flipping back to error?

I assume the slowly diverging altitude readings relate to the AoA stuck at 75 while pitot static reads ok...

Boeing's proposed fix includes shutting off the "required" MCAS when AoA disagrees so this is kind of important.

Whisky Tango Foxtrot guys? What's happening to that vane or the electrons/code behind it?
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Old 4th Apr 2019, 23:08
  #3202 (permalink)  
 
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Originally Posted by ktcanuck View Post
When all is said and done, the one common indicator is the AoA reading on the Cap's side. Debate everything else till your heart's content.

Why did it flip to ~75deg at 150kn? More interestingly, why did it flip back to "normal" for a moment just before end of flight before flipping back to error?

I assume the slowly diverging altitude readings relate to the AoA stuck at 75 while pitot static reads ok...

Boeing's proposed fix includes shutting off the "required" MCAS when AoA disagrees so this is kind of important.

Whisky Tango Foxtrot guys? What's happening to that vane or the electrons/code behind it?
Maybe it is just simple plain old grounding also why the other sensor messed up , combined with a very bad qa at boeing trying to push more and more of the planes out of the factory more mistakes made
Both incident planes had issues within months of delivery i would love to see the issues on the other 380 they had delivered
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Old 4th Apr 2019, 23:21
  #3203 (permalink)  
 
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There is something seriously wrong with this design.
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Old 4th Apr 2019, 23:32
  #3204 (permalink)  
 
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Originally Posted by AVAT View Post


Because Boeing stated MCAS inoperative with flaps extended. If crew extended flaps before reengaging electric stabilizer they could have prevented MCAS from operating. That would have allowed them to easily trim plane electrically. With known MCAS issue retracting flaps at 1000 agl seems too soon to me. At that altitude MCAS engagement more difficult to control.
Totally agree that, if the crew actually understood MCAS, and understood that they had a sensor or sensor signal failure that might cause it to activate, they would not have cleaned up when they did. I honestly don't think that the ET crew, or the vast majority of crews at that time, had anything like the understanding of MCAS that we have now, notwithstanding the Lion crash and the subsequent Boeing and FAA notices. At this point, I can't remember airspeed, etc. at the point they reengaged electric trim. I think it may have been high enough that extending flaps would not have come naturally to mind, but I'll have to look at the data again.
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Old 4th Apr 2019, 23:52
  #3205 (permalink)  
 
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More about the newly-disclosed 'minor' issue with safety-critical software, from the Washington Post:

Additional software problem detected in Boeing 737 Max flight control system, officials say

But later Thursday, Boeing confirmed to The Washington Post that it had found a second software problem that the Federal Aviation Administration has ordered fixed — separate from the anti-stall system that is under investigation in the two crashes and is involved in the worldwide grounding of the aircraft.

That additional problem pertains to software affecting flaps and other flight-control hardware and is therefore classified as critical to flight safety, said two officials who spoke on the condition of anonymity to discuss the probe.

The realization of a second software problem explains why the timeline that Boeing projected publicly last week for getting hundreds of the aircraft airborne again has slipped, the officials said.
Boeing initially said it planned to submit fixes for its stall-prevention system to the FAA for review last week. On Monday, an FAA spokesman said the agency instead expected to receive the final package of software “over the coming weeks.”

“Obviously, we ended up at a situation that in hindsight was not supposed to happen,” one of the officials familiar with the investigation told The Washington Post. “Now, you don’t want to be in a situation where there was one contributing factor to an accident, and then three weeks later you find another one.

In a statement, Boeing called the additional problem “relatively minor” but did not offer details of how it affects the plane’s flight-control system. “We are taking steps to thoroughly address this relatively minor issue and already have the solution in work to do that.”
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Old 5th Apr 2019, 00:42
  #3206 (permalink)  
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There is something seriously wrong with this design.
In all seriousness, I fear you're right.




We always used to say, if it looks right, it'll fly right, etc., but I can't recall ever having such an uneasy feeling about an aircraft's ground clearance. Remember, this is the core of the problem we're discussing.

Fuel, payload and a bit of the thumper will make it even more critical, and that's before a hefty cross wind.
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Old 5th Apr 2019, 00:46
  #3207 (permalink)  
 
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While it seems fair to say the crew made decisions that we, based on FDR plots and from the comfort of our homes, do not understand... I think it is also important to keep in mind the timespan and chaotic situation in the cockpit at the time. Pretty much in the time that it takes you to read the events as they happened, they had to analyze them and troubleshoot them with alarms going off. We can sit down with a cup of tea or coffee and take 10-15 minutes to think about it and decide what they should have done instead.

Pointing at the crew as the cause of this accident is not completely fair in my opinion. They may have made decisions that turn out to be questionable to the skygods here who have figured out exactly what they should have done. But... if they'd had a well designed plane without vague, ill designed and explained "safety" features one could also argue this accident would not have happened. Follow the problem down to the roots and you will IMHO always end up at Boeing and the design decisions made. The crew on this flight could be considered the last slice of the cliché swiss cheese with the hole lined up.
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Old 5th Apr 2019, 00:49
  #3208 (permalink)  
 
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Left side AoA sensor vs vertical acceleration (g)

I offer an explanation for the behaviour of the Left hand AoA indication throughout the flight.

If the vane had been lost the AoA sensor would become unbalanced about its usual axis of rotation. The internal balance weight** would then cause the axle to be subject to movement when the aircraft transitioned from +g to -g. +g would cause the indication of +AoA. (If I have got this the right way round

Looking at the FDR traces it can be seen that this appears to be the case. I have drawn four green vertical lines to indicate the transitions from +g to -g and vice versa. In each case they appear to align with a change in the direction of movement of the sensor in the correct sense. Remember that the data consists of discrete samples and we do not know the sample rate and I am assuming that any small discrepancies are due to errors introduced by the sampling.

I have (rather crudely) chopped out a period in the middle of the chart so that it is a bit narrower so that the scale markings can be easily seen. The horizontal blue line in the "g" section of the chart is coincidentally exactly on 0g.

It therefore seems quite likely that the vane was lost or perhaps damaged soon after take off, perhaps by a bird strike or otherwise. Note however that if the vane had been bent back its balance would be moved in the other direction and its aerodynamic influences would still have been felt so I think that the best conclusion consistent with the data is that the vane was lost.

** A post on PPRuNe regarding this incident showed an exploded view of the AoA sensor and it featured an internal balance weight. Unfortunately I cannot recall where it was now.




Last edited by jimjim1; 5th Apr 2019 at 02:15.
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Old 5th Apr 2019, 01:04
  #3209 (permalink)  
 
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Originally Posted by jimjim1 View Post
I offer an explanation for the behaviour of the Left hand AoA indication throughout the flight.

If the vane had been lost the AoA sensor would become unbalanced about its usual axis or rotation. The internal balance weight** would then cause the axle to be subject to movement when the aircraft transitioned from +g to -g. +g would cause the indication of +AoA. (If I have got this the right way round
Excellent post, the tracking is almost too perfect to be believed at first.

This fits with the AoA heater fault as well, whatever happened caused 2 symptoms that are hard to explain except as physical damage to the sensor since the heater supply is independent of the resolver.

Someone commented that the pilots would have heard a bird strike, given the lack of a full CVR transcript we don't know if anything was audible.
I would suspect that the press conference statement of 'no foreign object' would suggest but not prove none was audible.

Anyone know if a departing vane by itself would cause a heater open?

One other observation is that the flight deck actions seem to be mostly normal until the AP disconnect, almost as though the stick shaker was activated but not working or noticed.
If missing from CVR that probably would have been noted.



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Old 5th Apr 2019, 01:09
  #3210 (permalink)  
 
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The flight before LionAir crash, had the same symptoms, but did it have the AoA sensor vane missing or shaft failure?
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Old 5th Apr 2019, 01:12
  #3211 (permalink)  
 
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Originally Posted by xyze View Post
Checkmate - they were damned if they did (use trim cutout switches) and damned if they didn't. Seems that at the point they shut of the system the AND trim was more than the elevators could overcome and with ever increasing speed in the dive manual trim was not an option. what would you do? try flying inverted (as with the air Alaska accident )? May explain the last minute roll .
​​​​
If the column forces were too high, one option (if you hadn't worked out how to engage the manual trim, which these guys hadn't) is to turn the trim back on, correct it with the normal electric trim switches, and then turn it off again before MCAS kicks back in. You asked what I would do, that's what I would do.


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Old 5th Apr 2019, 01:17
  #3212 (permalink)  
 
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Originally Posted by AlexGG View Post
The flight before LionAir crash, had the same symptoms, but did it have the AoA sensor vane missing or shaft failure?
No but it was active with a constant ~20 degree offset to good sensor and no reported AoA heater fail, which would have led to it's replacement.

Clearly a different issue, simplest explanation would be it was stressed on installation causing the vane to slip on shaft but not known if that is possible.

Last edited by MurphyWasRight; 5th Apr 2019 at 01:20. Reason: deleted extra word.
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Old 5th Apr 2019, 01:29
  #3213 (permalink)  
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I just can't get to grips with the concept of a weight. Counter-balance or not. My recollection of checking the vanes was that they stayed put to wherever you shoved them. Having any weight would make them subject to accelerative forces - the weight of the vane itself would be almost totally air-supported and while still affected by g, it would be minuscule compared to the pressure of the air.
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Old 5th Apr 2019, 01:31
  #3214 (permalink)  
 
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Originally Posted by jimjim1 View Post
I offer an explanation for the behaviour of the Left hand AoA indication throughout the flight.
...
Looking at the FDR traces it can be seen that this appears to be the case. I have drawn four green vertical lines to indicate the transitions from +g to -g and vice versa. In each case they appear to align with a change in the direction of movement of the sensor in the correct sense. Remember that the data consists of discrete samples and we do not know the sample rate and I am assuming that any small discrepancies are due to errors introduced by the sampling.
You may be on to something. There are some other data points to consider:

The Left AOA divergence begins at 05:38:45 right as the Vertical Acceleration hits a new positive maximum in the climb.

At 05:41:22, that divergence is reduced just a little bit as the Vertical Acceleration hits a new low.

(I don't have enough posts to upload the images).
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Old 5th Apr 2019, 01:34
  #3215 (permalink)  
 
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Originally Posted by Loose rivets View Post
I just can't get to grips with the concept of a weight. Counter-balance or not. My recollection of checking the vanes was that they stayed put to wherever you shoved them. Having any weight would make them subject to accelerative forces - the weight of the vane itself would be almost totally air-supported and while still affected by g, it would be minuscule compared to the pressure of the air.
If they were not balanced they would not stay put wherever you put them, so either the vane itself was balanced or there would need to be a separate internal counter balance.
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Old 5th Apr 2019, 01:45
  #3216 (permalink)  
 
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New NY Times Story on ET Prelim Report

Ethiopian Crash Report Indicates Pilots Followed Boeing’s Emergency Procedures

[. . .]
“The captain was not able to recover the aircraft with the procedures he was trained on and told by Boeing,” said Dennis Tajer, a spokesman for the American Airlines pilots union and a 737 pilot, who read the report.
A bit of additional stuff, but mostly I thought Tajer's comment might be of interest.
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Old 5th Apr 2019, 02:05
  #3217 (permalink)  
 
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What I still find baffling is why both pilots kept pulling back on the controls and barely ever tried to trim the forces off.

I'd imagine if I felt the nose heavy (whether it was MCAS at it or not) I'd trim up, which would alleviate the forces AND stop MCAS. Do the pilots just give up trimming up at some point?

FO reported manual trim didn't work. Later on, we see electric trim up as though they'd re-energized the cutouts as a last resort to get the nose up, but the amount of trim used was negligible? Then MCAS with regained access went ahead and sent the plane into its final dive.

Something that I haven't seen pointed out in this thread is that with the MAX, Boeing changed stab trim cutout philosophy from the NG. Previously you'd have 2 switches, MAIN ELEC and AUTOPILOT channels, self explanatory. Now they are PRI and BU for Primary and Backup whereby the cutout of any of them leaves you with absolutely no electric trim, precisely what you don't need when they implement an automatic system capable of limitless authority over the horizontal stabilizer. One would think previous design would work better with MCAS (disable autopilot trim channel via cutout and keep the ability to use the electric trim).

Just my 2c.
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Old 5th Apr 2019, 02:06
  #3218 (permalink)  
 
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Question:
Why did Boeing give a FULL description of the NG’s , STS, in their FCOM but ONLY mention MCAS in the abbreviation section of the MAX FCOM.
A. Because a full description would highlight an unwanted flight characteristic forced onto a 1967 airframe, pushing it beyond the MAX. ( criminal)
B. It was considered better to cover up & hide this aerodynamic instability, which could open Pandora’s box and affect market $hare against the NEO. ( criminal)
C. Because their partner in crime the FAA, allowed them. ( criminal)
D. Because they thought their magic software would magically make their shoe horned design failure disappear.
E. All of the above.

The answer is always “ all of the above”.
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Old 5th Apr 2019, 02:07
  #3219 (permalink)  
 
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Just curious, what position are the vanes in at rest or when say taxiing?

Are they self zeroing or just in a random position until X speed?

When is their position considered live/legit is it based on airspeed?

This might help with what would happen with a damaged or missing vane. It definitely seems like something that would be accounted for in the design or software, but maybe the addition of MCAS meant unexpected side effects.
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Old 5th Apr 2019, 02:10
  #3220 (permalink)  
 
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Originally Posted by ktcanuck View Post
When all is said and done, the one common indicator is the AoA reading on the Cap's side. Debate everything else till your heart's content.

Why did it flip to ~75deg at 150kn? More interestingly, why did it flip back to "normal" for a moment just before end of flight before flipping back to error?

I assume the slowly diverging altitude readings relate to the AoA stuck at 75 while pitot static reads ok...

Boeing's proposed fix includes shutting off the "required" MCAS when AoA disagrees so this is kind of important.

Whisky Tango Foxtrot guys? What's happening to that vane or the electrons/code behind it?
WTF indeed. So when a critical data input sensor fails, take the system out of the picture. A system that was vital to the certification of the aircraft.

I think you’ll have to do better than that guys!
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