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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 8th Apr 2019, 08:56
  #3601 (permalink)  
 
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Originally Posted by fizz57 View Post
Getting a bit fed up with this canard. Not saying it shouldn't have been done, but what exactly would this have done to the throttle position in the accident scenario?
You miss my point.
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Old 8th Apr 2019, 08:56
  #3602 (permalink)  
 
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Originally Posted by ecto1 View Post
It's time to roll up our sleeves and produce good computer systems for planes. Systems in which sensor signals jumping straight up to off - the - scale - high are automatically rejected and have zero impact. For a start. Updating whichever rules as we go to keep a cheap but much safer solution.
In general, the industry does, and overall does a very good job of it. Input value filtering is normal, and even the A330 that nose-dived in cruise had computers which did. It was done inappropriately, but most of the time the erroneous values were rejected.

The problem here lies much earlier, in specifying the requirements for MCAS. More specifically, the failure modes and the severity and likelihood of each were not properly analysed.

We totally know how to do it right, it's just we forbid ourselves to do it by all sorts of rules and bureaucracy. Let's not accept another band aid.
Au contraire. Aviation is the industry which mandates appropriate techniques. They are well-known, and used.

Add-on systems that bring airliners back into compliance, which are not by themselves aerodynamically completely compliant to regulations, are literally as old as jet airliners themselves. Many types have stick-nudgers or stick-pushers, and they work fine, and are perfectly sensible to use. But that does not mean one can skip due diligence in developing them, which includes a thorough risk and hazard assessment.

Bernd
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Old 8th Apr 2019, 09:02
  #3603 (permalink)  
 
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Originally Posted by KyleRB View Post
Of course that’ll be the job of Boeing lawyers to mitigate and try to reduce liability but ultimately the main culprit will come down to MCAS, it’s design, certification, discreet implementation, lack of redundancy and ultimately its power to overcome pilot actions without crews being made fully aware of its capabilities.
CurtainTwitcher
Perhaps as the lawyers are debating and dissecting second by second details, they should do it with the continuous Stick Shaker loop in the background. It really is an asymmetric situation for the crews, if they don't do something (following the NNM for the stick shake and shove the nose down) and save the day at that point they get no credit, yet they don't do something else everyone wants to point to them and say see, pilot error. For the manufacturer it is a business decision to hang the crew if possible as it reduces their liability. They get to cherry pick the inevitable forced pilot errors in such difficult circumstances and gloss over the manufacturers errors and omissions.
Originally Posted by CodyBlade View Post
going to be judged not by your peers or professionals who can intepret the data objectively.But by mom and pop.
Not a lawyer. Presumably there are differing degrees/amounts of liability. More important are punitive damages, which can be many multiples of the actual damages claimed. All of these will be argued intensely, and the various role players will have differing inputs into this process, other than the dead pilots.
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Old 8th Apr 2019, 09:13
  #3604 (permalink)  
 
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Before rotation there is speed where AoA sensors should work in the intended way, why not check the values then and throw out the bad one. That does not help for events after rotation but would atleast check for ground damage.
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Old 8th Apr 2019, 10:03
  #3605 (permalink)  
 
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Hello all,

My understanding from these situations is one contributing factor may be that a pilot may ultimately get into a situation where he or she has little to no ability to correct a badly out of trim Max manually or electrically due to aerodynamic loads in certain flight situations. What I have not seen is anyone noting is if this inability, irregardless of how it came about, is the same in other aircraft from Boeing or Airbus. From a 320 to a 380 or a 737 NG or 747 to a 787 would a pilot have the ability to re-trim from the same situation these two flights faced?

TME
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Old 8th Apr 2019, 10:34
  #3606 (permalink)  
 
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Originally Posted by TeachMe View Post
My understanding from these situations is one contributing factor may be that a pilot may ultimately get into a situation where he or she has little to no ability to correct a badly out of trim Max manually or electrically due to aerodynamic loads in certain flight situations.
That appears to be true for manual (wheel) trim, but there's no evidence that it's the case for electric trim.

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Old 8th Apr 2019, 10:41
  #3607 (permalink)  
 
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Originally Posted by ecto1 View Post
I see more and more posts selecting solutions which are strictly inside the coordinates of what we "know" it is possible. (EG: more training, software patch...) and not seriously mentioning much better / obvious ones.

I say: listen to people from other industries. What we sometimes believe impossible in terms of complexity / cost / time may not be so.

It's time to roll up our sleeves and produce good computer systems for planes. Systems in which sensor signals jumping straight up to off - the - scale - high are automatically rejected and have zero impact. For a start. Updating whichever rules as we go to keep a cheap but much safer solution.

We totally know how to do it right, it's just we forbid ourselves to do it by all sorts of rules and bureaucracy. Let's not accept another band aid.

Somebody wrote many pages ago he spent 30 years in the industry and failed to change substandard quality. Sorry I forgot who. But to him and everyone else, we have to keep fighting, as futile as it may be. It's simply what we have to do.
Or we could get rid of the grandfathering loophole that Boeing and the FAA have abused, where software has been used as a bandaid. A320/ B787 etc that were designed to make full use of computer systems from the ground up are fine. It is the unholy mixing of digital & analogue on the MAX that is the issue.

Would a similar AOA failure on an NG cause a crash? Almost certainly not.

In my opinion, Boeing & FAA have stretched the definition of a derivative aircraft beyond what is reasonable. What will be interesting will be impact on 777X, I am guessing FAA will be reviewing all assumptions on that now, and we might even see the end of "NG" derivative programmes (as opposed to stretches) with authorities insisting that if you build a new plane, it meets current standards. This is how it works in contruction. Sure, you don't have to rebuild a 40 year old building to new standards, but you can't say "this design met earthquake standards in 1990, so I can just build a derivative of it"





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Old 8th Apr 2019, 10:43
  #3608 (permalink)  
 
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Exactly. The electric trim will still work as advertised. This was not, on the surface of it an unrecoverable situation.
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Old 8th Apr 2019, 10:53
  #3609 (permalink)  
 
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but there's no evidence that it's the case for electric trim.
Agreed. There is the suggestion though, that the pilots, after the again assumed switching back on, of the electrical trim... Only apply relatively short trim up commands... as perhaps the TRIM appears to be jammed to the pilots. The FDR doesn't seem to show any significant movement in TRIM to their last commands. The subsequent MCAS TRIM down, seems to have an immediate effect.
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Old 8th Apr 2019, 10:56
  #3610 (permalink)  
 
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Originally Posted by TeachMe View Post
Hello all,

My understanding from these situations is one contributing factor may be that a pilot may ultimately get into a situation where he or she has little to no ability to correct a badly out of trim Max manually or electrically due to aerodynamic loads in certain flight situations. What I have not seen is anyone noting is if this inability, irregardless of how it came about, is the same in other aircraft from Boeing or Airbus. From a 320 to a 380 or a 737 NG or 747 to a 787 would a pilot have the ability to re-trim from the same situation these two flights faced?

TME
I guess it depends on whether the A320 to A380 or a 737 NG or 747 to a 787 would also suddenly and arbitrarily trim itself into the sea/ground!
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Old 8th Apr 2019, 11:20
  #3611 (permalink)  
 
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Originally Posted by quentinc View Post
The FDR doesn't seem to show any significant movement in TRIM to their last commands.
I don't think you can deduce anything from that.

The previous application of trim, 9-10 seconds' worth at around 05:30:40 resulted in a tad under 2 units (degrees) of ANU stab movement. The final two bursts of trim command were no more than 1 second each, so any stab movement would have been a fraction of a degree (which is actually just discernable on the trace).
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Old 8th Apr 2019, 11:21
  #3612 (permalink)  
 
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DaveReidUK, #3647, Icarus2001, quentinc
“… no evidence that it's the case for electric trim.”

Indications in the EASA reference ‘Equivalent Safety Case’ suggest otherwise.
Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts.”

Also see the ‘forceful‘ arguments in https://www.satcom.guru/2019/04/stab...and-range.html

Where the Equivalent Safety Case relates to normal flight with an aircraft reasonably in trim - low tail forces - Peter L, initial diagrams with level tail +/- elevator.
However with serious trim malfunctions, the tail displacement together with adverse elevator load attempting to recover, might limit electric trim (latter diagrams). Add effects of elevator feel offset due to false stall warning, higher stick loads add to trim offset loads.

Discussions range between trim motor stall, and/or trim inhibit at large angles for trim runaway safety case.

Also remember that MCAS vs elect trim is 10 to 5 time ratio in favour of nose down.
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Old 8th Apr 2019, 11:37
  #3613 (permalink)  
 
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Originally Posted by alf5071h View Post
DaveReidUK, #3647, Icarus2001, quentinc
...

Indications in the EASA reference ‘Equivalent Safety Case’ suggest otherwise.
Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts.”

...

However with serious trim malfunctions, the tail displacement together with adverse elevator load attempting to recover, might limit electric trim (latter diagrams). Add effects of elevator feel offset due to false stall warning, higher stick loads add to trim offset loads.

...

Also remember that MCAS vs elect trim is 10 to 5 time ratio in favour of nose down.
I've seen this comment a few times, and I think there may be a misunderstanding about how MCAS and the electric trim switches interact.

While MCAS *can* operate for up to 10 seconds, it is inhibited by the activation of the wheel trim switches. When they are activated, MCAS stops until they are released, and waits 5 seconds. There isn't a ratio of time between one and the other. You can hold the trim switch for any length of time and MCAS will stay off until 5 seconds after it is released.

Nothing stops a pilot from trimming to neutral in one long press, then using the cutout switches, beyond the counterintuitive nature of that reaction.
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Old 8th Apr 2019, 11:44
  #3614 (permalink)  
 
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Originally Posted by bsieker View Post
Au contraire. Aviation is the industry which mandates appropriate techniques. They are well-known, and used.

Add-on systems that bring airliners back into compliance, which are not by themselves aerodynamically completely compliant to regulations, are literally as old as jet airliners themselves. Many types have stick-nudgers or stick-pushers, and they work fine, and are perfectly sensible to use. But that does not mean one can skip due diligence in developing them, which includes a thorough risk and hazard assessment.
I fully agree Bernd. As someone who has worked for years on aviation safety critical software, I am stunned at the poor specification and implementation for the MCAS software. The Ethiopian flight FDR showing an AoA of 75 – where is the limit check?? Even for software that was (incorrectly) not considered as DO-178C Level A, I still find it impossible to comprehend how someone somewhere in the development process did not suggest to put in a “if AoA > x deg, no trim”, perhaps only as a “just in case – belt and braces couple of lines of code”. It grieves me to think that if they had, we might not have lost 2 aircraft.

IMO the MCAS software has to be redeveloped from scratch as Level A, not just patched. The risk of a bug in the software that could cause an AND runaway has to be reduced to ALARP (As Low As Reasonably Practical).
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Old 8th Apr 2019, 11:57
  #3615 (permalink)  
 
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Originally Posted by alf5071h View Post
DaveReidUK, #3647, Icarus2001, quentinc
“… no evidence that it's the case for electric trim.”

Indications in the EASA reference ‘Equivalent Safety Case’ suggest otherwise.
Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts.”
This has already been discussed, the EASA document refers to the designed limits for electric trim, that do not allow you to trim electrically outside its designed range, even though the range available with mechanical trim is larger, and in some situations you need to access a larger range than manual electric trim allows. Those design limits don't apply when you trim electrically ANU to counter MCAS. They would only apply if you would try to trim further AND electrically, after you reached the AND design limit for manual electric trim (lower than 3.95 units on the 737-800, for example). The opposite direction, ANU, would be unaffected and should work just fine, even when you are trimmed AND outside the designed range for manual electric trim.
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Old 8th Apr 2019, 12:01
  #3616 (permalink)  
 
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Originally Posted by VicMel
The Ethiopian flight FDR showing an AoA of 75 – where is the limit check?? Even for software that was (incorrectly) not considered as DO-178C Level A, I still find it impossible to comprehend how someone somewhere in the development process did not suggest to put in a “if AoA > x deg, no trim”, perhaps only as a “just in case – belt and braces couple of lines of code”.
Consider AF447. The design team thought that an aeroplane could not be flying below 60KIAS, so they inhibited the stall warning... We know how that ended.

It could be possible to have an AOA of 75° and appreciate some assistance from the machine (MCAS) to keep the nose from getting there in the first place. The F up was making it reliant on only one sensor.
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Old 8th Apr 2019, 12:02
  #3617 (permalink)  
 
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Originally Posted by FrequentSLF View Post
A simple question from an engineer, why a designer should put a pilot in such situation, why the designer cannot prevent and design a solid system? And why if he is not able to do so the blame is on the operator? A wrong design is a wrong design, no matter how much you train the operators
A simple answer; a designer will do what his boss tells him to do and his boss will tell him to do what increases the share price. A simple answer is that neither Boeing nor the airlines were willing to pay for the cost of a full re-design.
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Old 8th Apr 2019, 12:08
  #3618 (permalink)  
 
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Originally Posted by alf5071h View Post
DaveReidUK, #3647, Icarus2001, quentinc
“… no evidence that it's the case for electric trim.”

Indications in the EASA reference ‘Equivalent Safety Case’ suggest otherwise.
Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts.”


Now why would a manufacturer limit the thumb switch trim range to even less than the autopilot trim authority, particularly when it could no longer be shown to trim throughtout the required envelope?
How about non compliance with the out of trim dive requirement as mentioned in said equivalent safety determination?
Perhaps the easiest solution was to use the 30lbf out of trim with the wheel rather than redesign the system to be compliant when 3 seconds of no load electric trim operation is applied?
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Old 8th Apr 2019, 12:10
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Originally Posted by MemberBerry View Post
The opposite direction, ANU, would be unaffected and should work just fine, even when you are trimmed AND outside the designed range for manual electric trim.
Yes, that was my point, supported by the FDR trace.
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Old 8th Apr 2019, 12:37
  #3620 (permalink)  
 
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Thanks MemberBerry, #3655

Your helpful description, interpretation of the EASA text, infers that the trim demand will be as required for the flight condition. Even in extreme ‘normal flight’, in-trim conditions, the trim system should not hold any significant load other than that required for a balanced aircraft, which will fly ‘hands off’.

The situation with MCAS malfunction is that the flight condition is not balanced - ‘abnormal flight’; the trim system is creating the flight path deviation which the crew are attempting to correct. A correcting force has to be applied via the trim motor and elevator; these forces are probably much higher than that for the near trim condition.
Thus are the differences in the required force and direction of motion, at or beyond the capability of the trim elect motor ?

The apparent anomaly towards the end of the FDR could be interpreted as a nose up elect trim demand, but no trim movement, yet shortly after MCAS trim demands nose down and the tail moves. This of course is not evidence of a nose up elect trim limit (stall), but remains a possibility.

Is this above a valid position, is it one which the EASA (FAA, Boeing) text might not have considered ?
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