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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 16:06
  #3741 (permalink)  

Keeping Danny in Sandwiches
 
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The easiest fix was to automatically apply a little nose down trim at high angles of attack.
I'm just an old retired pilot.
Couldn't the same effect have been achieved by inputting nose down elevator? At least the crew would be kept in the picture and no lasting aerodynamic input would have been involved.
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Old 9th Apr 2019, 16:18
  #3742 (permalink)  
 
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737 Driver, infrequentflyer789, I stand corrected. Thanks!

I have amended my previous post.

The posted diagrams are quite inconclusive because the various switches are only labelled with cryptic identifiers, and the outgoing signals not at all, and it is hard to tell what is what.

Bernd
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Old 9th Apr 2019, 16:45
  #3743 (permalink)  
 
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Originally Posted by sky9 View Post
I'm just an old retired pilot.
Couldn't the same effect have been achieved by inputting nose down elevator? At least the crew would be kept in the picture and no lasting aerodynamic input would have been involved.
That probably would have been the preferred option, however the elevator feel system cannot accept these inputs. I know that this is a very long thread, but this has been gone into in some depth earlier. Search for posts by FCEng84, which are particularly authoritative and illuminating.
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Old 9th Apr 2019, 17:33
  #3744 (permalink)  
 
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Could they just have switched autopilot on?
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Old 9th Apr 2019, 17:34
  #3745 (permalink)  
 
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Originally Posted by DaveReidUK View Post
For the mainstream media, and even some of the industry press, knowing that aircraft can stall and that it's generally considered a BAD THING represents about the limit of their understanding.
"Our airplane flies too well when the nose gets high, so the jealous authorities made us tamp it down. We overtamped."
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Old 9th Apr 2019, 17:38
  #3746 (permalink)  
 
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Originally Posted by YRP View Post
Surely the Tristar predated GPS by some years.
apologies. Quite right. INS ground speed. Just as accurate though. And on 737-200 we had Doppler ground speed - and 707. Pretty accurate too!
GPS ground speed will get you out of trouble in all these cases of chaotic airspeeds/stickshakers etc. Couple it with pitch and power and you actually cannot stall or over speed. Maintain ground speed at time of failure.
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Old 9th Apr 2019, 17:50
  #3747 (permalink)  
 
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Iíll
Originally Posted by dlen View Post
Could they just have switched autopilot on?
Switching AP ON in chaotic situations of unreliable airspeeds and concurrent stick shakers is deadly since itís inputs become unreliable. Better to fly the plane using the reliable information that is available PITCH. POWER. GROUNDSPEED. it works. Some time back (1980s) 737 suffered bird strike which took out AOA. And all air data including airspeed and altitude. Safe landing using above parameters. This became a standard training exercise. Fly a circuit with no speed info at all- except of course GPS which is a great help.
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Old 9th Apr 2019, 17:51
  #3748 (permalink)  
 
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Wouldn't it be possible to just measure the nose up tendency - be it caused by engine nacelle lift or thrust at low speed - by evaluating both pitch change rate and control column position, and apply ND trim accordingly?
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Old 9th Apr 2019, 17:57
  #3749 (permalink)  
 
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Originally Posted by weemonkey View Post
" the whole raison d'Ítre for MCAS is to modify stick forces when pulling back"

Is it? That seems to be in contradiction with everything I have read on the media.
That's because the media is clueless. The problem Boeing had with Max was that it didn't provide the same tactile feedback as NG (and other 737s) at high AoA, which is required for (grandfather?) certification. The reason is that the feel system could not produce enough force. In addition, Boeing promised to deliver Max which required no sim training to transition from NG. Nothing less, nothing more.

The very bright "solution" for the above problems from Boeing was to move the stab up to 6 units (from original 2 or so which were "not enough") in order to produce additional force on the stick. Yes, the bright heads at Boeing used one of the most essential flight control surfaces just to produce feedback force. Sending the plane straight to the ground in the process is just an overlooked side effect. Ooopsie. Well at least MCAS provided great (false!) tactile feedback of supposedly high AoA in the process - which was never there in the first place (a single faulty AoA vane).

Yes. Read it again. Let it sink.

Now imagine how a jury is going to react to that once Boing is sued by the victims families.
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Old 9th Apr 2019, 18:01
  #3750 (permalink)  
 
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Originally Posted by YRP View Post
Surely the Tristar predated GPS by some years.
you are correct of course. It was INS groundspeed. For those unfamiliar with how this works itís easy. Say VRef +5 =140. Surface wind headwind 30 kts. Fly approach at 110 g/s.
Target threshold speed less headwind. All the way down. Guarantees excellent speed control and avoids wind gradient shear effects.
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Old 9th Apr 2019, 18:01
  #3751 (permalink)  
 
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Originally Posted by dlen View Post
Could they just have switched autopilot on?
AP disengage when AoA disagree.
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Old 9th Apr 2019, 19:15
  #3752 (permalink)  
 
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trim drift

Another possible actor in this drama.

When boeing says to hold the trim wheel, when one study how the system is build and when one studies the data trace (and the misterious ND changes during multiple periods with no ND input).

I believe with sufficient pull and sufficient speed the trim motor clutch will slip and the trim wheels will creep ND.

If you blip the yoke switch and see the trim wheel not moving at a good pace and then you release and you see it slowly moving BACKWARDS, the whiskey tango foxtrot moment may be enough to make you forget about the pedestal switches.

Just another disturbing thought.
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Old 9th Apr 2019, 19:20
  #3753 (permalink)  
 
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Originally Posted by dlen View Post
Wouldn't it be possible to just measure the nose up tendency - be it caused by engine nacelle lift or thrust at low speed - by evaluating both pitch change rate and control column position, and apply ND trim accordingly?
Pitch and AOA are not the same. Pitch and AOA can change independently during maneuvers, which is why tracking pitch changes will not help determine rates of AOA changes, nor will it help determine the absolute value of AOA. In order to do this properly, you need a full blown computer system that integrates all of these sensor inputs, plus GPS, airspeed, etc. Those are fitted in an A350 or B787, and their price is more than twice that of a B737.

Originally Posted by derjodel View Post
AP disengage when AoA disagree.
Someone should write a thread showing all of the things that do not work when there is AOA disagree. IMO it would also simplify checklists if there were a single comprehensive one for AOA disagree, instead of parts of other checklists for procedures which bear some relevance to the actual flight conditions.

Edit: @ecto1 IMO Boeing needs to produce some hard data about how the trim system actually operates under real world conditions, not how manuals written in 1968 say it should work.
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Old 9th Apr 2019, 19:26
  #3754 (permalink)  
 
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Originally Posted by ecto1 View Post
Another possible actor in this drama.

When boeing says to hold the trim wheel, when one study how the system is build and when one studies the data trace (and the misterious ND changes during multiple periods with no ND input).

I believe with sufficient pull and sufficient speed the trim motor clutch will slip and the trim wheels will creep ND.

If you blip the yoke switch and see the trim wheel not moving at a good pace and then you release and you see it slowly moving BACKWARDS, the whiskey tango foxtrot moment may be enough to make you forget about the pedestal switches.

Just another disturbing thought.
There is no evidence that anything like this happened. I suppose with enough time you could sit and make up all sorts of possible problems, but let's stick to the evidence as we have it.

- GY

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Old 9th Apr 2019, 20:10
  #3755 (permalink)  
 
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I wonder if Boeing would not be better off taking their losses with MCAS and the trim system kludge by designing something appropriate for the narrow stick feel purpose, even if it means the certification of new hardware. A further software attempt is unlikely to impress and the equivalent of recertification and pilot sim training will be required anyway.
Loss of face, loss of money and admission of guilt are all water under the bridge.
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Old 9th Apr 2019, 20:22
  #3756 (permalink)  
 
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[QUOTE=Lost in Saigon;10443422]Just about everything you have read in the media is wrong. MCAS is not stall protection. MCAS does not counter the additional thrust of the more powerful engines. It is only there because the larger engine nacelles of the B737 MAX cause an aerodynamic pitch up moment at high angles of attack that did not meet FAA longitudinal stability and stick force certification standards.

The easiest fix was to automatically apply a little nose down trim at high angles of attack. (Quote)

I agree with all in the first paragraph but believe it needs to be expanded a little bit.

The B737 sat low on its undercarriage.For fuel efficiency the MAX was powered with much larger engines which had to be slung more forward than the old version. In addition the nose gear was raised by eight inches. This therefore meant that in order to satisfy the longitudinal stability requirements for certification, some fix was necessary. This then gave birth to MCAS. A quick fix through electronics control technology. The only point of its application naturally became the point of maximum effect, namely the horizontal stab. Accordingly the whole thing is one that addresses the original design concept, everything else would appear to be an attempt to overcome/control this in built longitudinal instability issue.
With it now comes an additional instrument, the AoA indicator wired into the whole system of automation. This piece of instrumentation did have its rightful place in the cockpit of a fighter jet, with its all moving horizontal stab, but does it really also have a place in a civil transport aircraft. Especially as a command function. After all to what is known so far about this particular event is that it was the stubborn stab that took the aircraft down despite all the crews efforts to encourage it away from its determined course.

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Old 9th Apr 2019, 20:25
  #3757 (permalink)  
 
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AoA Failure Rate

I hope the investigators will look into the AoA failure rate pre MAX. How many have been pulled for repair or replacement? Birds and ramp rash did not begin with the MAX.

Pre MAX, an AoA failure seems to produce "only" stick shaker and IAS disagree which every 737 crew handled successfully (until ET and JT). I suspect there's been a considerable number.
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Old 9th Apr 2019, 20:31
  #3758 (permalink)  
 
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Originally Posted by ecto1 View Post
Another possible actor in this drama.

When boeing says to hold the trim wheel, when one study how the system is build and when one studies the data trace (and the misterious ND changes during multiple periods with no ND input).

I believe with sufficient pull and sufficient speed the trim motor clutch will slip and the trim wheels will creep ND.

If you blip the yoke switch and see the trim wheel not moving at a good pace and then you release and you see it slowly moving BACKWARDS, the whiskey tango foxtrot moment may be enough to make you forget about the pedestal switches.

Just another disturbing thought.
A better explanation for the slight drift down while the electrical trim was disabled is that the pilots were attempting to move the wheel by rocking it but were not able to.
The deleted video clearly show how this might happen.
Just because the prelim report has no mention of activity during this time does not mean it was absent, we do not have a CVR transcript just a few excerpts.

The jackscrew cannot back drive the manual trim wheel for the same reason a screw jack holding a car will not slowly lower the car.
.
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Old 9th Apr 2019, 20:39
  #3759 (permalink)  
 
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Originally Posted by dlen View Post
Could they just have switched autopilot on?
That's what they tried to do, but it didn't engage.
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Old 9th Apr 2019, 21:16
  #3760 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
A better explanation for the slight drift down while the electrical trim was disabled is that the pilots were attempting to move the wheel by rocking it but were not able to.
The deleted video clearly show how this might happen.
Just because the prelim report has no mention of activity during this time does not mean it was absent, we do not have a CVR transcript just a few excerpts.

The jackscrew cannot back drive the manual trim wheel for the same reason a screw jack holding a car will not slowly lower the car.
.
That's called irreversibility. A screw Jack is irreversible. A ball recirculation screw Jack is reversible. The steering of a truck as an example of the latter.

And to the poster who suggested to stick to what happened and not to what may have happened, I think he is right and I would try to post only probable theories, not merely possible ones. The trim drift is bugging me inmensely but it is probably irrelevant and the rocking explanation also fits.
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