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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 9th Apr 2019, 14:31
  #3741 (permalink)  
 
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Originally Posted by bsieker View Post
Not all opposing electric trim is stopped. Only main electric (control wheel switch) and autopilot trim. STS is not the autopilot.
[...]
And if you insist that something "broke" the column cutout switch design philosophy, STS did it first. It also trims against stick deflection, explicitly to "increase control column forces".

Bernd
Sorry, with the greatest respect, I think you are mistaken.

STS is not the autopilot, and it only operates with autopilot off, but it operates using the autopilot trim signal. It is therefore cutout by (a) the "autopilot" console cutout switch on NG, and (b) the column cutout switch (if in opposing direction). I have multiple references all clearly stating this and functional diagrams showing it, but I'll quote just one, from the NG AMM 27-41-00:

The column cutout switches stop the stabilizer trim actuator when the pilot moves the control column in a direction opposite to the trim direction.
Stop the actuator, for opposing trim, period, regardless of signal source.

But not on the MAX - the cutout is bypassed when MCAS enabled (and that new bypass wouldn't be needed if STS already bypassed it).
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Old 9th Apr 2019, 14:39
  #3742 (permalink)  
 
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Avionista

It should be possible to calculate the turning moment about the HS pivot point for a range of ND trim units at various aircraft speeds and elevator positions. This should determine the range of forces which has to be resisted by the HS trim jackscrew. A ground test rig capable of applying this range of forces to a jackscrew could then be used to check the ability of the manual trim (both electric and crank wheel) to apply NU trim under all circumstances.
I sincerely hope that Boeing engineers are doing this as we speak, otherwise they won't have a leg to stand on if this ever comes to trial. Saying that it was designed and calculated to do X and Y back in 1968, is not going to impress a jury...
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Old 9th Apr 2019, 14:40
  #3743 (permalink)  
 
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Originally Posted by weemonkey View Post
" the whole raison d'être 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.
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.


25.173 Static longitudinal stability.

Under the conditions specified in 25.175, the characteristics of the elevator control forces (including friction) must be as follows:

(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC, whichever is appropriate, or lower than the minimum speed for steady unstalled flight.

(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in § 25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in § 25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section.

(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.

(d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.
*

Last edited by Lost in Saigon; 9th Apr 2019 at 14:52.
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Old 9th Apr 2019, 14:53
  #3744 (permalink)  
 
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Originally Posted by Lost in Saigon View Post
Just about everything you have read in the media is wrong. MCAS is not stall protection. 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 stick force certification standards.
I fear you are fighting a losing battle in your campaign.

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.

Start talking about FARs and stick force gradient certification standards and their eyes will start to glaze over ...
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Old 9th Apr 2019, 14:55
  #3745 (permalink)  
 
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The easiest fix was to automatically apply a little nose down trim at high angles of attack.
But a little [0.6] became a lot [2.5], semi-persistently/repetitively?
Am I reading that correctly?
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Old 9th Apr 2019, 15:01
  #3746 (permalink)  
 
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Originally Posted by DaveReidUK View Post
I fear you are fighting a losing battle in your campaign.

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.

Start talking about FARs and stick force gradient certification standards and their eyes will start to glaze over ...
Heard BBC Radio4 news the other day saying that MCAS was a system to prevent the ENGINES stalling. I wrote them a stiff email...!
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Old 9th Apr 2019, 15:06
  #3747 (permalink)  
 
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Originally Posted by MPN11 View Post
But a little [0.6] became a lot [2.5], semi-persistently/repetitively?
Am I reading that correctly?
Yes, and Boeing has admitted that it was a little too much: “Upon reflection on what has occurred, it appeared the system could present a high-workload environment—and that’s not our intention."

It appears as though they have now remedied the situation.
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Old 9th Apr 2019, 15:06
  #3748 (permalink)  

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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, 15:18
  #3749 (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, 15:45
  #3750 (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, 16:33
  #3751 (permalink)  
 
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Could they just have switched autopilot on?
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Old 9th Apr 2019, 16:34
  #3752 (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, 16:38
  #3753 (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, 16:50
  #3754 (permalink)  
 
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I’ll
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 it’s 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, 16:51
  #3755 (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, 16:57
  #3756 (permalink)  
 
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Originally Posted by weemonkey View Post
" the whole raison d'être 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, 17:01
  #3757 (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 it’s 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, 17:01
  #3758 (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, 18:15
  #3759 (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, 18:20
  #3760 (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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