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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Old 26th Jun 2019, 06:22
  #661 (permalink)  
 
Join Date: Feb 2015
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Originally Posted by Icarus2001 View Post
Not if the pilot uses their thumb on the trim switch on the control column, that will stop MCAS. Or if a pilot holds the centre console trim wheel to stop it moving.
Oh good. There’s nothing to worry about then. The Max should be back on line pretty soon.
bill fly is offline  
Old 26th Jun 2019, 06:53
  #662 (permalink)  
fdr
 
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Originally Posted by Smythe View Post

1. I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel.

2. If, at high angle of attack, or low speeds, if, due to certain areas of the wing the airflow is going supersonic, the resulting lack of laminar airflow over the wing (as stated by the Boeing wind tunnel tests) induces a stall...

3. The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for.

4. Look at what they tried to do, adding vortex tabs, changing the wing design....

5. Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall?
The differential pressure on the yoke is a RESULT of the stall.

6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.

7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?

8. In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”

9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA. “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).


10. In reality, MCAS is anti-stall.
1. "I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel."

stick force is a consequence of aerodynamics, control system architecture and modifiers.

2. (A) "...going supersonic..."

High AOA is inconsistent with high mach numbers in general, wings tend to fall off Par 25 planes when pulling high AOA at high mach, available AOA is constrained by buffet in such cases, so the terminology used is vague... At low speeds, there is no transonic flow on a BAC447-450 type section at any AOA. The BAC airfoil section may have gone supersonic in the Silk air bingle, and possibly Adam Air splash, but otherwise it is rather unlikely to get to a point where the section is supersonic, e.g., has an oblique shock formation. It will usually have a normal shock at around 0.5c-0.6c in cruise, at say M0.78 and above, at 2.3 AOA.

(B) "...lack of laminar flow...";

irrespective of what is being smoked, and how much was spent on the design, there is no laminar flow worth noting on any RPT jet transport. If you want laminar flow, go and look at a standard sail plane. the slat TE destroys laminar flow, as does the first rivet head, screw head etc that exists on the slat. For the first 1% of the chord, which more or less is in the radius of the LE, around the Kutta point, there may be laminar flow dependent on when the plane was last washed. FYI, the slat TE eats up the boundary layer, it is a discontinuity in the surface, and causes separation in the presence of a shear, and that always gives an initial span-wise vortex structure which is highly unstable as is any flow behind an aft facing step... That vortex structure starts to shed with a Strouhal number that is identifiable as a harmonic of the how frequency vibration that arises from the instability of the normal shock and the associated SBLI foot, which is observable oscillating on the wing in steady flight. Look out the window with the sun aligned down the mid chord span and you will have a Schlieren iamge of the shadow of the densty change in the foot of the shock. In essence, at any time, your Boeing, or Bus doesn't have laminar flow anywhere except in the idealised models of the airfoil in simplified CFD modelling. Sorry. That is not to say it can't be improved...

(C ) "...induces a stall..."

not the way flow works. Sorry. A turbulent BL has one benefit over laminar and that is it can cope with adverse pressure gradient perturbations before becoming messy. Flow behind a shock is separated near the surface, but that is not a stall as such, which is defined in the regs... as a number of specific conditions that occur. separation may be a pain, particularly geographic ones, but it is a normal part of life. Stalls are stalls... per the regs. Taking your comment to an extreme, pulling say 10 AOA at M0.82 will still not end up in a stall, it will take the wings right off the aircraft, but before you get to that point, flow separation will have resulted in severe buffet, and reducing Cl/AOA, which put you back toward the beginning...

3. "...The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for..." what is your evidence for that statement? If that was so, then the aircraft would have rather high V speeds, as they are a function of Vs.

There is an interaction between the nacelle and the wing, and was ever thus. If you look on the side of a B737 Classic, NG A320, 330, B777 B787 etc etc etc, you will find, there is a strake on the side of the nacelle around the location of 10 to 2 o'clock, more often on the inner side of the nacelle, but it can be on both sides. The strakes are agnostic, they are on CF6, RR, GE, CFMI engines etc, and they are there as the standard, pre Max and the GTF designs did indeed have an interaction between the nacelle and the wing abaft of the nacelle that suppressed Cl/AOA at modest to high AOAs, and that caused performance decrements. The strake develops a very powerful vortex filament that passes over the top of the LE HLD's and controls the extent of the interaction. It is a thing of beauty, and pretty to watch on a high humidity day. The Vortex itself is not doing the work, it stops the flow adjacent to the filament from being discombobulated.

The Max issue is entirely different, in fact, it goes in the opposite direction, and it increases lift of the section effectively, or at least in the aggregate results in an reduction in the component Cm of the span-wise region behind the nacelle, which is exhibited as a reduction in stick force per g... If you have a back of an envelope you will see that in order to achieve a reduction in Cm, for the location on the planform there is a high likelyhood that the design actually increased local CL, at the same time as moving the Cp forward on that section, and that gives the Cm outcome. Now, Bill B could cure the deal by reducing or removing the strake that is on the max engine, but that comes with a performance penalty, the Max had better performance than it would have otherwise due to the nacelle design, it just came with a side effect that the regs would need a SAS input to meet longitudinal control reqts. Had that been done well, then the Max would have been acknowledged as a great design. Heck, even now, it can be, if Boeing corporate management grows a spine and get their house in order on ethical matters. The design of the MCAS and its changes did not happen in a vacuum, they occurred as a result of the attitude of the beancounters and the ethics that flow down the sewer that is the board room to the poor sod at the coal face.

4. Look at what they tried to do, adding vortex tabs, changing the wing design....

That is what happens with almost all designs, including Busses etc. The dog tooth on the Bus arises from a surprise in testing... VG's are a good tool for curing issues of separation and shock issues, they don't cook means well, but they have their uses. I would be happier if they did do more work on the wing, there is a lot of room for improvement on all of these designs which are the industrial engineers mass produced product rather than the ASW-21 or other embodiment of elegant design.

5. "...Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall? The differential pressure on the yoke is a RESULT of the stall..."

There are a number of manners that the design could be tweaked in testing to attempt to achieve compliance with the stick force gradient requirements. As commented above, removing the strake would get rid of the issue, but comes at a cost, whereas MCAS had no cost had it been implemented competently. There are designs out there that have specific application of VGs to meet the same requirement. Almost every aircraft on the ramp has flow modifiers on them, and they are all specific in their application, what the defect was that was underlying their implementation. Each conventional VG has a drag count penalty, according to NASA research of around 0.0002Cd, so they are used sparingly and only when the gain from their use offsets the Cd increase from their installation. Not sure what a differential pressure on a yoke refers to, I only speak english, but if you are referring to the stick force gradient non linearity, that is not caused by stall. Consider that any premature stall around the nacelle will actually improve handling qualities of any aircraft; it will ensure that the lateral control requirements are met, that the aircraft will pitch down, that buffet on the airframe from impingement of wake on the stab and elevators is more likely to be encountered. These are good things. The GTF nacelle doesn't cause a premature stall, it does the opposite... Now if the wing tips stall, then you get great entertainment, and that is not the problem, nor would it arise from the nacelle.... Think of why simple wings have wash out, and why dog tooths, VG's vortilons etc are used on swept wing aircraft. I don't think I agree with your paragraph on that matter...

6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.

If the aircraft was unstable at any point, which is pretty hard to see how that would be achievable, given the margins that exist between the normal aft envelope and the neutral point, then in any case, no TP would have signed off on the acceptability of the aircraft. It is a very straight forward matter to ascertain if the design is or was unstable, and for the record the data that has been published already is sufficient to show that the aircraft was statically and dynamically stable. Long ago I looked at an RPT aircraft that was on the limit of stability in flight, and it is not hard to detect in the data. THE MAX IS NOT UNSTABLE. PERIOD. Don't take my word for it, look at the public domain data on the control inputs. It had a trim issue, and that is all.

7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?

Again, as the aircraft IS NOT UNSTABLE, an autopilot doesnt need MCAS to meet any stick force per g compliance matters. That is the first big clue that the issue is and has always been about the force gradient compliance. MCAS would be redundant, a double negative, nonsense for an autopilot engaged condition.


8. "....In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”

Autopilots are born of man (or woman etc... )per the bard and fail. The reported issue was a pitch down excursion which is an event that is required to be covered in certification, being an aspect that is considered in the minimum engagement and disengagement heights for autopilots. Had the MCAS not resulted in public misinformation and hysteria, then this particular matter would have been kept in its proper place, that being an APFD anomaly, with no association to the MCAS issue.

9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA. “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).


The crews getting into the Max deserved more than an Ipad briefing. However, the comment on the APFD anomaly has nothing to do with MCAS.

10. In reality, MCAS is anti-stall.

Nope, not even close. Refer preceding.



cheers,
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Old 26th Jun 2019, 07:10
  #663 (permalink)  
 
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Originally Posted by fdr View Post
1. "I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel."

stick force is a consequence of aerodynamics, control system architecture and modifiers.

2. (A) "...going supersonic..."

High AOA is inconsistent with high mach numbers in general, wings tend to fall off Par 25 planes when pulling high AOA at high mach, available AOA is constrained by buffet in such cases, so the terminology used is vague... At low speeds, there is no transonic flow on a BAC447-450 type section at any AOA. The BAC airfoil section may have gone supersonic in the Silk air bingle, and possibly Adam Air splash, but otherwise it is rather unlikely to get to a point where the section is supersonic, e.g., has an oblique shock formation. It will usually have a normal shock at around 0.5c-0.6c in cruise, at say M0.78 and above, at 2.3 AOA.

(B) "...lack of laminar flow...";

irrespective of what is being smoked, and how much was spent on the design, there is no laminar flow worth noting on any RPT jet transport. If you want laminar flow, go and look at a standard sail plane. the slat TE destroys laminar flow, as does the first rivet head, screw head etc that exists on the slat. For the first 1% of the chord, which more or less is in the radius of the LE, around the Kutta point, there may be laminar flow dependent on when the plane was last washed. FYI, the slat TE eats up the boundary layer, it is a discontinuity in the surface, and causes separation in the presence of a shear, and that always gives an initial span-wise vortex structure which is highly unstable as is any flow behind an aft facing step... That vortex structure starts to shed with a Strouhal number that is identifiable as a harmonic of the how frequency vibration that arises from the instability of the normal shock and the associated SBLI foot, which is observable oscillating on the wing in steady flight. Look out the window with the sun aligned down the mid chord span and you will have a Schlieren iamge of the shadow of the densty change in the foot of the shock. In essence, at any time, your Boeing, or Bus doesn't have laminar flow anywhere except in the idealised models of the airfoil in simplified CFD modelling. Sorry. That is not to say it can't be improved...

(C ) "...induces a stall..."

not the way flow works. Sorry. A turbulent BL has one benefit over laminar and that is it can cope with adverse pressure gradient perturbations before becoming messy. Flow behind a shock is separated near the surface, but that is not a stall as such, which is defined in the regs... as a number of specific conditions that occur. separation may be a pain, particularly geographic ones, but it is a normal part of life. Stalls are stalls... per the regs. Taking your comment to an extreme, pulling say 10 AOA at M0.82 will still not end up in a stall, it will take the wings right off the aircraft, but before you get to that point, flow separation will have resulted in severe buffet, and reducing Cl/AOA, which put you back toward the beginning...

3. "...The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for..." what is your evidence for that statement? If that was so, then the aircraft would have rather high V speeds, as they are a function of Vs.

There is an interaction between the nacelle and the wing, and was ever thus. If you look on the side of a B737 Classic, NG A320, 330, B777 B787 etc etc etc, you will find, there is a strake on the side of the nacelle around the location of 10 to 2 o'clock, more often on the inner side of the nacelle, but it can be on both sides. The strakes are agnostic, they are on CF6, RR, GE, CFMI engines etc, and they are there as the standard, pre Max and the GTF designs did indeed have an interaction between the nacelle and the wing abaft of the nacelle that suppressed Cl/AOA at modest to high AOAs, and that caused performance decrements. The strake develops a very powerful vortex filament that passes over the top of the LE HLD's and controls the extent of the interaction. It is a thing of beauty, and pretty to watch on a high humidity day. The Vortex itself is not doing the work, it stops the flow adjacent to the filament from being discombobulated.

The Max issue is entirely different, in fact, it goes in the opposite direction, and it increases lift of the section effectively, or at least in the aggregate results in an reduction in the component Cm of the span-wise region behind the nacelle, which is exhibited as a reduction in stick force per g... If you have a back of an envelope you will see that in order to achieve a reduction in Cm, for the location on the planform there is a high likelyhood that the design actually increased local CL, at the same time as moving the Cp forward on that section, and that gives the Cm outcome. Now, Bill B could cure the deal by reducing or removing the strake that is on the max engine, but that comes with a performance penalty, the Max had better performance than it would have otherwise due to the nacelle design, it just came with a side effect that the regs would need a SAS input to meet longitudinal control reqts. Had that been done well, then the Max would have been acknowledged as a great design. Heck, even now, it can be, if Boeing corporate management grows a spine and get their house in order on ethical matters. The design of the MCAS and its changes did not happen in a vacuum, they occurred as a result of the attitude of the beancounters and the ethics that flow down the sewer that is the board room to the poor sod at the coal face.

4. Look at what they tried to do, adding vortex tabs, changing the wing design....

That is what happens with almost all designs, including Busses etc. The dog tooth on the Bus arises from a surprise in testing... VG's are a good tool for curing issues of separation and shock issues, they don't cook means well, but they have their uses. I would be happier if they did do more work on the wing, there is a lot of room for improvement on all of these designs which are the industrial engineers mass produced product rather than the ASW-21 or other embodiment of elegant design.

5. "...Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall? The differential pressure on the yoke is a RESULT of the stall..."

There are a number of manners that the design could be tweaked in testing to attempt to achieve compliance with the stick force gradient requirements. As commented above, removing the strake would get rid of the issue, but comes at a cost, whereas MCAS had no cost had it been implemented competently. There are designs out there that have specific application of VGs to meet the same requirement. Almost every aircraft on the ramp has flow modifiers on them, and they are all specific in their application, what the defect was that was underlying their implementation. Each conventional VG has a drag count penalty, according to NASA research of around 0.0002Cd, so they are used sparingly and only when the gain from their use offsets the Cd increase from their installation. Not sure what a differential pressure on a yoke refers to, I only speak english, but if you are referring to the stick force gradient non linearity, that is not caused by stall. Consider that any premature stall around the nacelle will actually improve handling qualities of any aircraft; it will ensure that the lateral control requirements are met, that the aircraft will pitch down, that buffet on the airframe from impingement of wake on the stab and elevators is more likely to be encountered. These are good things. The GTF nacelle doesn't cause a premature stall, it does the opposite... Now if the wing tips stall, then you get great entertainment, and that is not the problem, nor would it arise from the nacelle.... Think of why simple wings have wash out, and why dog tooths, VG's vortilons etc are used on swept wing aircraft. I don't think I agree with your paragraph on that matter...

6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.

If the aircraft was unstable at any point, which is pretty hard to see how that would be achievable, given the margins that exist between the normal aft envelope and the neutral point, then in any case, no TP would have signed off on the acceptability of the aircraft. It is a very straight forward matter to ascertain if the design is or was unstable, and for the record the data that has been published already is sufficient to show that the aircraft was statically and dynamically stable. Long ago I looked at an RPT aircraft that was on the limit of stability in flight, and it is not hard to detect in the data. THE MAX IS NOT UNSTABLE. PERIOD. Don't take my word for it, look at the public domain data on the control inputs. It had a trim issue, and that is all.

7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?

Again, as the aircraft IS NOT UNSTABLE, an autopilot doesnt need MCAS to meet any stick force per g compliance matters. That is the first big clue that the issue is and has always been about the force gradient compliance. MCAS would be redundant, a double negative, nonsense for an autopilot engaged condition.


8. "....In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”

Autopilots are born of man (or woman etc... )per the bard and fail. The reported issue was a pitch down excursion which is an event that is required to be covered in certification, being an aspect that is considered in the minimum engagement and disengagement heights for autopilots. Had the MCAS not resulted in public misinformation and hysteria, then this particular matter would have been kept in its proper place, that being an APFD anomaly, with no association to the MCAS issue.

9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA. “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).


The crews getting into the Max deserved more than an Ipad briefing. However, the comment on the APFD anomaly has nothing to do with MCAS.

10. In reality, MCAS is anti-stall.

Nope, not even close. Refer preceding.



cheers,
Great post!
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Old 26th Jun 2019, 07:40
  #664 (permalink)  
 
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Originally Posted by zzuf View Post
Great post!
Yes but I am afraid it will be wasted on the intended recipient
bill fly is offline  
Old 26th Jun 2019, 07:41
  #665 (permalink)  
 
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fdr,
Thank you for an informative and educational post # 663, representing the very best professional interchange, with benefit for individuals, forum, and industry.
I have learnt from the technicalities, similarly from the style and manner of presentation; a personal credit for you and for everyone in enhancing the credibility of this forum.
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Old 26th Jun 2019, 09:32
  #666 (permalink)  
 
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They're getting desperate for storage space now. I take it the pink spreader pads are because the carpark isn't sufficently strong enough to bear the weight.




https://www.independent.co.uk/travel...-a8973726.html
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Old 26th Jun 2019, 12:04
  #667 (permalink)  
 
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As a matter of fact, there are two rows of three aircraft in that car park. Three for TUI and three for Icelandair.
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Old 26th Jun 2019, 12:35
  #668 (permalink)  
 
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Great, detailed post by fdr there. Hopefully, we can put to bed all the claims that the 737 is inherently "unstable."

On the issue of MCAS activation during TCAS, GPWS, and windshear events, a few more thoughts.

First, all of these maneuvers are hand-flown. Not only are they hand-flown, but they are dynamic situations in which there are significant changes in thrust and airspeed. As such, the pilot should be trimming constantly through the maneuver using the yoke trim switch. As we know, the yoke trim switch will always override MCAS. If the pilot doesn't know how to properly fly and trim, then that is a training issue, not a design issue.

Second, properly flown a TCAS maneuver should never enter stick shaker region. Windshear and GPWS maneuvers may be in and out of the stick shaker, but they should not be flown with a continuous stick shaker because you certainly don't want to stall the aircraft during the recovery. MCAS has some threshold values in terms of both AOA and time to trigger activation, so it is unclear whether MCAS will even activate if you are only triggering the stick shaker intermittently.

Third, I think there is too much focus on what happens when MCAS activates at a time when it is not needed as it did with the accident aircraft. Outside of the high AOA regions, the MCAS input will increase control force gradient above what would be normally expected/wanted. Approaching the high AOA region for which it was designed, the MCAS input will attempt to maintain the control force gradient as it offsets the extra lift generated by the engine nacelles. In other words, the controls should feel something close to normal when MCAS works as designed.

Finally, the aircraft is not flown with trim, nor does MCAS override the pilot's ability to trim. In the case of any hand-flown maneuver the pilot should use the primary flight controls to set the aircraft attitude, set the thrust to the desired power setting (full thrust in case of windshear or GPWS warning) and then trim accordingly to relieve control pressures. I say again, the pilot should establish the desired attitude and power setting and then trim accordingly! If the new and improved MCAS was making a trim input that the pilot did not want, that pilot could easily override it. Any consideration into aircraft design will assume that the pilot will employ standard hand-flying procedures.
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Old 26th Jun 2019, 12:49
  #669 (permalink)  
 
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Originally Posted by Icarus2001 View Post
You need to understand that Airbus, Boeing and Embraer all use FBW technology that would prevent excessive pitch attitudes in an EGPWS recovery.
Slight correction here. I can't speak for newer Boeing's like the 777 & 787, but the 737MAX is not really a FBW aircraft, and you can demand more pitch than required in these maneuvers. There is some automated control logic inserted into the spoiler and rudder controls, but not for the elevator or ailerons.
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Old 26th Jun 2019, 13:49
  #670 (permalink)  
 
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If you read my post again you will see that I mention that the 737 is not FBW. We are in agreement.
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Old 26th Jun 2019, 14:04
  #671 (permalink)  
 
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Yoko1:

As we know, the yoke trim switch will always override MCAS.
That is a fairly sweeping statement that may well be true, for all I know. However, I'm still wondering about the short NU trim commands visible on the FDR print-outs from both accident aircraft. They appear so similar that I wonder if there is some feature or limitation of the manual electric trim system that has yet to be revealed. Hopefully, the accident reports may provide some explanation.
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Old 26th Jun 2019, 14:52
  #672 (permalink)  
 
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Originally Posted by Avionista View Post

However, I'm still wondering about the short NU trim commands visible on the FDR print-outs from both accident aircraft.
A Bjorn’s Corner blog provided a pretty detailed analysis that would explain those “blips.” There is no evidence that the pilot’s yoke trim switch failed to work any time that it was used.
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Old 26th Jun 2019, 15:25
  #673 (permalink)  
 
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Originally Posted by DaveReidUK View Post
In the sense that the FAA will almost certainly feel the need to get EASA on side before lifting the grounding, then yes, it's down to politics.
With the FAA under fire for seeing it’s role as lobbying on behalf of Boeing as much as being an impartial regulator, it needs to stay well away from EASA and let Boeing deal with certification outside the US itself.
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Old 26th Jun 2019, 15:34
  #674 (permalink)  
 
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blips

"There is no evidence that the pilot’s yoke trim switch failed to work any time that it was used."

Oh contraire, there is more than sufficient evidence to demonstrate that, on the balance of probability, the trim system was unable to overcome (in the ANU moment) the AND aerodynamic forces present at that time.

You are more than welcome to dispute this conclusion of course, but you must be prepared to challenge numerous experts from multiple CAA's around the world in order to do so in any meaningful way. Until that occurs, the aircraft is grounded indefinitely.
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Old 26th Jun 2019, 15:49
  #675 (permalink)  
 
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Originally Posted by Just the fax maam View Post

Oh contraire, there is more than sufficient evidence to demonstrate that, on the balance of probability, the trim system was unable to overcome (in the ANU moment) the AND aerodynamic forces present at that time..
I have no idea what you mean by “balance of probability.” The pilot’s trim worked or it didn’t. There is no “probable” about it.

To be clear, we are not talking about the manual trim here. We are referring to the main electric trim controlled by the pilot’s yoke switch.

Please show me any “sufficient evidence” from one of these experts that supports the contention that the main electric trim did not work when used or that it could not override MCAS.
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Old 26th Jun 2019, 15:49
  #676 (permalink)  
 
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Originally Posted by Speed of Sound View Post
With the FAA under fire for seeing it’s role as lobbying on behalf of Boeing as much as being an impartial regulator, it needs to stay well away from EASA and let Boeing deal with certification outside the US itself.
Well that's one way of looking at it.

Let's wait and see. I'll be amazed if, in due course, the grounding isn't lifted simultaneously by the FAA and EASA on an agreed date.

If not, which do you think will be first ?

Originally Posted by Just the fax maam View Post
"There is no evidence that the pilot’s yoke trim switch failed to work any time that it was used."

Oh contraire, there is more than sufficient evidence to demonstrate that, on the balance of probability, the trim system was unable to overcome (in the ANU moment) the AND aerodynamic forces present at that time.
Could you quote some of this evidence, please ?

All I've seen are indications that manual (wheel) trim can't provide enough mechanical advantage to move the stab back from extreme AND, no suggestion that electric trim (if enabled) couldn't.

Oh and it's "au contraire", by the way.
DaveReidUK is offline  
Old 26th Jun 2019, 16:36
  #677 (permalink)  
 
Join Date: Mar 2006
Location: England
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yoko1, # 676

The FDR printout suggests that the electric trim was operable at that time, however there is no evidence that the tail moved - possibly too short a time scale. Thus probability enters the debate; more accurately we don’t know.

Reconsider the EASA question about lack of electric trim throughout the flight envelope (NG). The Boeing response indicated that this was a design intention to minimise the risk of trim runaway, thus use manual wheel. This has not been confirmed or refuted; again we don’t know.

Thus either the elect switch indication on the FDR remains operable, but by design not the trim motor; or whether the inhibition applies in both tail directions. Alternatively if the trim was operable, then the motor was unable to move the tail due to an accumulation of adverse forces, not identified in certification or in the EASA query due to extreme tail position, e.g. large tail nose-down plus elevator nose-up, which as referenced in Borns blog as hydraulic jack stall; and an indication of the magnitude of forces involved.

A further speculative view is of an adverse interaction between manual trim wheel from one pilot and elect trim from the other; what is the priority re clutching mechanism; does manual input de-clutch the electric motor ?


PEI_3721 is offline  
Old 26th Jun 2019, 16:48
  #678 (permalink)  
 
Join Date: Feb 2019
Location: All-At-Sea
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on balance

Originally Posted by yoko1 View Post
I have no idea what you mean by “balance of probability.”

This is likely one source of misunderstanding here, as there are many posters here unable to fully comprehend why the grounding is ongoing or what the long delay is a result of.

In the absence of conclusive evidence that demonstrates with absolute certainty that the main electric trim system was 100% operative throughout all phases of both fatal accidents [for the record, there is none], investigators are left drawing probable conclusions based on the available evidence:

1> It is not very plausible that two separate crews comprised four trained, experienced, pilots all elected not to attempt to trim ANU (more than a sub-second blip) when presented with life-threatening ongoing opposing trim in clear conditions in sight of the ground, whilst simultaneously commanding nose up at forces likely not previously experienced.
2> The "blips" are present in the last seconds of both fatal accidents.
3> The "blips" are consistent with an initial ANU trim command, but each has no resulting actual ANU trim of any significance whatsoever, certainly nothing like that required to improve the situation they were presented with. See 1> above.
4> The "blips" are entirely consistent with a shorted and/or overpowered motor.
5> The "blips" look exactly as we would expect a command to an overpowered motor to record; a power spike then null.
6> The manual trim was also overpowered under the aerodynamic loads experienced at that time.
7> "Trim with me" is not the announcement one would expect to be made by a PF who's trim is functioning correctly.
8> The actions of the MS crew, including attempting to briefly trim AND in such a situation, are consistent with a crew dealing with an inoperable main electric trim in the ANU moment.
9> Both crews were aware that an 'auto' trim was trimming against them but were ultimately unable to prevent their aircraft from trimming them into the ground to their certain death and the demise of all onboard.
10> XAA's all around the world have grounded all aircraft of this type until further notice.

I could go on, however... given just the above, and all the other facts as known at this time, we can safely conclude that ON THE BALANCE OF PROBABILITY the main electric trim was totally inoperable in the ANU moment during the last seconds or minutes of the fatal accidents.

That conclusion inexorably leads to some uncomfortable truths, and it is understandable that many here do not wish to go there. But, just as the Soviets learnt the hard way at Chernobyl, in the end, only by shining a bright light on all of the flaws can real progress be made towards rectification.

This aircraft remains grounded worldwide until that happens, despite protestations otherwise, whether on this forum or elsewhere.
Just the fax maam is offline  
Old 26th Jun 2019, 18:18
  #679 (permalink)  
 
Join Date: Jun 2009
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Salute!

All the FDR traces we have seen show the control col thumb trim switches worked up until the lasts seconds of flight or when the crew turned off all the trim power using the new procedure. So the tiny trim wheel is the last resort. I love it.

I can't rely on 15,000 hours of time in the plane because a new "feature" has been implemented and I do not know about it or the conditions that activate the feature. Ohhh baby....

Oh yeah, the decades-old switches on the col that could override the AP or STS don't work if MCAS is doing its dirty work. Gotta love it, huh?

There's folks here that claim :MCAS won't work if "flaps are down", "MCAS won't work if AP is engaged:, amd so on. Fer chrissakes, if the damned system was supposed to work as intended ( not finally designed and implemented), the stick shaker onset would have been notice to the crews that MCAS would also be in effect. MCAS? What's that, Gums? Oh yeah, forgot to tell you......... GASP!

The accidents happened when MCAS was not supposed to be a player of the plane when approaching a high AoA but below the stall AoA stick shaker threshhold. Right? So with a maxed out AoA doofer we get airspeed warning lights, stick shaker, and the unknown MCAS thing commanding nose down with out me doing a thing except analyzing the warning llghts and stick shaker and,,,,,,,,

Gums sends...

Last edited by gums; 26th Jun 2019 at 18:30.
gums is offline  
Old 26th Jun 2019, 18:31
  #680 (permalink)  
 
Join Date: Feb 2015
Location: The woods
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And then there's folks here that claim :MCAS won't work if flaps are down", "MCAS won't work if AP is engaged:, amd so on.

Well that’s what we have all been told from the first thread here - Haven’t heard it disputed properly yet.
Or do you mean it could be triggered by a fault even when the conditions above are met?

Greetings
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