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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 24th Nov 2019, 00:40
  #4101 (permalink)  
 
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unacceptable stick force gradient at high alpha.
My mistake. Is that better?
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Old 24th Nov 2019, 00:41
  #4102 (permalink)  
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Can you elaborate on the difference between “unacceptable stick force” and “non linear stick force . . .
Takwis, just reading it with conventional usage, I'd jump to the conclusion that the former was unacceptably high, whereas the opposite is the truth under specific circumstances.
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Old 24th Nov 2019, 01:22
  #4103 (permalink)  
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Originally Posted by jimtx View Post
I envision two cases where the MAX's stick force non linearity might come into play. It might make a steep turn easier if you fly the attitude you want to put the performance where you want. It also might be a problem in a clean off autopilot windshear escape. Would have to see that in the airplane or simulator.
The trigger mechanisms for the original MCAS had speed (mach) and aoa conditions. Those equate to conditions such as a wind up turn, which is quite a way from a wind shear avoidance and/or recovery case.

As far as what is and what isn't acceptable in force and gradient, AC25.7D covers the requirements, examples and suggested methods to ascertain § 25.173 Static longitudinal stability. Para C is of interest but so is A, B and D. For the amended MCAS, § 25.145 Longitudinal control may have been part of the compliance issue that led to the unfortunate turn of events in the repurposing of the MCAS from a high speed/manoeuver SAS to a low speed SAS system.

The regs provide for an average stick force gradient to be met, but there is additionally a requirement on “local” reversals in the stick force versus airspeed relationship over the range of airspeeds tested. That is shown in the AC, in figure 7-1 (page 7-4) and 7-2 (page 7-5).

Most times planes behave like darts, but they can get awkward, like the F-101 pitch up issue, the overbank of a BUFF, (where wing sweep and high bank angles with slip results in a lateral directional divergence, think Czar 52 @ Fairchild, limitations in the Dash-1 etc. [Your T-38 got a mention in PIO history for the condition you note, NASA/WPAFB put out a great doc on the subject, and an instrumented 38 gave impressive data].











REF: AC25.7D

§ 25.145: page 5-15 to 5-19
§§ 25.173 and 25.175 pages 7-1 through 7-5



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Old 24th Nov 2019, 04:41
  #4104 (permalink)  
 
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Originally Posted by AerocatS2A View Post

If certification requirements were unnecessarily strict you could have a system that failed to meet certification requirements but were not actually a problem. That’s my reading of it anyway.
That is what I was trying to say. A little lightening of stick force/gradient in some rarely visited regimes might not be a problem for most pilots especially if it was noted in their training. But I'm in the dark as to the regime and the need to pencil whip it. It might be a more egregious and pervasive problem or not. But while it might be more egregious, currently I would fly the MAX without MCAS after seeing the Boeing YouTube video of the aircraft in regime they were trying to pencil whip. But they haven't put that out.

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Old 24th Nov 2019, 04:53
  #4105 (permalink)  
 
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Originally Posted by fdr View Post
The trigger mechanisms for the original MCAS had speed (mach) and aoa conditions. Those equate to conditions such as a wind up turn, which is quite a way from a wind shear avoidance and/or recovery case.

As far as what is and what isn't acceptable in force and gradient, AC25.7D covers the requirements, examples and suggested methods to ascertain § 25.173 Static longitudinal stability. Para C is of interest but so is A, B and D. For the amended MCAS, § 25.145 Longitudinal control may have been part of the compliance issue that led to the unfortunate turn of events in the repurposing of the MCAS from a high speed/manoeuver SAS to a low speed SAS system.

The regs provide for an average stick force gradient to be met, but there is additionally a requirement on “local” reversals in the stick force versus airspeed relationship over the range of airspeeds tested. That is shown in the AC, in figure 7-1 (page 7-4) and 7-2 (page 7-5).

Most times planes behave like darts, but they can get awkward, like the F-101 pitch up issue, the overbank of a BUFF, (where wing sweep and high bank angles with slip results in a lateral directional divergence, think Czar 52 @ Fairchild, limitations in the Dash-1 etc. [Your T-38 got a mention in PIO history for the condition you note, NASA/WPAFB put out a great doc on the subject, and an instrumented 38 gave impressive data].











REF: AC25.7D

§ 25.145: page 5-15 to 5-19
§§ 25.173 and 25.175 pages 7-1 through 7-5
Why is MCAS not needed if the autopilot is on. Does longitudinal stability not apply to the autopilot? Or does everyone know the MCAS protected regime is not on where the autopilot could be engaged?
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Old 24th Nov 2019, 05:55
  #4106 (permalink)  
 
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The autopilot is a machine that will apply the required input to achieve the desired result. It doesn’t have expectations or past experience to fool it into doing the wrong thing. It doesn’t care if the “stick forces” change as it applies more elevator, it just does what needs to be done.
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Old 24th Nov 2019, 08:02
  #4107 (permalink)  
 
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Originally Posted by AerocatS2A View Post
The autopilot is a machine that will apply the required input to achieve the desired result. It doesn’t have expectations or past experience to fool it into doing the wrong thing. It doesn’t care if the “stick forces” change as it applies more elevator, it just does what needs to be done.
This is only true for a system that is basically stable and where a linear input produces a proportional linear output. Controllers for systems which have less or no inherent stability are highly complex and their design needs to incorporate more factors and functions and limitations than a "simple" proportional controller. It took years for instance to develop the stability controller of a Harrier jump-jet in the hover state. In an aircraft, a crossfeed of the other axes and/or configurations may also be required (e.g up-stick when the plane rolls into a turn). An autopilot that needs less up-stick in a standard turn than in a tight one might not be easy to implement.

Last edited by clearedtocross; 24th Nov 2019 at 12:55. Reason: spelling
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Old 24th Nov 2019, 08:35
  #4108 (permalink)  
 
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Originally Posted by AerocatS2A View Post
The autopilot is a machine that will apply the required input to achieve the desired result. It doesn’t have expectations or past experience to fool it into doing the wrong thing. It doesn’t care if the “stick forces” change as it applies more elevator, it just does what needs to be done.
What happens when the AP disengages in a regime where hand 'stick forces' or gradient are not in the 'certifiable' range ?
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Old 24th Nov 2019, 09:04
  #4109 (permalink)  
 
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Aircraft flight behavior has to follow the FARs not the other way around.
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Old 24th Nov 2019, 10:22
  #4110 (permalink)  
 
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For all the analysis, the expert and intelligent opinion on this thread alone, it’s clear the 737 has been designed right upto, and perhaps slightly over what the overriding aircraft type is capable of accepting.

it isnt the only type that has gone up the edge of what is possible. But engineering fix over engineering fix in the name of progress and improvement is one thing, but time will tell whether the unfortunate incidents were sciences way of stepping in with the 737 and saying enough is enough, possibly decades after someone in Boeing should have made the ballsy decision.

Posters mention “murky” in relation to the desperate teams over in the US trying in as cost effective way as they are able to navigate the MAX back into service, One thing that is extremely clear to me my close family my friends, mostly non aviation linked - is that the companies who end up with the MAX or unfortunately already own the MAX and can’t cost effectively rid it from their fleets without loss of face or substantial amounts of loss of finance, then its going to be extremely difficult to encourage joe public onto these jets. I’m speaking outside our sphere of understanding about the finer points of PoF and complex jet control systems. Basic general public level of understanding.

A few “software updates” and supplemental sim training may satisfy the American regulators, and name changes may disguise the fact which aircraft the MAX actually is (I think it’s safe to say the media is in the starting blocks, ready for the exclusives with this particular part of the strategy) however, from a personal level, I know I shall check every airlines fleet list to ascertain who has these jets, and won’t be booking short haul flights with any company with these types. I am absolutely sure I won’t be alone with this strategy. There’s an old Latin saying somewhere that roughly translates to buyer beware that fits this scenario perfectly.

I strongly suspect this this may be the much bigger problem than the reported “murky” dealings trying to get this jet back into service.

It most likely will go back into service, possibly early next year. As stated, I won’t be getting on one, neither will many people I have spoken to recently either. I’ve personally not engaged in conversation other than to say make your own mind up.

How Boeing fix this is going to make interesting viewing. The cost to buy these jets will now be bargain basement prices, so there will be the bait for airlines to buy them. But at what risk. Losing customers to competitors without MAX in their fleets, may be the biggest economic decision many airlines make in their lifetime.
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Old 24th Nov 2019, 11:44
  #4111 (permalink)  
 
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MAX test flights Someone is doing an awful lot of checking out over the last two days.
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Old 24th Nov 2019, 11:53
  #4112 (permalink)  
 
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Originally Posted by ktcanuck View Post
MAX test flights Someone is doing an awful lot of checking out over the last two days.
Like some WWII fighters, they seem to prefer left turns...

Last edited by Fly Aiprt; 24th Nov 2019 at 12:41.
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Old 24th Nov 2019, 12:07
  #4113 (permalink)  
 
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Originally Posted by Three Lions View Post
For all the analysis, the expert and intelligent opinion on this thread alone, it’s clear the 737 has been designed right upto, and perhaps slightly over what the overriding aircraft type is capable of accepting.

it isnt the only type that has gone up the edge of what is possible. But engineering fix over engineering fix in the name of progress and improvement is one thing, but time will tell whether the unfortunate incidents were sciences way of stepping in with the 737 and saying enough is enough, possibly decades after someone in Boeing should have made the ballsy decision.

Posters mention “murky” in relation to the desperate teams over in the US trying in as cost effective way as they are able to navigate the MAX back into service, One thing that is extremely clear to me my close family my friends, mostly non aviation linked - is that the companies who end up with the MAX or unfortunately already own the MAX and can’t cost effectively rid it from their fleets without loss of face or substantial amounts of loss of finance, then its going to be extremely difficult to encourage joe public onto these jets. I’m speaking outside our sphere of understanding about the finer points of PoF and complex jet control systems. Basic general public level of understanding.

A few “software updates” and supplemental sim training may satisfy the American regulators, and name changes may disguise the fact which aircraft the MAX actually is (I think it’s safe to say the media is in the starting blocks, ready for the exclusives with this particular part of the strategy) however, from a personal level, I know I shall check every airlines fleet list to ascertain who has these jets, and won’t be booking short haul flights with any company with these types. I am absolutely sure I won’t be alone with this strategy. There’s an old Latin saying somewhere that roughly translates to buyer beware that fits this scenario perfectly.

I strongly suspect this this may be the much bigger problem than the reported “murky” dealings trying to get this jet back into service.

It most likely will go back into service, possibly early next year. As stated, I won’t be getting on one, neither will many people I have spoken to recently either. I’ve personally not engaged in conversation other than to say make your own mind up.

How Boeing fix this is going to make interesting viewing. The cost to buy these jets will now be bargain basement prices, so there will be the bait for airlines to buy them. But at what risk. Losing customers to competitors without MAX in their fleets, may be the biggest economic decision many airlines make in their lifetime.
I believe you overestimate the public concern. A few weeks after the reintroduction, no one will be checking aircraft types any more.
Of course, the whole saga will be replayed anyways in gory detail the next few times one crashes.
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Old 24th Nov 2019, 12:48
  #4114 (permalink)  
 
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The frequent fliers won't forget quickly. And it will be interesting to see how the American public react if the FAA so 'yes' and Europe, China, Australia and Canada say 'not proven'.
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Old 24th Nov 2019, 13:30
  #4115 (permalink)  
 
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Originally Posted by Fly Aiprt View Post
I'd say any test pilot will perform the tests he is asked to perform, but they don't have command of what becomes of their flight reports.
Gone are the Concorde days when test pilots were part of the development team...
Now be it in the US or not, the issue will be how will they ensure that the test aircraft is 'MCAS-free'.

On the other hand, what do those continuous test flights on Flight24 mean, whereas Boeing says their fixes have been ready for weeks now.
Or does the leaked email really mean that new issues are still appearing day after day ?
Actually 2 questions:

How will MCAS be disabled for the test, simplest would likely be a software load but that might raise questions of what else changed. Might also be possible by a HW/wire mod.
Either one of course puts thing squarely in "test pilot" territory.

Vefying MCAS free should be possible by taking it to conditions where it should kick in and verifying no trim action.

The continuing flight may be collecting more data for simulator or exposing additional pilots to real aircraft response.

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Old 24th Nov 2019, 13:44
  #4116 (permalink)  
 
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Three Lions: For all the analysis, the expert and intelligent opinion on this thread alone, it’s clear the 737 has been designed right upto, and perhaps slightly over what the overriding aircraft type is capable of accepting.
I had come to that same conclusion, as soon as I first heard the name of the new model, MAX. I do hope your family does not get on it, but I think the general public will gradually forget, and fill the plane to capacity most of the time.

So it goes.





Last edited by Takwis; 25th Nov 2019 at 01:24.
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Old 24th Nov 2019, 15:14
  #4117 (permalink)  
 
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why not fix a control force issue directly with a control force fix?

If Boeing's 737-MAX has a column control force issue, in a close to stall situation to abide by FAR's. Why did they did they take the scenic path to move the Stabilizer to fix it? Why didn't they just modify the Elevator Feel computer system to load up the control column correctly? That would be the most direct fix, and you wouldn't deal with a trim runaway issue if an AOA fails, just more force to deal with on the column suddenly. Pilots would notice the issue quickly, and instinctively fly the plane. You can control the force applied so as to not over power the pilots ability to fly the plane. You wouldn't have the danger of the plane getting far out of trim. That seems to me, to be the simple, direct, and safer fix. What am I missing?
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Old 24th Nov 2019, 15:21
  #4118 (permalink)  
 
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Originally Posted by GroundedDinosaur View Post
If Boeing's 737-MAX has a column control force issue, in a close to stall situation to abide by FAR's. Why did they did they take the scenic path to move the Stabilizer to fix it? Why didn't they just modify the Elevator Feel computer system to load up the control column correctly? That would be the most direct fix, and you wouldn't deal with a trim runaway issue if an AOA fails, just more force to deal with on the column suddenly. Pilots would notice the issue quickly, and instinctively fly the plane. You can control the force applied so as to not over power the pilots ability to fly the plane. You wouldn't have the danger of the plane getting far out of trim. That seems to me, to be the simple, direct, and safer fix. What am I missing?
AFAIK the B737 elevator feel computer is a hydraulic mechanism, not an electronic system. Reverse engineering it would create many more certification obstacles than it would solve. It would also be like adding FBW to an aircraft never designed for it. Oh wait, haven't we been down this road before...

Edit: See: B737 Elevator Feel Shift Module
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Old 24th Nov 2019, 15:43
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Originally Posted by GordonR_Cape View Post
AFAIK the B737 elevator feel computer is a hydraulic mechanism, not an electronic system. Reverse engineering it would create many more certification obstacles than it would solve. It would also be like adding FBW to an aircraft never designed for it. Oh wait, haven't we been down this road before...
One can imagine they found it easier to just add some code to the STS, no hardware etc.
And yet, moving the stabilizer just to correct a control column feel issue....
When one thinks the stick-nudger solution had already been implemented in some airplanes...

Last edited by Fly Aiprt; 24th Nov 2019 at 16:15.
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Old 24th Nov 2019, 15:56
  #4120 (permalink)  
 
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RUNAWAY STAB.....

I never got more glass and push-button qualified than my last nine years on the 744, thus well out of date on any later stuff. However I did begin my 707 school with Boeing at Seattle.
A 'Runaway Stabiliser' had been briefly an issue with earlier versions, and it was always a priority memory item emergency drill practised on most recurrent training exercises. The first
action made a lot of sense. It was for each pilot to lean over and physically grab the opposite trim wheel bloody quick. It certainly hurt a bit, but life might depend on this.
Different aircraft, but is the strength required to halt the rotation not possible with the Max? Can't quite recall the 707 cut-off switches, but the follow up drill was Stab,
Mach Trim, and Autopilot circuit breakers. Looks like the 737 Max has two Stab Trim cut off switches; 'Main Electric' and 'Autopilot'. Also would be surprised if sudden
strong back pressure on the yoke wouldn't auto-brake in the forward runaway situation. That Idea was to allow time to get the handles out for manual re-wind prior to
risk releasing the yoke again. There must be 737Max experienced pilots who can explain to those of us out of the loop why grabbing the wheel is not an obvious option?
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