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

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

Old 17th Dec 2019, 11:52
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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures Mk II

It is not clear (to me at least) how exactly the modified FC software (including tamed MCAS) can allow re-certification.

1) The original certification was heavily based on "grandfather rights" (an arrangement under which an aircraft can be approved per a variation of the original Type Certificate, simplifying certification and allowing to ignore some new requirements that the original did not have to meet). It is not sure to what degree the FAA will now allows that, much less all other certification authorities. Loss of all grandfather right would immensely complicate return to service, and the following.

2) MCAS is there to mitigate at least an unsatisfactory stick feel, which would be un-certifiable (without MCAS) under the current type certificate. Some speculate MCAS may even be required to mitigate a deeper nose-up tendency in some corners of the flight envelope. Thus MCAS should be highly reliable. But the current hardware is not meant for that: only two FC and AOA vanes, and some scenarios disable MCAS leaving the plane without its protection.

3) The elevator can't be actuated manually (using trim wheel) in some (abnormal) conditions of the flight envelope (heavy mistrim at high speed), because the muscular force of the pilots is just not high enough. This always has been a serious issue with the 737, worsened with design changes (smaller trim wheel) of the NG also applying to the MAX. There is evidence that this issue was involved in the ET302 crash. Partial mitigation (yoyo maneuver) is not longer taught to pilots, many simulators do not attempt to simulate trim wheel force, and it surfaced that those that do had not been properly simulating that.

4) It surfaced that in the opinion of a panel of FAA engineers, the MAX did not meet standards of protection against shrapnel that could be thrown by engine malfunction and could severe essential cables, leading to loss of control: the larger engines increase the risk, and the standards became more stringent. This judgment was apparently overruled, but the facts remains, and 1 seems to make it worse from a re-certification standpoint.

What's Boeing plan on these items ? I vaguely see a tenable position that with extra pilot qualification, the plane is flyable with MCAS disabled, and pilots have time enough to react so that the plane does not enter the abnormal conditions making the trim wheel inop. Can that can take care of 2/3, facing 1? And what about 4 facing 1?

Last edited by fgrieu; 19th Dec 2019 at 18:01. Reason: Polish
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Old 17th Dec 2019, 18:59
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Financial Times, today:

Boeing’s 737 Max suspension hits global supply chain

Sarah Provan and Archie Hall in London

Boeing’s decision to suspend production temporarily of its 737 Max airliner has hit the shares of UK and European suppliers to the US group, as concern deepens over when the aircraft will return to the skies.

Shares in France’s Safran, the world’s third-largest aerospace supplier, were one of the steepest fallers on the Stoxx 600 aerospace and defence index on Tuesday, declining about 3 per cent.

The Paris-based company warned in September that the grounding of the 737 Max would hit its cash flow by about €300m a quarter, up from the €200m that it estimated in the first quarter of the year.

Its forecasts were based on the Max being back in the air by the end of this year. The timetable for when regulators will allow the 737 Max, which was grounded after two fatal crashes, to return to service remains unclear.

Safran produces engines for Boeing through a joint venture with General Electric called CFM International.

In the UK, Senior, an engineering company that counts Boeing as one of its top customers, was the biggest decliner among suppliers, falling 9 per cent. The FTSE 250 group warned in August that margins at its aerospace business were going to be squeezed by the prolonged grounding of the Max.

The Hertfordshire-based company, which makes sensors and other high-tech components for equipment manufacturers in the aerospace, defence and power sectors, said on Tuesday that it will provide a further update on “the potential implications to its 2020 performance once it has clarification from its customers”. It added that its expectations for its performance in 2019 are unchanged.

Shares in a rival UK supplier Meggitt fell almost 2 per cent. The company said in November that margins would be “constrained” by the Max grounding, and would be towards the lower end of the 17.7 per cent to 18.2 per cent it had forecast.

Sheila Kahyaoglu, an analyst at Jefferies, said the decision would likely result in lay-offs across the supply chain. “The supply chain is unlikely to carry workers for 2-3 months in furloughs . . . that’s why this decision was so difficult and that’s why [Boeing] waited to December to do it.”

Boeing’s 737 Max supply chain also runs through a range of smaller suppliers in the UK. Relative minnows like Aeromet International, a Worcester-based supplier of advanced aluminium cast parts, and Maher, a Sheffield steel machinist, have also done work on the 737 Max.

Airlines have already been forced to delay their plans to return the Max to their fleets until March, a year after the second of two fatal crashes that killed 346 and forced the grounding of the fleet. Boeing’s plans to suspend production of the 737 Max from next month comes as the company grapples with a lengthy regulatory review. The US group is to provide information regarding the production halt when it releases quarterly earnings next month.

Europe’s Airbus, Boeing’s major rival, was one of the index’s few risers, with a gain of 0.5 per cent.
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Old 17th Dec 2019, 19:03
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Checklist - delay

The reported of poor performance in checklist action during simulator tests might identify further problems.
Assuming that MCAS 2.0 is sufficiently protected from AoA input failure - MCAS trim is inhibited, then why should checklist items be of great concern.

Considering the MAX as an extension of the NG, the baseline alerts with AoA failure are the Air Data disagree alerts (speed, altitude), Feel Diff Pressure - higher stick force, together with AP disengagement and continuous Stick Shake. Also, there will be changes in the EFIS display of speed and altitude and the low speed awareness symbology, and AoA display if shown.
The addition of AoA (mis compare) alert might not be a significant burden; it is available for the NG.

We might confidently assume that there will be an MCAS ‘OFF’ (fail /inhibit) alert, but the consequences of this on flight restriction is not known.
Why should there be discussion about additional checks; would this involve further layer of trim inhibit - manual switching.
If so why; MCAS inhibit should be sufficiently robust so that pilots do not have to disable electric trim (assume no AP/FD due to Air Data Issues).

Why was there reference to a reset procedure; why should MCAS be reset. It might be expected that the aircraft can be flown safely - away from the edges of the flight envelope, and thus for landing similar to any other system malfunction. If not … what is the nature of the residual problem - stability, manual handling, stick force (higher forces due to Feel Diff).

If the problem has its roots in the combination of many alerts, then the after takeoff situation might be most limiting, irrespective of the accident crews performance. There is no need to inhibit MCAS (trim system) immediately if the flap inhibit works, but if it is inhibited, then it may be overly difficult to manage the flight and several drills without electric trim. The NG would be similar except for inhibiting the trim; which could strengthen any argument that pilot trimming is a high workload task in combination checklist actions.

Also see discussion on trim issues in Flydubai crash at RVI final report out
Would the trim characteristics add problems with manual trim wheel operation and stick centering/feel, particularly when resolving stick shake and unreliable airspeed issues with higher stick forces ?
Would higher stick force (feel diff) change the feel - the ease of operating the trim wheel ?
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Old 18th Dec 2019, 18:20
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Originally Posted by fgrieu View Post
It is not clear (to me at least) how exactly the modified FC software (including tamed MCAS) can allow re-certification.

1) The original certification was heavily based on "grandfather rights" (an arrangement under which an aircraft can be approved per a variation of the original Type Certificate, simplifying certification and allowing to ignore some new requirements that the original did not have to meet). It is not sure to what degree the FAA will now allows that, much less all other certification authorities. Loss of all grandfather right would immensely complicate return to service, and the following.

2) MCAS is there to mitigate at least an unsatisfactory stick feel, which would be un-certifiable (without MCAS) under the current type certificate. Some speculate MCAS may even be required to mitigate a deeper nose-up tendency in some corners of the flight envelope. Thus MCAS should be highly reliable. But the current hardware is not meant for that: only two FC and AOA vanes, and some scenarios disable MCAS leaving the plane without its protection.

<snip>
This is my basic understanding too. However I think it might be the case that without MCAS, the MAX could not be certified today because of its pitch instability at high AoA. The MAX can’t be certified, anywhere, so long as it is dependent on AoA sensors for pitch stability in parts of the envelope, and only has two. To be certified, it has to have three. Yes I’m sure you could code laws to figure out which sensor is lying and which isn’t by comparison with the AHRS or whatever but no-one will certify that.

So the MAX will never fly again in commercial operation until it gets a third AoA sensor, the necessary flight control system to use three, and recertification. That’s a couple of years, at least. Heck, it might even be that it just can’t be certified. An Airbus in Direct Law is still certified...
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Old 18th Dec 2019, 18:33
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Originally Posted by medod View Post


This is my basic understanding too. However I think it might be the case that without MCAS, the MAX could not be certified today because of its pitch instability at high AoA. The MAX can’t be certified, anywhere, so long as it is dependent on AoA sensors for pitch stability in parts of the envelope, and only has two. To be certified, it has to have three. Yes I’m sure you could code laws to figure out which sensor is lying and which isn’t by comparison with the AHRS or whatever but no-one will certify that.

So the MAX will never fly again in commercial operation until it gets a third AoA sensor, the necessary flight control system to use three, and recertification. That’s a couple of years, at least. Heck, it might even be that it just can’t be certified. An Airbus in Direct Law is still certified...
What pitch instability? It's just a nonlinear control response which might or might not affect the pilot. It apparently does not affect the autopilot although maybe MCAS is not needed for the autopilot because the autopilot is not capable of entering the flight regime which Boeing is keeping to itself.
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Old 18th Dec 2019, 18:48
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Originally Posted by jimtx View Post
What pitch instability?
That seems to be a question that has not yet been adequately answered -- at least not clearly and publicly. From the JATR report:

Recommendation R3.4: The FAA should review the natural (bare airframe) stalling characteristics of the B737 MAX to determine if unsafe characteristics exist. If unsafe characteristics exist, the design of the speed trim system (STS)/MCAS/elevator feel shift (EFS) should be reviewed for acceptability.

Observation O3.4-A: The original implementation of MCAS was driven primarily by its ability to provide the B737 MAX with FAA-compliant flight characteristics at high speed. An unaugmented design would have been at risk of not meeting 14 CFR part 25 maneuvering characteristics requirements due to aerodynamics.

Observation O3.4-B: Extension of MCAS to the low-speed and 1g environment during the flight program was due to unacceptable stall characteristics with STS only. The possibility of a pitch-up tendency during approach to stall was identified for the flaps-up configuration prior to the implementation of MCAS.

Finding F3.4-A: The acceptability of the natural stalling characteristics of the aircraft should form the basis for the design and certification of augmentation functions such as EFS and STS (including MCAS) that are used in support of meeting 14 CFR part 25, subpart B requirements.

Recommendation R3.5: The FAA should review 14 CFR 25.201 (Stall Demonstration) compliance for the B737 MAX and determine if the flight control augmentation functions provided by STS/MCAS/EFS constitute a stall identification system.

Finding F3.5-A: The nose-down pitch identified during Boeing flight tests for stall appears to the JATR team to be the product of system augmentation with flaps and gear up, and is likely due to stabilizer motion from the MCAS function.

Finding F3.5-B: The FAA-accepted Boeing flight test technique of freezing column deflection at the onset of EFS was perceived by the JATR team as possibly not meeting the requirements of § 25.201 for natural stall identification from nose-down pitch, not readily arrested. Column/elevator deflection data indicates that there may be an insufficient column input to attempt to arrest the nose-down pitch created by system augmentation.

Finding F3.5-C: The JATR team considers that the STS/MCAS and EFS functions could be considered as stall identification systems or stall protection systems, depending on the natural (unaugmented) stall characteristics of the aircraft. From its data review, the JATR team was unable to completely rule out the possibility that these augmentation systems function as a stall protection system

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Old 19th Dec 2019, 01:33
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So the MAX will never fly again in commercial operation until it gets a third AoA sensor, the necessary flight control system to use three, and recertification. That’s a couple of years, at least. Heck, it might even be that it just can’t be certified. An Airbus in Direct Law is still certified.
That is a very big call. I cannot say I agree with you Medod but who knows. The sharemarket is stil positive.
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Old 19th Dec 2019, 08:44
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Originally Posted by Icarus2001 View Post

That is a very big call. I cannot say I agree with you Medod but who knows. The sharemarket is stil positive.
You think it’s a big call? Boeing and the certifying authorities are well advised to not cock this up again by taking any shortcuts. If they do, and there is yet another accident which can be traced back to MCAS and/or basic problems with the aerodynamic stability of the airframe, Boeing is going to be toast, at least in the short/med haul sector.
Sure, the stock market may still hope that there is a quick short term solution for the MAX problem. But that very same stock market is not very loyal and will drop Boeing like a hot piece of charcoal on any new mishap.

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Old 19th Dec 2019, 15:29
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Originally Posted by jimtx View Post
What pitch instability? It's just a nonlinear control response which might or might not affect the pilot. It apparently does not affect the autopilot although maybe MCAS is not needed for the autopilot because the autopilot is not capable of entering the flight regime which Boeing is keeping to itself.
It doesn't matter for the AP because the AP doesn't care about stick force gradient.
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Old 19th Dec 2019, 15:30
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fgrieu, #1, medod, #4,
The basic aircraft was certificated on the basis of MCAS. The theory and inservice experience confirmed that this option is satisfactory, excepting for AoA malfunction.
Thus the approval to fly again depends on the robustness of the changes to protect the system from AoA malfunction; evaluated as required against existing requirements and recommendations from accident investigations.
Technically the second iteration of modification appears to be satisfactory. However, there appears to be issues with pilot interaction, using checklists, presumably after the system has failed safe, #3 considers these.

The aircraft is not inherently unstable; there is a reduction in stability margin in small areas of the flight envelope, which MCAS alleviates.
There should not be any significant concerns about abnormal flight without MCAS - after an inhibit - an assumption. The aircraft has ‘normal’ stability characteristic in most areas of the flight envelope - as demonstrated with lengthy time in service - but how often did MCAS work. Crews can be alerted to the need for care in the less stable ‘corner points’ when flying without MCAS.

The remaining oddity is trim. Historically the 737 trim has been ‘different’, which has changed (for the worse) with series development - extensive tech log discussion.
In the NG, a trim runaway appears to be acceptable, but depends on quick pilot reaction - trim inhibit, and as a last resort a yo-yo recovery manoeuvre (not in all situations - certification probability argument for acceptability).

The Max might have crossed the boundary of acceptability for timely pilot intervention and/or an increased need for, or impractical yo-yo.
How far does the trim run before MCAS shuts down, is this offset acceptable in every possible part of the flight envelope; with consideration of the variability and physical range of human ability to operate the trim wheel, (5sec ?)
How does a non MCAS Max differ from the NG; do aerodynamic differences effect the ability to trim with the wheel ?

The certification requirements involve judgement of how much piloting contribution can be assumed towards mitigating failures - with all alerted conditions coincident with, and consequential to an AoA failure. Whatever ‘piloting’ arguments have been used for NG have been negated by the two accidents in the Max; thus assessments are now clean sheet reappraisals by an authority under national and international pressure to demonstrate the highest quality processes.
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Old 19th Dec 2019, 15:46
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Originally Posted by PEI_3721 View Post
fgrieu, #1, medod, #4,
The basic aircraft was certificated on the basis of MCAS. The theory and inservice experience confirmed that this option is satisfactory, excepting for AoA malfunction.
Thus the approval to fly again depends on the robustness of the changes to protect the system from AoA malfunction; evaluated as required against existing requirements and recommendations from accident investigations.
Technically the second iteration of modification appears to be satisfactory. However, there appears to be issues with pilot interaction, using checklists, presumably after the system has failed safe, #3 considers these.

The aircraft is not inherently unstable; there is a reduction in stability margin in small areas of the flight envelope, which MCAS alleviates.
There should not be any significant concerns about abnormal flight without MCAS - after an inhibit - an assumption. The aircraft has ‘normal’ stability characteristic in most areas of the flight envelope - as demonstrated with lengthy time in service - but how often did MCAS work. Crews can be alerted to the need for care in the less stable ‘corner points’ when flying without MCAS.

The remaining oddity is trim. Historically the 737 trim has been ‘different’, which has changed (for the worse) with series development - extensive tech log discussion.
In the NG, a trim runaway appears to be acceptable, but depends on quick pilot reaction - trim inhibit, and as a last resort a yo-yo recovery manoeuvre (not in all situations - certification probability argument for acceptability).

The Max might have crossed the boundary of acceptability for timely pilot intervention and/or an increased need for, or impracticable, yo-yo.
How far does the trim run before MCAS shuts down, is this offset acceptable in every possible part of the flight envelope; with consideration of the variability and physical range of human ability to operate the trim wheel, (5sec ?)
How does a non MCAS Max differ from the NG; do aerodynamic differences effect the ability to trim with the wheel ?

The certification requirements involve judgement of how much piloting contribution can be assumed towards mitigating failures - with all alerted conditions coincident with, and consequential to an AoA failure. Whatever ‘piloting’ arguments have been used for NG have been negated by the two accidents in the Max; thus assessments are now clean sheet reappraisals by an authority under national and international pressure to demonstrate the highest quality processes.


Overall, excellent summary

But what to do about the bolded section above?

The masses of reader opinion are calling for a massive crackdown on the FAA-Boeing interaction process regarding product certification. But where is the update to the assumption about pilot responses to unreliable instruments and their aircraft response?

How can we expect a manufacturer to make a product that is airworthy yet confuses a crew? Is that in the certification review of the FARs or just a trust by the regulator that they can ultimately find blame outside their own processes?

Methinks this is not a Boeing problem to be buried in a grounding until the questions stop and enough pain endured
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Old 19th Dec 2019, 15:54
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The aircraft is not inherently unstable; there is a reduction in stability margin in small areas of the flight envelope, which MCAS alleviates.
Could you expand on this statement? This sounds like the sort of thing that the shipyard's lawyer says after the boat tips over...
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Old 19th Dec 2019, 16:00
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If it can't be fixed within a year can it ever be fixed?
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Old 19th Dec 2019, 16:26
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Originally Posted by PEI_3721 View Post
fgrieu, #1, medod, #4,
The basic aircraft was certificated on the basis of MCAS. The theory and inservice experience confirmed that this option is satisfactory, excepting for AoA malfunction.
I don't think that's accurate. First, by definition, theory can't confirm anything. And it is doubtful that MAX with MCAS 1.0 saw enough inservice time to confirm that it was satisfactory except for AoA failure.

Technically the second iteration of modification appears to be satisfactory.
We have nothing but weakly-sourced reports about MCAS 2.0

The aircraft is not inherently unstable; there is a reduction in stability margin in small areas of the flight envelope, which MCAS alleviates.
Again, none of us have seen data on the aerodynamics of the bare airframe. And many, including JATR, think it would be wise to find out just how it behaves in those corners without MCAS.

There should not be any significant concerns about abnormal flight without MCAS - after an inhibit - an assumption.
Yes, so far, just an assumption.

The aircraft has ‘normal’ stability characteristic in most areas of the flight envelope - as demonstrated with lengthy time in service
The MAX has not had "lengthy time in service."

- but how often did MCAS work.
If anyone has any clue as to the answer to that question, it certainly hasn't been made public.

Crews can be alerted to the need for care in the less stable ‘corner points’ when flying without MCAS.
Assuming that the airplane can be certified to fly without MCAS.

How does a non MCAS Max differ from the NG; do aerodynamic differences effect the ability to trim with the wheel ?
Unless the regulations change, one way it differs is that it can't be certified.

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Old 19th Dec 2019, 17:32
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lomapasseo,

Much of the judgement about human-system interaction is in the application of AMC 25.1302 ‘Installed Systems and Equipment for Use by the Flight Crew’ (I assume that you are familiar).
The application and processes for the 737 Max failed.

The requirement is ‘relatively’ new such that early versions of the 737 may not have had a full evaluation. The industry has struggled (continues to do so) with the human aspects. The dependence on judgement vs hard regulation requirers that both manufacturers and regulators understand and agree critical assumptions and justifications. This process was weakened with self certification and poor regulatory oversight, and thus the appropriate application of the requirement is central to the 737 Max re-evaluation.
The task involves both aircraft modification and reviewing / repairing the regulatory process, the latter requiring more regulators, trained and conversant with the 737, in regulation interpretation, and processes for evaluation, approval; … time.

The ‘massive crackdown on the FAA-Boeing interaction process regarding product certification‘ is required, but it’s not directly part of the current regulatory process; who oversees the regulators. Max flights don’t depend on this as it appears that the manufacturer is largely excluded from current ‘judgements’.

Water pilot, certification stability requirements depend on increasing stick force with speed reduction (fixed trim). This is particularly important at low speed where increasing pull helps identify approach to the stall. If the force required reverses - decreases, or the aircraft exhibits tendency to pitch-up into the stall, these identify unacceptable stability characteristics without being ‘unstable’. The details are within the lengthy and complex requirements of CS / FAR part 25.
The Max appears to differ in the nature of the low speed characteristics and also when manoeuvring at high speed - use of AoA / Mach vs speed; there are few precise details.

P.S. You or others might wish to see the free course https://www.futurelearn.com/courses/flight-mechanics stability is in week 4

Last edited by PEI_3721; 19th Dec 2019 at 22:32. Reason: P.S.
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Old 19th Dec 2019, 18:35
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From Bloomberg:

An Irish company that buys and leases airplanes sued Boeing Co. in the U.S. to void contracts for almost two dozen 737 Max aircraft and to demand at least $185 million in damages, citing design flaws that led to two deadly crashes.

More
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Old 19th Dec 2019, 19:11
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OldnGrounded,
Certification approval is a mixture of theory, design, engineering, documentation, proof of concept, assessment, and testing. It does not depend on being inservice for some time.

There are some significant assumptions; reading between the lines, and extrapolation of what is known publicly …, … joining up the dots; but first what is a ‘dot’.

Normal stability throughout the flight envelope had to be demonstrated in certification. No abnormalities were identified (excluding the MCAS flight envelope). Indeed the Max ‘had to be the same’ as previous series.

The task is not to provide type certification without MCAS; the ‘naked’ 737 Max does not meet requirements. Within the certification process it is necessary to asses an aircraft for abnormal operation after a required system fails, e.g. 737 single engine flight is within certification, being ‘acceptable’ for emergency operation, safety recovery and landing.

The extent of 737 Max approval for abnormal operation with MCAS inhibited is not known; we assume that it would not involve significant flight restrictions - slow down, select flaps - trim. However, within that assumption, the extent and use of checklists appears to be a problem; human machine interaction.

The abnormal piloting task has to consider the overall operation with AoA disagree, other failed systems; simultaneously no MCAS - offset trim, recover attitude, continuous Stick Shake, Air Data disagree - use st by instruments, incorrect low speed awareness, increased stick force (feel diff), manual operated wheel trim.
The approval of this operation requires judgement, of piloting capability, in context, at any time, to public satisfaction via the FAA, with or without published reasoning.
First the FAA has to reacquire public trust, not change regulations.
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Old 19th Dec 2019, 19:23
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Is it possible for Boeing to somehow support its own stock price?
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Old 19th Dec 2019, 19:36
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Originally Posted by jimtx View Post
And I as a hand flying pilot might not worry about a change in gradient. I use the column to put the attitude where I want it. But I can envision conditions where I might overcontrol in a dynamic state. I would want to see a clean windshear escape for one.
You are a cool dude not worrying about stick gradients, but when I handfly it is usually at minimum maneuvering speeds, that is high(er) AOA and I also make turns and look out the window as well so I do worry about stick gradients.
I also don’t follow your reasoning for windshear, that is instrument flying, overcontrolling is a known threat in any aircraft. It requires tunneling on pitch/ flightdirector guidance and stickforces are different from day to day operation already..
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Old 19th Dec 2019, 19:56
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Certification approval is a mixture of theory, design, engineering, documentation, proof of concept, assessment, and testing self-imposed as a result of successful lobbying to the FAA.

Sorry, I couldn't resist! Seriously though, the trust aspect is a major factor now, especially outside the US.

FAA and Boeing has a bit of work to do in rebuilding this trust, and so far I don't know if there's been much progress. Considering how a "released cargo door" can turn out to be a split fuselage (777x wing loading), I'm not sure how to interpret light stick load in relation to pitch up tendencies of the Max. I would prefer to see results of the bare airframe stall characteristics (as asked for by JATR in recommendation R3.4) before taking Boeings word for what is the purpose of MCAS.
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