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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 31st Oct 2019, 14:56
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Originally Posted by Zeffy
Senate hearings have often/typically/nearly-always been about political posturing. If anyone fancies enduring three hours of it again, here is the C-SPAN link:
​​​​​​https://www.c-span.org/video/?465677...ax-safety&live


Time might be better spent on this 12 minute video by Juan Browne -
737 Max hearings on Capitol Hill 29 Oct - Analysis and Opinion
https://youtu.be/QZO7sIbWrX8
Interesting comments, but unfortunately several inaccuracies. How he finally asserts the MAX will be the safest airplane out there, I don’t know.
More noise with some truth and some baloney

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Old 31st Oct 2019, 15:05
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Originally Posted by Maninthebar
The KNKT report states that Boeing did test MCAS misfire in the sim, but without any associated cause and therefore without the warnings and other symptoms associated with whatever triggered MCAS in any real world case. It is not clear from the report whether those operating (flying) the sim were aware of MCAS in advance. On the basis of their testing Boeing assessed the risk of unintended MCAS operation to be controlled.
Don't have the reports at hand right now, but I seem to remember they replaced the MCAS 'misfire' with just running the trim at the rate the MCAS was supposed to.
Hence hardly an actual 'misfire' test, rather just a plain runaway trim event.
But I may be wrong.

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Old 31st Oct 2019, 15:13
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Originally Posted by Fly Aiprt
Don't have the reports at hand right now, but I seem to remember they replaced the MCAS 'misfire' with just running the trim at the rate the MCAS was supposed to.
Hence hardly an actual 'misfire' test, rather just a plain runaway trim event.
But I may be wrong.
My reading of it is that Boeing carried out the tests, first on the low speed (high mach) version, found these to be manageable and then reassessed for the high speed (low mach) version. In each case this was done without any associated failures so, yes, it would resemble runaway trim.

KNKT analysis points out that no simulation was carried out as to the outcome on crew effectiveness if MCAS was once allowed to run to its maximum 2.5 deg maximum
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Old 31st Oct 2019, 17:36
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Originally Posted by BDAttitude
Hazard Analysis being done as a pro forma excercise to sign off an existing design, not as part of discovering subsystem requirements before implementing something.!
That appears to be the norm for most of the systems I've worked on. The same seems true for most of the "design" documents.
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Old 31st Oct 2019, 17:54
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Originally Posted by Grebe
<snip>
But the PR-legal buzzwords re ' open door' 'no retaliation' re org changes etc still sell well to the outsiders
That all aligns with my experiences in any company. In general managers frown on people speaking up and even worse going over there heads.
The worst was active duty military.

I've heard the lip service given, and felt the disincentives, but never had to test it.
As it is, I stay out of the managers offices and under their radar as much as possible.
I've been lucky? in that I've been able to work out issues without management involvement. I hope that continues.
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Old 31st Oct 2019, 17:55
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Originally Posted by ST Dog
But they do. MCAS is neither main electric trim nor autopilot trim.
My understanding is that STS is not included either.
MCAS is an extension of STS and STS runs through the autopilot trim.

There is an STS system which includes speed trim functions and MCAS functions,
STS function will be cutout via the control column cutout switch. By design MCAS will not. The only reference to bypassing the column cutout is using the stab trim override switch.

However, the AOMs and FCOMs don’t explain the exception for MCAS, obviously.
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Old 31st Oct 2019, 18:06
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MCAS is powered through autopilot trim. Some time ago, in a thread far away, someone posted a wiring diagram showing MCAS trim running through the STAB TRIM OVRD relay, thereby bypassing the control column cutout switches.
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Old 31st Oct 2019, 18:56
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Originally Posted by Takwis
MCAS is powered through autopilot trim. Some time ago, in a thread far away, someone posted a wiring diagram showing MCAS trim running through the STAB TRIM OVRD relay, thereby bypassing the control column cutout switches.
I think that may have first appeared as post #737
https://www.pprune.org/rumours-news/621879-maxis-return-delayed-faa-reevaluation-737-safety-procedures.html#post10504031

Other, but not all, wiring diagrams on #750, #780 & #947
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Old 31st Oct 2019, 19:02
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Originally Posted by Peter H
I think that may have first appeared as post #737
https://www.pprune.org/showthread.php?p=10504031

Other, but not all, wiring diagrams on #750, #780 & #947
All relevant wiring diagrams with a comparison NG/MAX can be found here:
https://www.satcom.guru/2019/08/conn...to-action.html
The override relay function is shown here:




Last edited by spornrad; 31st Oct 2019 at 19:41. Reason: better diagram
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Old 31st Oct 2019, 19:07
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Thanks both of you; I thought all that was in one of the closed threads. Also, I had not read that particular article from Peter Lemme... chock full of relevant, accurate information.

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Old 31st Oct 2019, 23:17
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Originally Posted by Maninthebar
You have comprehensively missed point.

JATR are recommending that certifying authorities (re)consider the handling characteristics of the airframe in the case where MCAS does not activate WHEN IT IS DESIGNED TO.

Can you address that?
Boeing have not got those talking points ready for staff yet
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Old 31st Oct 2019, 23:42
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Do we know the trigger AoA for MCAS, and if it would stop commanding AND if AoA dropped back below that trigger value?
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Old 31st Oct 2019, 23:47
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Originally Posted by medod
Do we know the trigger AoA for MCAS, and if it would stop commanding AND if AoA dropped back below that trigger value?
For my understanding, the orginal MCAS design would stop and return the stab to the original position
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Old 1st Nov 2019, 02:34
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From a design point of view, calling MCAS as similar to STS does not seem to be correct. If it were similar, MCAS should have only reduced the rate of movement of the Stabilizer under high AoA conditions.
They call it an "Augmentation"System, but in fact it is more a "Correction" system.
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Old 1st Nov 2019, 03:39
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Originally Posted by GlobalNav
Interesting comments, but unfortunately several inaccuracies. How he finally asserts the MAX will be the safest airplane out there, I don’t know.
More noise with some truth and some baloney
Take out the “safest plane” comment and what did he say that’s misleading? He’s a current type rated pilot on a medical. Not the smoothest delivery but outside of that final comment where are the “several inaccuracies” and “Baloney”?

Last edited by Drc40; 1st Nov 2019 at 12:35.
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Old 1st Nov 2019, 04:45
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Originally Posted by FrequentSLF
For my understanding, the orginal MCAS design would stop and return the stab to the original position
Is the logic tree of the MCAS system a trade secret? I have never seen a clear and detailed explanation of the inputs, decisions, and effects. We're all just guessing.

I'm sure it's complicated, but a little transparency would be appreciated. They post electrical circuits, why not logic circuits too.
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Old 1st Nov 2019, 09:56
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Leeham has an interesting article about the most ancient version of a MCAS-style system, in the B707-400 !Little known because totally innocent is her "stick nudger". In contrast to MCAS, however, it was integrated reasonably at the elevator...https://leehamnews.com/2019/11/01/a-...0s/#more-31553

...In this respect Airbus has a distinct advantage with the A320neo family, who’s digital flight control software can be amended, to a limited extent, to take into account varying aerodynamic profiles. Airbus’ decision to incorporate a fly by wire control system in the 1980s was a bold but ultimately wise decision.It is still not clear when the 737MAX will return to service or what additional modifications will be required by the FAA and EASA. The various investigations underway internally within Boeing and separately by the US Government and associated parties, such as EASA, will ultimately bring about seismic changes to procedures. The lessons learnt during this period will only make current and future generation aircraft safer for the travelling public however we should also remember that many of the problems encountered today were first resolved 60 years ago.
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Old 1st Nov 2019, 10:50
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No technical news, but provides a broad context to the MAX story. Worth reading: https://www.forbes.com/sites/hershsh...greek-tragedy/
Boeing’s CEO Dennis Muilenburg, is spending this week on Capitol Hill testifying before a series of Congressional committees about the failures related to the 737 MAX. The saga of the 737 MAX is akin to a classic Greek tragedy featuring heroes with tragic flaws, along with a host of general lessons.
This week, much attention is being paid to the culture at Boeing, especially its willingness to focus on profit at the expense of safety. Doing so is instructive, especially if we identify the broader psychological issues that impacted Boeing’s decision makers and which apply to decision makers at many firms.

The broad issues pertain to Boeing decision makers’ mindsets as they faced strong competitive pressures from Airbus. In part, this pressure arose because Boeing executives were late to recognize the Airbus threat, partly because of their own hubris. In particular, Boeing executives failed to put controls in place in order to manage normal human tendencies that people face when they feel themselves to be operating in the domain of losses. These human tendencies involve the inclination to take imprudent risks in an attempt to avoid having to accept a sure loss. Just as unsuccessful racetrack bettors tend to back long shots late in the day, in the hope of breaking even, Boeing took risks with the 737 MAX by cutting corners, in an effort to avoid having to cede the market to Airbus.
Lying at the root of the behavior patterns described in the previous paragraph are a small number of psychological phenomena. The list includes excessive optimism, overconfidence, and confirmation bias: optimism for underestimating the probability of commercial airline pilots being able to deal with MCAS problems, overconfidence about the risks attached to allowing for a single point of failure, and confirmation bias by downplaying the problem after two 737 MAX planes had crashed.

Aversion to accepting a sure loss, excessive optimism, overconfidence, and confirmation bias are general phenomena that impact many organizations. Boeing’s experience is currently under scrutiny, but it is hardly unique. The firm’s missteps serve as a cautionary warning for all firms facing strong competitive pressures about the need to practice prudent risk management.
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Old 1st Nov 2019, 11:57
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spronrad,

I think you will find that D P Davies required the stick nudger on the B707-320s registered in the UK (page 262, Handling the Big Jets). He also required a stick nudger on the 747s registered in the UK - it worked a treat.

I always wondered why Boeing did not fit one to the 737MAX.
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Old 1st Nov 2019, 12:34
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Originally Posted by Bergerie1
spronrad,

I think you will find that D P Davies required the stick nudger on the B707-320s registered in the UK (page 262, Handling the Big Jets). He also required a stick nudger on the 747s registered in the UK - it worked a treat.
Peter Lemme
I always wondered why Boeing did not fit one to the 737MAX.
In Satcom Guru: Flawed Assumptions Pave a Path to Disaster
https://www.satcom.guru/2019/10/flaw...-disaster.html
Peter Lemme says:
In Stall identification (augmentation) is an action - it is doing something to the airplane to push the
nose down. Stick pushers and stick nudgers are commonly used for stall identification. A stick
nudger would have performed nicely in place of MCAS, but that would have required redesign of
the feel system.


I wonder how much it would have cost in terms of: delay, engineering, per-plane hardware -- and how
that compares to the costs of some of the high-tech solutions being proposed (genuine question).
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