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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 1st May 2019, 21:50
  #4721 (permalink)  
 
Join Date: Jan 2012
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Originally Posted by sadtraveller View Post
Not a 737 driver so I have no vested interest in defending my plane/livelihood. Also neither American nor European, so I could not care less about defending my "team". What I am is a frequent traveller who would prefer not to become a smoldering crater in the ground -- preferably not even if I am flying on a plane flown by the most poorly trained, least-skilled and most sleep-deprived (but properly licensed) pilot.

I've read this whole thread and then some, and thus far my reading of the facts of these incidents is as follows:

1. (Fact) Boeing was caught with their pants down by the A320neo and needed to come up with a quick 737 update with better fuel economy (i.e. with minimal engineering changes).

2. (Fact) Rather than produce a clean-sheet design or update the 737 undercarriage/wings to incorporate an extended main landing gear, Boeing extended the nose gear and bolted on larger engines by moving the engines forward relative to COG, in spite of the negative impact that this had on aircraft stability/handling.

3. (Fact) The 737 stabilizer has much greater vertical authority than the elevator (I have seen estimates of about 3x as much)

4. (Fact) Due to the enlarged and forward-positioned engines, the Max exhibits poor aerodynamic/flight handling characteristics (tendency to pitch up) at high AOA when approaching stall. As communicated by at least one purported Boeing engineer much further up this thread (post 1000 or thereabouts?), this tendency to pitch up is so extreme that elevator input alone would not have been sufficient to make the aircraft adequately controllable, hence why a stick pusher was considered and rejected.

5. (Speculated) For the same reason, once in a stall, the 737 Max is quite likely to be extremely difficult to bring back under control through normal control surface inputs, without the rapid application of an extreme amount of stab trim (i.e. by MCAS). (Has anyone flown a Max into a stall at MTOW/rear COG and would they be able to provide input on whether it is possible to bring it back under control without trim input/MCAS?)

6. (Fact) The FAA/other authorities require positive elevator feel at high AOA to provide stall protection.

7. (Fact) MCAS is designed to provide positive elevator feel at high AOA to meet regulator requirements.

8. (Fact, deduced from 6 and 7) MCAS provides stall protection.

9. (Fact) The certified limit of MCAS authority (one burst of 0.6 deg) was also not sufficient to make the aircraft adequately controllable at high AOA. Boeing increased MCAS authority to 2.5 degrees per cycle, with an unlimited number of allowed cycles, without informing regulators/obtaining certification.

10. (Fact) The FAA was so understaffed/feckless that they did not discover/were not made aware of this severe deviation from certification.

11. (Fact) Based on various pilot reports and according to archaic 737 manuals and flight training, past a certain limit that is inversely related to airspeed, manual authority over trim is impossible for pilots of less than super-human strength, so e trim is the only option available.

12. (Fact) Boeing shrunk the size of the Max trim wheel, reducing leverage and further increasing strength required to manually trim. They failed to inform regulators of this change.

13. (Fact, deduced from numbers 10 and 3) Under certain portions of the flight envelope, pilots lose all vertical authority without electric trim.

14. (Opinion) Based on its essentiality to maintaining vertical authority under all allowed portions of the flight envelope, electric trim should be considered a safety-critical component and should be subject to all of the requirements thereof (e.g. redundant computers, motors, etc.)

15. (Fact) Boeing redesigned the cutoff switches to make it impossible to disable MCAS without also disabling e trim. They then failed to describe these changes in the manual or provide this information to pilots.

16. (Unverified, but extremely likely) Based on the inconsistent application of upward electric trim by the pilots during the two accidents, something was likely preventing the pilots from continuously applying upward trim. If your life is flashing before your eyes and you are battling to get your aircraft to climb, your thumb is going to be glued to the trim switch, not making the bare minimum in small (and inconsistent) blip applications. As far as I know it has not been conclusively reported, but it appears that based on the trim traces that MCAS is in fact capable of overriding thumb trim and not the other way around, despite claims to the contrary.

17. (Speculation) Many have speculated that the electric trim motor is also incapable of controlling the stabilizer under certain portions of the flight envelope (e.g. high speed, extreme trim, opposing elevator). If this is true, then there are actually portions of the flight envelope in which pilots lose all vertical authority even if electric trim is operational.

18. (Opinion) If there are regions of the 737 flight envelope (e.g. extreme trim, opposing elevator) under which pilots lose vertical authority, the whole 737 fleet, both maxs and NGs (assuming that they are also affected by the same issue), should be grounded until such a time as trim deflection is mechanically limited to prevent entry into these uncontrollable regions of the flight envelope.

19. (Fact) Boeing did not advise pilots of the limits of manual trim authority or the need to e trim to neutral before cutting the trim switches.

20. (Fact) Boeing decided to base the activation of MCAS on a single AOA sensor (rather than making use of the two installed sensors) and a single computer, without even basic sanity checking.

21. (Fact) A large number of recent reports have identified safety-critical manufacturing defects and foreign object debris in recently-constructed Boeing aircraft as a result of lax manufacturing standards, and articles from yesterday indicate that a whistleblower reported to the FAA on April 5th that FOD had resulted in damage to Max AOA wiring in at least one instance.

22. (Fact) Boeing did not inform airlines or pilots about MCAS, did not include it in the manual, and did not provide any MCAS related training.

23. (Fact) Boeing chose not to provide an aural/visual MCAS activation warning.

24. (Fact) Boeing opted to sell the AOA indicator display as an optional extra rather than a built-in safety feature.

25. (Fact) Boeing disabled the AOA disagree warning for customers who did not purchase the optional AOA indicator display. Previous 737 models had functional AOA disagree warnings. As reported recently by the SWA pilots assoc, Boeing did not inform airlines/pilots of this change.

26. (Fact) Boeing did not make or arrange for the production of an adequate number of Max simulators, hence the extreme resistance to any changes that would require sim training.

27. (Fact) Pilots did not receive any information about MCAS or sim/flight training for dealing with a possible MCAS activation prior to these incidents (they did not even know that the system existed).

28. (Inferred based on events) The incident pilots were inadequately trained to handle and responded very poorly to MCAS activation incidents. (Note: is it the pilots' fault that they were inadequately trained on a system that they did not know existed on simulators that were not available?)

29. (Fact) After the Lion air accident, Boeing provided only the bare minimum in information regarding MCAS and refused to acknowledge any problem with the plane, placing the blame entirely on the dead pilots.

30. (Fact) After the Ethiopian accident, Boeing resisted efforts to ground the aircraft and refused to acknowledge any problem with the plane, again placing the blame entirely on the dead pilots.

31. (Fact) After the Ethiopian incident and following a meeting between Muilenburg and Trump, Trump's FAA stooge resisted requests to ground the Max.

32. (Fact) Only after foreign regulators began grounding the Max en masse did the FAA also follow suit and ground the Max.

33. (Fact) Boeing, as evidenced by Muilenburg's recent press conference, still refuses to acknowledge any problem with the plane, and continues to place blame solely on the dead pilots. Meanwhile they are working on a software "improvement" for MCAS, without any reports of hardware fixes in the works.

Conclusion:

In my opinion, Boeing has acted disgracefully in this situation and should be prosecuted criminally for manslaughter (perhaps this is an opportunity for Barr to prove that he is not a Trump stooge). Meanwhile the FAA has been completely compromised and corrupted by the kleptocracy that is taking over America. These are systemic failings rather than a one-off incident, and they raise the question of how many other similar failures remain lurking in the shadows due to negligent management practices and oversight in a country that is rapidly losing any respect for the rule of law. No outcome short of a complete (and transparent) overhaul of Boeing's safety culture, prosecution and incarceration of senior management, and possibly even a break-up of the company (e.g. splitting off commercial aviation from defense) will make me comfortable flying on any recently-produced Boeing metal. I'll be putting my money where my mouth is by exclusively booking Airbus until these changes are made. I'm not holding my breath, so it looks like I'll be flying Airbus for some time to come.
What a truly superb analysis. I suspect this could very well be plaigerised in its entirety and feature in various court cases in the future.

Regards


Last edited by Alchad; 1st May 2019 at 22:01.
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Old 1st May 2019, 22:11
  #4722 (permalink)  
 
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Originally Posted by slacktide View Post
Yes, it was. The "automatics" failed by providing undesired and uncommanded control inputs, but this was not the initiating failure that the pilots failed to react to. MCAS did not become a factor until a full minute and fifteen seconds into the flight. The inital failure that was presented to the pilots was stickshaker on takeoff with UAS. Rather than manually flying the aircraft using pitch and power as required by the memory items on the UAS NNC checklist, the accident pilots tried to re-engage the autopilot multiple times, and relied on the autothrottle to manage power, which put them above VMo. This directly contradicts the UAS NNC checklist which has the pilots disengage the autopilot, autothrottle, and flight director as memory items for the first three steps, and further goes on to state "Do not use the the autopilot, autothrottle, or flight directors." Look at the timeline:

5:37:34 ATC clears flight for takeoff
5:38:44 (just after liftoff) AOA disagree, airspeed disagree, altitude disagree, and stickshaker are indicated in FDR
5:38:46 Master Caution light illuminates.
5:38:58 Pilots attempt to engage autopilot
5:39:00 Pilots attempt to engage autopilot
5:39:22 Pilots successfully engage autopilot
5:39:45 Flap retraction begins
5:39:55 Autopilot disengages
5:40:00 MCAS begins MCAS-ing

So, it was a minute and fifteen seconds between the initial indication of a failure, and the undesired, uncommanded trim input. During that time, the pilots did not execute a single step of the UAS NNC checklist that they should have been following from memory. But they sure spent a lot of time heads-down button-pushing that autopilot. It's clear that they were not comfortable manually flying the aircraft as the checklist requires. To be noted, until 5:40:00, the airplane would have been behaving exactly as an NG would during the same type of AOA vane failure, and the procedure to follow during UAS is identical.


1. Excerpt from Boeing FCOM Philosophy and Assumptions; “• the full use of all automated features (LNAV, VNAV, autoland, autopilot, and autothrottle). This does not preclude the possibility of manual flight for pilot proficiency where allowed” in other words become masters of automation.

2. “startle Factor” + prolonged confusion = Panic

3. Panic = return to comfort Zone.

4. Hence at 50’ LNAV selected and 400’ VNAV selected and Autopilot attempted, “HAL” will fix it.

5. What would that comfort zone be? Automation of course. That’s what the industry teaches.

6. Manual flight with a flight director ON does little to solve the issue, it must be flight director and autothrottle off to achieve the familiarity and thus comfort zone. Do we want pilots flying commercial jets as if in a Cessna 152? Catch 22.

Question

1. At 100’ what was the difference in IAS Left vs Right? Is it clear which is erroneous? Could your Trim sheet/ Load sheet be wrong?

2. At what timeline should they have identified UAS and not an approach to stall?

3. If UAS identified should they have powered back to 80% N1 and Pitch 10”?

4. How much does the stick shaker noise, vibration, low altitude and low time FO affect you, and when was the last time you trained for this?

5. Hundreds of hours and discussion on this thread and no consensus, but this flight lasted 6 minutes and was doomed by 3 minutes.

With what we know now, all on this thread could survive the same scenario, how about before?

Pilots are so trained to follow the memory items; it doesn’t allow them to fly on instinct in panic mode. Panic Mode starts with wings level, pitch and power and let the brain settle.


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Old 1st May 2019, 22:15
  #4723 (permalink)  
 
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Originally Posted by GordonR_Cape View Post
There are two followup aspects to your question, depending on the version of MCAS 1.0 and MCAS 2.0:
- MCAS 1.0 was designed to satisfy the criteria that you describe, though not smoothly, due to the 5 second pause.
- MCAS 2.0 may remove the option of repeated trim during the same high AOA "event", depending on the definition of event. This would not satisfy the 25.173 criteria, if it allowed the pilots to do a second pull into the high AOA region, after interrupting MCAS by blipping the stabiliser trim switches.

Long ago I posted this assertion: It is logically impossible to design a system that satisfies both limited MCAS activation and 25.173 criteria simultaneously. One of the two limits has to be broken in some scenarios, for example pilots repeatedly entering the high AOA condition. I have not seen any evidence to contradict my assertion.
I think everyone is missing the the point of MCAS. MCAS is supposed to operate behind the scenes to add just a little nose down trim at high angles of attack to replicate the "feel" of a "normal" aircraft" as the pilot pulls the nose up WTHOUT TRIM. FAA 25.173 requires it for nose up, neutral trimmed high angles of attack. It is not needed if the flaps are out or if the autopilot is on. It is also not needed if the pilots decides to trim on his own.

When MCAS has been triggered at high angles of attack, as soon as the pilot touches the trim, MCAS is disabled because the pilot now has decided to trim for whatever trim feel they want. There is now no need for MCAS as there is no FAA mandated "feel" that needs to be demonstrated. MCAS does not operate for 5 full seconds after each application of trim.

So yes, I believe MCAS can satisfy both limited activation and 25.173 criteria simultaneously.
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Old 1st May 2019, 22:19
  #4724 (permalink)  
 
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The Max crashes might have to do with pilots shortcomings but never with the airline, country or origin. Af447 made this clear once and for all.
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Old 1st May 2019, 22:23
  #4725 (permalink)  
 
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Can anyone here definitively state what Reg MCAS was intended to provide compliance with??

I have seen several different sources cite to 14CFR 25.203 "Stall Characteristics" ("No abnormal nose-up pitching may occur.") and others to 14CFR 25.173(c) Static Longitudinal Stability (The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.)

I'm working on something and it would be immensely helpful to have a factual basis for the certification requirements that MCAS was intended to provide compliance with. This is actually somewhat of a big deal- I had been working off the predicate that it was for "increasing feel" approaching high alpha, but I see now with either of those Regs that might not be correct.

Whatever the reg is that MCAS was implemented for it seems clear that the system as designed is not a feel enhancement, it is a flight control system, which raises a huge number of questions if you think about it for too long...

Thank you in advance for your time and for passing on any relevant information-
dce
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Old 1st May 2019, 22:26
  #4726 (permalink)  
 
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sadtraveller, Ethiopian airliner down in Africa
A few quibbles, but I won’t.
A ‘factual’ review, credible of a determined safety case to improve our industry.

Opinion. Recall that “opinion is really the lowest form of human knowledge. It requires no accountability, no understanding.” (B. Bullard) Yet your views provide the very necessary grounding for logical reappraisal, which at this stage of the thread has been lacking.

Last edited by safetypee; 2nd May 2019 at 06:57. Reason: link update
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Old 1st May 2019, 22:39
  #4727 (permalink)  
 
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Wonkazoo, I don’t have an answer to your direct question but erring towards flight control systems, I think that 25.672 is also relevant.
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Old 1st May 2019, 22:41
  #4728 (permalink)  
 
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Originally Posted by wonkazoo View Post
Can anyone here definitively state what Reg MCAS was intended to provide compliance with??
I have never seen any official reference to the actual FAA requirement.

This is the most detailed explanation I have seen: 737 MAX Maneuvering Characteristics Augmentation System (MCAS)

MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.


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Old 1st May 2019, 23:00
  #4729 (permalink)  
 
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Yo gums,
We need some test community inputs to this discussion.’

Not sure how any further input from the test community or any other could contribute.
A distant, philosophical view is that with hindsight people view situations as they wish, based on knowledge (often inaccurate) and (irrelevant) past experiences, then fit these to interpretations of incomplete information - a narrow window from FDR and CVR, together with the hazard of ‘internet’ belief (social media).
We rarely question our own thoughts.
This might be the greater threat to the industry than any technical malfunction if these aspects relate to all flight crews (no reason why not - just being human).

Whilst it is unreasonable to expect everyone to have deep knowledge of CS/FAR 25 etc, there should be general confidence that most, if not all aspects of design, certification, and testing have been considered. Rarely and unfortunately in this instance, these processes are not without mistake; similar to the ‘mistakes’ observed in operation. These do not warrant blame, and rarely can ‘cause’ be identified, which would be meaningless anyway - a social construct.

Experiences from investigation of serious incidents (non fatal), with the benefit of pilot interview, conclude that humans behave rationally according to how they saw the situation at that time (irrational with hindsight to an external observer).
Subsequent review of the FDR enable crews to re-evaluate their understanding, not changing what they did, nor providing understanding of why they acted as they did (they don’t know - don’t recall why), but significantly they are able to realign time frames (wildly misjudged), and the event order according to individual viewpoint (no such thing as a shared mental model).
Thus after the event, there is no way of being sure that any discussion represents anything relevant to crew thought, analysis, belief, and action; even accident reports.

A way forward is to consider what can be learnt from these accidents; of course including the comments above, but where speculation - what if - is a basis for safety improvement. Irrespective of any relevance to these accidents, because they will not occur in exactly the same way again.

Test community input; might readjust the views on the crew’s contribution in minimising the effects of malfunction, particularly with ‘grandfather rights’ aircraft. We are not as good as we think we are.
For the future, be very concerned about the balance between new design (or modification) and pilot ability; technology advances faster than crew training / adaptation.
Human performance will limit the advance of technology; but technology will further erode the human role because of lower cost.

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Old 1st May 2019, 23:27
  #4730 (permalink)  
 
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Agreed! and they think the MAX will fly anytime soon? if their are bookies taking bets let me know how to contact them.
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Old 1st May 2019, 23:58
  #4731 (permalink)  
 
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Salute!

Thanks, PEI. I was and am still looking for the aero explanation that is more relevant than basic control column feel.

Unless you are flying a Mk 1, Mod 1. version 737, you need to try the new suckers. The plane is not the one your grandfather flew.

"Lost" quoted the British dot org explanation, and that that is one that I support most of all.

Gums sends...



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Old 2nd May 2019, 00:42
  #4732 (permalink)  
 
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As far as I know it has not been conclusively reported, but it appears that based on the trim traces that MCAS is in fact capable of overriding thumb trim and not the other way around, despite claims to the contrary.
Completely wrong.
See the Boeing bulletin posted above, trim use will always override MCAS.
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Old 2nd May 2019, 00:59
  #4733 (permalink)  
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Lost in Saigon #4673

It has been said time and time again: MCAS is not for stall protection.
Yes, thank you. After reading what is probably nearer 6,000 posts, and many other articles, I think I've got the hang of what MCAS is, or is supposed to be.

*********

Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.

I also mentioned one of my main bee's in bonnets. The second (underfloor) column switch - which may or may not be there, and if it is, allowed to function at all times. In the same question, they made an eye-opening statement about the switch function. It's going to take time to absorb the circuitry at the bottom of the article, but again, it's not as straight forward as I'd thought.

Crash: Ethiopian B38M near Bishoftu on Mar 10th 2019, impacted terrain after departure

The posting, again, of the old out-sourced fabrication issue a few posts back. Could this be connected, though some time after the associated whistle-blowing.
14 - Russia's MAK revoked the certificate of airworthiness for the entire 737 family (from 737-100 to 737-900) three years ago claiming they found an issue in the pitch/altitude control system of the aircraft (suggesting that at least the Tatarstan crash in Kazan as well as the Flydubai crash in Rostov may have been the result of that weakness) but did not receive a satisfactory response by the FAA and Boeing, also see News: Russia suspends airworthiness certification for Boeing 737s, but does not prohibit operation of 737s. What was the issue they found?
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Old 2nd May 2019, 01:29
  #4734 (permalink)  
 
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Originally Posted by Loose rivets View Post

Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.

.
I have no doubt that this is true, but it is also largely irrelevant from a procedural viewpoint. The pilots don’t need to be able to read a wiring diagram and tell you all the things that happens when they throw the cutout switches. They just need to know when they need to throw the cutout switches - as in the case of the runaway stab trim procedure.


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Old 2nd May 2019, 01:43
  #4735 (permalink)  
 
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Originally Posted by sadtraveller View Post
Conclusion:

In my opinion, Boeing has acted disgracefully in this situation and should be prosecuted criminally for manslaughter (perhaps this is an opportunity for Barr to prove that he is not a Trump stooge). Meanwhile the FAA has been completely compromised and corrupted by the kleptocracy that is taking over America. These are systemic failings rather than a one-off incident, and they raise the question of how many other similar failures remain lurking in the shadows due to negligent management practices and oversight in a country that is rapidly losing any respect for the rule of law. No outcome short of a complete (and transparent) overhaul of Boeing's safety culture, prosecution and incarceration of senior management, and possibly even a break-up of the company (e.g. splitting off commercial aviation from defense) will make me comfortable flying on any recently-produced Boeing metal. I'll be putting my money where my mouth is by exclusively booking Airbus until these changes are made. I'm not holding my breath, so it looks like I'll be flying Airbus for some time to come.
sadtraveller Thank you for this neat summation of the facts. As a fellow frequent flyer, I'm with you in wanting a safe flight despite non-optimal staffing in the front seats.

I'd also question the focus on MCAS alone. Boeing did not react to Lion Air as if a known risky design choice had been exposed. My own impression was that MCAS hadn't been on their minds, and they were surprised and slightly irritated that it had caused problems. Ergo, I assume that the MCAS design was of a piece with their general design standard. If this is the case, it's reasonable to think that there may be more potential bear-traps in the MAX design. Are there any other systems dependent on a single sensor? Are there any other silently disabled features that Boeing haven't mentioned? Can any other flight characteristics be affected by features undisclosed to the pilot?

If I were an aviation authority, I'd want those questions answered. It's a shame the FAA isn't leading the way.
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Old 2nd May 2019, 02:28
  #4736 (permalink)  
 
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Originally Posted by 737 Driver View Post
Originally Posted by Loose rivets
Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.
.
I have no doubt that this is true, but it is also largely irrelevant from a procedural viewpoint. The pilots don’t need to be able to read a wiring diagram and tell you all the things that happens when they throw the cutout switches. They just need to know when they need to throw the cutout switches - as in the case of the runaway stab trim procedure.
Unfortunately the AV herald "question" referenced above commingled the under floor cutout switches with the 2 center 'stab cutout' switches. If carefully parsed it might be correct but very hard to read.
Here are the relevant facts on the cutout switches:

The 737 NG the cutout switches:
left cutout kills all electric trim (protects against stuck pilot switches as well as some faults in motor module)
right disable automatic trim only.

On the 737 MAX either cutout switch disables all electric trim.
The 28V power to the main trim motor 115V 3 phase relay is in series through both switches. The av herald shows some function not identified secondary contacts that are probably status inputs to other systems.

The 737 MAX labels were changed to 'primary and 'backup', the changed functionality was not mentioned in at least some version of the 'i-pad conversion course.

One theory is that the ET pilots used the right switch and then discovered they had no manual electric trim, which they would have had on NG.

Others have posted that the runaway trim procedure was changed from first using the right switch only to using both a few years ago for unknown reasons.
No one has posted a rational sounding reason why the switch functions were changed.
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Old 2nd May 2019, 02:55
  #4737 (permalink)  
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Originally Posted by Loose rivets
Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.
.
737 Driver #4736
I have no doubt that this is true, but it is also largely irrelevant from a procedural viewpoint. The pilots don’t need to be able to read a wiring diagram and tell you all the things that happens when they throw the cutout switches. They just need to know when they need to throw the cutout switches - as in the case of the runaway stab trim procedure.

I know, and I concede I've always been a 'know every nut and relay' kind of bloke. But this, if true, means the function of the BU switch is much more subtle than I'd realised.

My first reaction was of curiosity, wondering where that would take us. It seemed to imply only throwing the RH switch might be significant.

However, it'll be 03 Sparrows again before I get my head down - about par for the course since November. I'll look at the AV H's list and circuits again tomorrow as I usually have a photographic memory for systems, but these logic changes are leaving me in a fog. Don't like it. Hope it's not age related!!
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Old 2nd May 2019, 03:19
  #4738 (permalink)  
 
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Originally Posted by MurphyWasRight View Post

The 737 NG the cutout switches:
left cutout kills all electric trim (protects against stuck pilot switches as well as some faults in motor module)
right disable automatic trim only.

<snip.

Others have posted that the runaway trim procedure was changed from first using the right switch only to using both a few years ago for unknown reasons.
No one has posted a rational sounding reason why the switch functions were changed.
Small correction. On the NG, the left switch disables the main electric (pilot actuated) trim and the right switch disable all automatic inputs (Speed Trim/Mach Trim/Autopilot Trim). We used to try to identify the offending system and actuate these switches separately in the runaway stab trim procedure depending on the situation. Somewhere along the way, Boeing changed their philosophy on the procedure (probably part of a larger trend of moving away from “troubleshooting” type actions) and now we always use both switches when necessary. I have previously speculated that two switches were retained in the MAX for both redundancy against possible relay welding and to simply harmonize the procedural aspects of runaway trim between the NG and the MAX. That is, both aircraft have two switches (for different reasons) and you always use both switches at the same time.

737 Driver is offline  
Old 2nd May 2019, 03:20
  #4739 (permalink)  
 
Join Date: Dec 2006
Location: Whanganui, NZ
Posts: 135
Originally Posted by sadtraveller View Post
[snip]

18. (Opinion) If there are regions of the 737 flight envelope (e.g. extreme trim, opposing elevator) under which pilots lose vertical authority, the whole 737 fleet, both maxs and NGs (assuming that they are also affected by the same issue), should be grounded until such a time as trim deflection is mechanically limited to prevent entry into these uncontrollable regions of the flight envelope.

[snip]
This
My reading of this whole catastrophe is that the certification of the B737MAX and the B737NG may now be thrown into question.

The FAA will undoubtedly approve "MCAS 2.0" in as quick a time as they consider respectable, but the evidence shows that there is such a degree of regulator capture that this will be hard to see as an objective evidence-based process.
Others, particularly CAAC and EASA, may not be so easily convinced. The Russian regulator would probably be onside with that judging by their actions a couple of years ago to attempt remove certification from the B737NG.

That would be catastrophic for Boeing and for the air transport industry world-wide: half the global narrow-body fleet grounded indefinitely hardly bears thinking about.
kiwi grey is offline  
Old 2nd May 2019, 04:19
  #4740 (permalink)  
 
Join Date: Jul 2003
Location: In da Big Smoke
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My reading of this whole catastrophe is that the certification of the B737MAX and the B737NG may now be thrown into question.

The FAA will undoubtedly approve "MCAS 2.0" in as quick a time as they consider respectable, but the evidence shows that there is such a degree of regulator capture that this will be hard to see as an objective evidence-based process.
Others, particularly CAAC and EASA, may not be so easily convinced. The Russian regulator would probably be onside with that judging by their actions a couple of years ago to attempt remove certification from the B737NG.
I think the Whistle blower testimony may impact what decision that the FAA is even able to make. I'll bet that both Boeing and the FAA are keen to just patch MCAS and move on.
However if they start getting expert whistle blower testimony that brings in to question the whole certification process of MCAS and the reasons for its existence the FAA will be out of options other than some sort of re certification process which is a whole new can of worms.
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