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Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed

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Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed

Old 29th Mar 2019, 00:29
  #441 (permalink)  
 
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PS about the crashed AOA sensors.
No troubleshooting will be possible. They are outside, they are on the nose (outside cockpit area) => Ever seen an AOA vane that hit the deck at 400 kts?
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Old 29th Mar 2019, 00:47
  #442 (permalink)  
 
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Sorry if I missed this question:
! wonder how many times the MCAS system activated and kept aircraft safe? Would the aircraft be safe without the system and if not, there in lies the problem.
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Old 29th Mar 2019, 03:21
  #443 (permalink)  
 
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Why not have an emergency override switch that allows pilots to command the stabiliser RAPIDLY to a precise target position? Or one of a few conveniently spaced positions within its total excursion?

That way if MCAS does something unwanted, or other issues arrive that make fast manual retriming desirable the pilots just select the desired target position and get back to sanity ...

Edmund
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Old 29th Mar 2019, 03:41
  #444 (permalink)  
 
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Originally Posted by edmundronald View Post
Why not have an emergency override switch that allows pilots to command the stabiliser RAPIDLY to a precise target position? Or one of a few conveniently spaced positions within its total excursion?

That way if MCAS does something unwanted, or other issues arrive that make fast manual retriming desirable the pilots just select the desired target position and get back to sanity ...

Edmund
No, not a good idea at all. First of all the pilots HAVE a switch that overrides MCAS while it is trimming: the electric trim switch on the control wheel. Second, the last thing you want is an instant reset of a fully nose-down trimmed THS while the pilots are pulling full up, it is pretty much guaranteed to pull the wings off. There is a reason the trim speed isn't faster.
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Old 29th Mar 2019, 04:54
  #445 (permalink)  
 
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Originally Posted by edmundronald View Post
Why not have an emergency override switch that allows pilots to command the stabiliser RAPIDLY to a precise target position? Or one of a few conveniently spaced positions within its total excursion?
As others have stated repeatedly: Adding complexity to an inherently reliable system does not enhance safety. On the contrary, it is likely to introduce new and unanticipated failure modes.

Ironically this is exactly the outcome of the MCAS system. Trying to add another fix, to counter the problem created by MCAS, is the height of absurdity. Sorry if that is too blunt!
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Old 29th Mar 2019, 09:25
  #446 (permalink)  
 
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Originally Posted by GordonR_Cape View Post
As others have stated repeatedly: Adding complexity to an inherently reliable system does not enhance safety. On the contrary, it is likely to introduce new and unanticipated failure modes.

Ironically this is exactly the outcome of the MCAS system. Trying to add another fix, to counter the problem created by MCAS, is the height of absurdity. Sorry if that is too blunt!
Exactly.

Technically, the most practical solution would be to completely delete mcas and upgrade the stall feel mechanism from on/off to gradual. One little box involved, possibility of mayhem kept the same (very low), feel improved also in real life scenarios, requirement passed.

(For those like me looking for an elegant solution: Tiny fixed lift devices (vanes) at the lower sides of the plane near the tail (cirrus vision style) are not hard to tune to be lift neutral or even generating a bit of downforce at low aoa but increase its contribution to the pitch down moment by adding lift quicker than the stabilizer as aoa increases (thats the key, otherwise simply enlarging the stabilizer 1% would do). But this system would retain the fundamental MCAS fault: it will perhaps only work at one range of load factors, perfect for certification, not so perfect for real life).

Bureaucratically who knows what makes sense now.
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Old 29th Mar 2019, 09:40
  #447 (permalink)  
 
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Gordon to the point
Also see posts #2730-2732 and 2734 Ethiopian airliner down in Africa
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Old 29th Mar 2019, 13:04
  #448 (permalink)  
 
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Originally Posted by PEI_3721 View Post
Also see posts #2730-2732 and 2734 Ethiopian airliner down in Africa
Thanks. I missed that whole page (sometimes happens when there are multiple updates and thread renumbering at the same time). Edit: Posts that were held back for moderation and suddenly appear in the thread, also increment the counter.

This story seems to be unraveling quickly this week, reflecting badly on both Boeing and the regulators, but giving us (non-experts) a lot more clarity on how/why/when/where/what. Pity we had to have two crashes to find this all out...

Last edited by GordonR_Cape; 29th Mar 2019 at 13:34.
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Old 29th Mar 2019, 14:00
  #449 (permalink)  
 
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The 737Max has an uncertifiable characteristic whereby elevator control force decreases as AoA increases towards stall. It is stated that this is the sole reason for MCAS.

The 737 has an Elevator Feel Computer whose purpose is to increase elevator control force as airspeed increases. The control forces are applied hydraulically and are separate from Speed Trim System whose function is to maintain control force neutral.

Is there a simple reason why the EFC could not have been provided with an AoA proportional input so that this existing system could compensate for the control force reversal?

MCAS as a solution is a brutal method of stick force compensation. When operating it almost makes the aircraft impossible to stall. One might wonder what the Max stall characteristics are like.

Or am I being too cycnical?

Last edited by ktcanuck; 29th Mar 2019 at 14:55.
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Old 29th Mar 2019, 14:07
  #450 (permalink)  
 
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Presumably MCAS (a requirement for certification) doesn't appear in the MEL? Meaning, if it's broken or compromised, no pilot would ever know about it and therefore be at the mercy of luck in case they encountered a high AoA situation...The mind boggles.
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Old 29th Mar 2019, 16:16
  #451 (permalink)  
 
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ktcanuck

The 737 has an Elevator Feel Computer whose purpose is to increase elevator control force as airspeed increases. The control forces are applied hydraulically and are separate from Speed Trim System whose function is to maintain control force neutral.

Is there a simple reason why the EFC could not have been provided with an AoA proportional input so that this existing system could compensate for the control force reversal?
That question was comprehensively answered a few pages back in this thread. The elevator feel computer does not have enough hydraulic power to generate the feedback forces for certification: Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed

2. There is not enough column / elevator feel system adjustment range to provide the compensation that would be needed. This is particularly true at high Mach / speed where the nominal setting for the feel system is rather high to begin with. There is not enough range left in the system from there to max pressure to sufficiently offset the Cm-alpha characteristic that must be addressed to meet the FARs.
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Old 29th Mar 2019, 17:48
  #452 (permalink)  
 
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tail vanes 738/739



Vortex tabs-MAX wings..somethings just dont go away..



Did they have MCAS turned off!


Last edited by Smythe; 29th Mar 2019 at 20:54.
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Old 29th Mar 2019, 19:34
  #453 (permalink)  
 
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Originally Posted by Takwis View Post
MCAS does not appear in the MAX MEL.
MCAS is not a device or equipment. It is software code.
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Old 29th Mar 2019, 20:10
  #454 (permalink)  
 
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[QUOTE=GordonR_Cape;10433552]ktcanuck



That question was comprehensively answered a few pages back in this thread. The elevator feel computer does not have enough hydraulic power to generate the feedback forces for certification: /QUOTE]

My apologies. I thought I was following this thread comprehensively!
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Old 29th Mar 2019, 23:53
  #455 (permalink)  
 
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Originally Posted by GordonR_Cape View Post
Thanks. I missed that whole page (sometimes happens when there are multiple updates and thread renumbering at the same time). Edit: Posts that were held back for moderation and suddenly appear in the thread, also increment the counter.

This story seems to be unraveling quickly this week, reflecting badly on both Boeing and the regulators, but giving us (non-experts) a lot more clarity on how/why/when/where/what. Pity we had to have two crashes to find this all out...
Interesting assertion in a 737NG pilot's YT video - the KC-46 also has an MCAS-like system but it relies on input from both alpha vanes and disconnects on opposite control column input. Big difference in CG I understand but if true, why would Boeing not carry the same philosophy over to the MAX?

Well, would appear to be accurate.

http://www.airforcemag.com/Features/...AS-System.aspx
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Old 30th Mar 2019, 01:02
  #456 (permalink)  
 
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Originally Posted by Smythe View Post
Did they have MCAS turned off!
They didn't need to turn it off. Not stalling. Pitch was high, yet with an empty aircraft, velocity vector was high too.
It does raise a related issue: What pitch angle and/or vertical speed should MCAS be safely disabled?
I might have implemented a disable for pitch angle < 6 degrees, and possibly add an AND condtion with vertical speed < 0 which would keep the nose from auto-trimming down when descending at a low or negative pitch angle.
What condition would you disable MCAS down-trim on? Boeing isn't buying any of that of course, yet doesn't keep us from suggesting something else.

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Old 30th Mar 2019, 01:36
  #457 (permalink)  
 
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exactly...what are the parameters?
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Old 30th Mar 2019, 04:42
  #458 (permalink)  
 
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Originally Posted by b1lanc View Post
Interesting assertion in a 737NG pilot's YT video - the KC-46 also has an MCAS-like system but it relies on input from both alpha vanes and disconnects on opposite control column input. Big difference in CG I understand but if true, why would Boeing not carry the same philosophy over to the MAX?

Well, would appear to be accurate.

Air Force Magazine
The use of MCAS on the KC-767 (KC-46) was broken 4 days ago by Aviation Week, and posted 4 pages previously in this very thread:
https://aviationweek.com/defense/boe...d-pitch-system
Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed

Last edited by GordonR_Cape; 30th Mar 2019 at 04:55. Reason: Typo.
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Old 30th Mar 2019, 09:12
  #459 (permalink)  
 
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Originally Posted by QuagmireAirlines View Post
They didn't need to turn it off. Not stalling. Pitch was high, yet with an empty aircraft, velocity vector was high too.
It does raise a related issue: What pitch angle and/or vertical speed should MCAS be safely disabled?
I might have implemented a disable for pitch angle < 6 degrees, and possibly add an AND condtion with vertical speed < 0 which would keep the nose from auto-trimming down when descending at a low or negative pitch angle.
What condition would you disable MCAS down-trim on? Boeing isn't buying any of that of course, yet doesn't keep us from suggesting something else.
MCAS (a proper one) would need to be active at any pitch angle and vertical speed. One can pull to a stall while going up, level or down.

And for those suggesting that artificial feel is not powerful enough, I don't agree, it makes one sweat if it goes haywire, does it not. That's not the problem, the problem is that it uses all its power in some circumstances now so it would be saturated if we superposed a new one. (And if it wouldn't, it would be too strong to physically overcome anyway).

Obvious solution: we would need to reduce the feel by a factor of 0.8 across all range and then use the new 20% headroom for MCAS. The rule is concerned about the linearity, I am almost sure that the feel in the 737 has a lot of margin to be softened.

I'm talking a fairly small double sided cylinder with 20% more area in the downstream side, feel computer connected to one chamber, feel swinging assembly to the other; and a big ass solenoid to supplement the pressure in the event of an MCAS event. One for each feel hydraulic circuit.

The feel "computer" (impressive bunch of pistons and springs and little pipes) would not need upgrading. And feel system is always possible to handle if something goes completely wrong (remember it is a peculiar system, force is always zero in the center by design).

Still I would prefer an aero solution like the vanes described before, it would involve much harder design but much easier retrofit.
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Old 30th Mar 2019, 10:02
  #460 (permalink)  
 
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Originally Posted by Lake1952 View Post
MCAS is not a device or equipment. It is software code.
From posts elsewhere, it would appear to be a sub routine of the Flight Control Computer ergo any MEL alleviation would be listed against the said Flight Control Computer, if no MEL listing for FCC, then it is non-despatchable.
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