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airhansa
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 6:20 pm

kalvado wrote:
airhansa wrote:
I think it's ridiculous to suggest that designing an airplane that is not able to aerodynamically fly without the input of automated controls (MCAS) is acceptable. Boeing's mistake was not that the automated system was malfunctioning, but rather than the plane itself cannot fly as it is expected to by nearly everyone.

we live in the world where software patches to help things work properly are the norm for complex systems. Especially when we're talking about edge cases, like MCAS was supposed to adress.


The problem is that there are patches required in the first place. A patch should only be a temporary or exceptional solution to a problem - it's okay to rivet a sheet of metal onto a plane to cover a hole, but can you imagine the fury if a plane was designed like that? A software patch should not have been implemented before the plane even got handed to the customer. Furthermore these planes are expected to be flown manually without input from the automated system. MCAS might be a good idea only in a situation where a plane is flown automatically. It doesn't speak wonders for the technical ability of the Boeing team when they can't design a plane that can fly aerodynamically - it's one of the basic concepts of a plane and constantly having to go into stall recovery mode is not a flyable plane..
 
889091
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 6:22 pm

As the saying goes, "It takes two to tango."

By that I mean - how could a certifying authority be hoodwinked to the level that the FAA was, when it signed off on the MAX.

We've seen heads roll/more will roll(?) at Boeing. Are we to expect the same from the FAA side of things?
 
kalvado
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 6:28 pm

airhansa wrote:
kalvado wrote:
airhansa wrote:
I think it's ridiculous to suggest that designing an airplane that is not able to aerodynamically fly without the input of automated controls (MCAS) is acceptable. Boeing's mistake was not that the automated system was malfunctioning, but rather than the plane itself cannot fly as it is expected to by nearly everyone.

we live in the world where software patches to help things work properly are the norm for complex systems. Especially when we're talking about edge cases, like MCAS was supposed to adress.


The problem is that there are patches required in the first place. A patch should only be a temporary or exceptional solution to a problem - it's okay to rivet a sheet of metal onto a plane to cover a hole, but can you imagine the fury if a plane was designed like that? A software patch should not have been implemented before the plane even got handed to the customer. Furthermore these planes are expected to be flown manually without input from the automated system. MCAS might be a good idea only in a situation where a plane is flown automatically. It doesn't speak wonders for the technical ability of the Boeing team when they can't design a plane that can fly aerodynamically - it's one of the basic concepts of a plane and constantly having to go into stall recovery mode is not a flyable plane..

Do you drive? If you do, very likely your car has anti-locking brakes and vehicle stability assist - or similar/advanced systems, whatever manufacturers call them. Bigger vehicles have their own versions of such systems, so you likely use those even if you're not driving. Both are pretty much like MCAS - kick in when the driver screws up to help straighten things up.
So this is exact same story - software patch to help things work, and possibly not fully meeting regulations with those systems disables. But if properly designed, it works just fine. Doesn't mean that design may be lousy - but correcting edge cases is usually OK
I am sure there are bunch of other examples of such software around you...
 
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SomebodyInTLS
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 6:37 pm

Ertro wrote:
It is pretty clear that at that moment when major architectural decisions were made about the landing gear and engine placement nobody saw the problems they are going to run into years later and the need to have anything like MCAS at all.

[...]

Redesigning landing gear was not seen as an easy upgrade so it was left out. Just hang the engines on the wings and that's it. All done.


I beg to differ. The threads at the time were full of discussion how it was going to be impossible to fit the NEO engines without extending the gear, plus there were many mentions of folding or telescopic gear as possible solutions. We also knew that doing that was going to incur additional work and certification, including the need to add slides and probably re-certify for evacuation.

The point was that everyone in those discussions realised that trying to squeeze the new engines into the available space *was* going to compromise the aero and other dynamics. Boeing seemed to think the performance hit would be worth it over the additional time and costs associated with changing the gear, but that doesn't change the fact that we already knew it was going to perform sub-optimally.

Also, it was clear that it would be a lot harder than "just hang the engines and all done" - the engines were obviously going to be a lot further forward and higher up than for any other conventional design, with all the complications that entails.

The only surprise is that "sub-optimally" is putting it mildly, and that Boeing - themselves surprised at how bad it really was - attempted to conceal it.
Last edited by SomebodyInTLS on Wed Jan 15, 2020 6:43 pm, edited 4 times in total.
"As with most things related to aircraft design, it's all about the trade-offs and much more nuanced than A.net likes to make out."
 
WIederling
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 6:38 pm

AirBoat wrote:
The reason for not doing structural modifications on the max is that any change will trigger a complete re analysis and design of every single structure, which could take years..


Changes on structure are further reaching but not too visible.
structure for the complete tail was changed. not much talk about it "1% reduced drag"
better airflow on the tail should also have improved stab and control surface effectiveness.
( they dropped a handful of vortex generators.)

From fallout elsewhere into public knowledge:
pickle forks were changed.( probably just the production process .. or more? )
probably a range of other little "optimizations" all around.
cockpit and e-bay stuffing seems to have changed/been rearranged.

All waved away per Jedi mind trick :-))
Last edited by WIederling on Wed Jan 15, 2020 6:40 pm, edited 1 time in total.
Murphy is an optimist
 
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par13del
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 6:45 pm

kalvado wrote:
Well, bit flip is not punitive.

Remind us again on the risk factor of the bit flip analysis that the FAA tested, and please don't leave out any of the zero's in the numbers, since safety and risk factors are at the heart of this grounding.
 
AirBoat
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 6:59 pm

I will assume that boeing can optimize a structure to be as light as possible, ie all parts are near maximum allowable stress.
This is the problem. If aero changes are now made which cause increased loading, they are overstressed and up the creek.
This is why any changes which lead to additional loading on structures are non-starters.
 
Ertro
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 7:17 pm

DenverTed wrote:
Ertro wrote:
...

I'm not sure that the FAA was giving them the option to build a new wingbox, longer gear, and keep all the old substandard stuff and still call it a 737. As Keesje points out, what was supposed to be the threshold of changes for complete review and compliance with current regulations?


I am not claiming that FAA gave them options. I was making the argument that most probably Boeing never went even asking for it since Boeing most probably did not want to do any of those things themselves either. So FAA not giving them options is not any deciding pivotal point in the matter.

SomebodyInTLS wrote:
Ertro wrote:
It is pretty clear that at that moment when major architectural decisions were made about the landing gear and engine placement nobody saw the problems they are going to run into years later and the need to have anything like MCAS at all. [...] Redesigning landing gear was not seen as an easy upgrade so it was left out. Just hang the engines on the wings and that's it. All done.


I beg to differ. The threads at the time were full of discussion how it was going to be impossible to fit the NEO engines without extending the gear, plus there were many mentions of folding or telescopic gear as possible solutions. We also knew that doing that was going to incur additional work and certification, including the need to add slides and probably re-certify for evacuation.

The point was that everyone in those discussions realised that trying to squeeze the new engines into the available space *was* going to compromise the aero and other dynamics. Boeing seemed to think the performance hit would be worth it over the additional time and costs associated with changing the gear, but that doesn't change the fact that we already knew it was going to perform sub-optimally.

Also, it was clear that it would be a lot harder than "just hang the engines and all done" - the engines were obviously going to be a lot further forward and higher up than for any other conventional design, with all the complications that entails.

The only surprise is that "sub-optimally" is putting it mildly, and that Boeing - themselves surprised at how bad it really was - attempted to conceal it.


Boeing and a.net might have had different opinions on the matter. Boeing could have been more optimistic and a.net more pessimistic. I don't buy that Boeing foresaw all the problems and brick wall and went for it anyway. To me more sensible theory is that Boeing thought they can make it work and that means without extending the landing gear since that is what was part of the grand plan. Boeing clearly disagreed with A.net since if they would have agreed then they would not have started the MAX project in the way that they did.

The need for MCAS coming as a surprise to Boeing is pretty clear? Is it not? First the high speed MCAS and then the slow speed MCAS. Both as a surprise. They thought they can tweak the aerodynamics with some metal pieces and to avoid high speed MCAS. And since MCAS was a surprise that means the need to extend landing gear would have been also a surprise. They thought they can make it work with original landing gear and without MCAS at all even the high speed one. That was the original plan. I Think.
Last edited by Ertro on Wed Jan 15, 2020 7:24 pm, edited 1 time in total.
 
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par13del
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 7:22 pm

DenverTed wrote:
Do you think the bit flip issue could be found on the NG as well? I am surprised it is only an issue on the MAX. Not that I think they need to fix it on the NG, but they should just say that the NG is grandfathered in at a lower safety factor if that is the case.

We have had a lot of articles so things are lost in the shuffle, but according to the articles the ability for but flip to occur on the NG is roughly the same as the MAX.
The real question would be did the FAA use a real world scenario to fail MCAS 2.0 and to get other improvements to the MAX, and why as the regulator they thought this was the best way to get improvements on the cash cow product for Boeing. We have a number of posters now talking about not being able to do additional changes to the a/c because of grandfathering, is this a similar situation?

"What the FAA was testing when it discovered this new vulnerability was esoteric and remote. According to the person familiar with the details, who asked for anonymity because of the sensitivity of the ongoing investigations, the specific fault that showed up has “never happened in 200 million flight hours on this same flight-control computer in [older model] 737 NGs.”

https://www.seattletimes.com/business/b ... -controls/
 
XRAYretired
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 7:37 pm

par13del wrote:
kalvado wrote:
Well, bit flip is not punitive.

Remind us again on the risk factor of the bit flip analysis that the FAA tested, and please don't leave out any of the zero's in the numbers, since safety and risk factors are at the heart of this grounding.

Which bit (no pun intended) do you still not get? Bit flip is a standing requirement for decades. The discovery of catastrophic failure modes has required compliance to be re-assessed. As with all catastrophic failure modes, it must be demonstrated that the probability better than E-9 is achieved for occurrence (consistently). The test defined and performed by Boeing, no doubt because it their responsibility (not FAA), did not achieve this. Just in case you are worried, I would expect that 5 bits is consistent with the calculated number of bits that could be flipped in a flight leg with an E-9 probability based upon the number of bits at risk/area at risk and the flux density of high energy neutrons during the selected flight profile.

Just in case you are still wondering, its worth stating again that the E-9 is applicable to any and all catastrophic failure modes nothing special here.

Just in case you are still wondering, bit flip is random and therefore worst case selection, of 5 bits, is made so you only have to do the test once (i.e. not for every possible combination of 5 bits).

Now, it may be that the assessment was based on Hazardous modes rather than catastrophic or as well as, in which case, replace E-9 with E-7 Its precisely the same, since the result was fail.

Ray
Last edited by XRAYretired on Wed Jan 15, 2020 7:45 pm, edited 1 time in total.
 
Checklist787
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 7:42 pm

airhansa wrote:
I think it's ridiculous to suggest that designing an airplane that is not able to aerodynamically fly without the input of automated controls (MCAS) is acceptable. Boeing's mistake was not that the automated system was malfunctioning, but rather than the plane itself cannot fly as it is expected to by nearly everyone.


No. The mistake is just that there were not enough AOA's.

Otherwise the 737MAX flies very well!
"No limit to my poooWer!!!
Do it! "...
 
KarlB737
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 7:45 pm

I add this as it will be always be debatable:

Courtesy: Fox News

Will Boeing's 737 Max be safe if it ever returns to the skies?

""The airplane will be safe once the FAA, in fact, does certify. They're going to be crossing all their T's and dotting all their I's," said Kyle Baily, who is a pilot and former Federal Aviation Administration (FAA) Safety Team representative.

However, troubling internal emails from Boeing employees and apparent delays in the FAA's decision to approve the aircraft for service are raising questions among pilots and passengers.

Baily noted that the public does not yet know the reason for an apparent delay in the 737 Max returning to service."

https://www.foxnews.com/media/boeing-737-max-jet-safe-faa
 
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PW100
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 7:55 pm

phollingsworth wrote:
. . .
Because of the history of the 737 the STS uses only one Pitot-Static system at a time (I believe there are two computers that can be switched between). Therefore if the pitot-static system fails to a low speed the system will runaway nose down. Boeing use the now infamous 'bad maths' to show that this STS was ok. It turns out that it probably is, even if the pilots are less likely to diagnose it properly and quick as the failure rate of the system is much lower than the original estimates.


Thanks for your excellent and well layed out posts. It's really enjoyable to read them amongst some of the garbage being trown around here.

Just want to add that apart from the low STS failure rate, another consideration is that for most STS failure modes control column inputs override/stops STS (unlike MCAS, as that would defeat its primary purpose).
Immigration officer: "What's the purpose of your visit to the USA?" Spotter: "Shooting airliners with my Canon!"
 
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par13del
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 8:06 pm

XRAYretired wrote:
Which bit (no pun intended) do you still not get?
Ray

...the part where folks are going around saying the NG is proven safe yet it is vulnerable to the same cosmic ray bit flip error and the FAA has not grounded the NG nor so far required Boeing to enact the same bit flip protections for the NG.

In case you are not following the point I was trying to address, the bit flip failure that the FAA used to fail MCAS in my opinon was not a real world test scenario.
https://www.seattletimes.com/business/b ... -controls/

If they wanted to make the MAX safer, unless restricted by grandfathering, there were better ways to get the dual computer use other than using MCAS which already had the loss of almost 400 lives on its plate.
Last edited by par13del on Wed Jan 15, 2020 8:09 pm, edited 1 time in total.
 
smartplane
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 8:08 pm

PW100 wrote:
phollingsworth wrote:
. . .
Because of the history of the 737 the STS uses only one Pitot-Static system at a time (I believe there are two computers that can be switched between). Therefore if the pitot-static system fails to a low speed the system will runaway nose down. Boeing use the now infamous 'bad maths' to show that this STS was ok. It turns out that it probably is, even if the pilots are less likely to diagnose it properly and quick as the failure rate of the system is much lower than the original estimates.


Thanks for your excellent and well layed out posts. It's really enjoyable to read them amongst some of the garbage being trown around here.

Just want to add that apart from the low STS failure rate, another consideration is that for most STS failure modes control column inputs override/stops STS (unlike MCAS, as that would defeat its primary purpose).

Unfortunately, Boeing tinkered with a perfectly stable, satisfactory STS, and retrospectively added MCAS-related functionality.

Hopefully the joint FAA/EASA tests will include exploring the flying characteristics with MCAS off, and then MCAS and STS off.
 
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par13del
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 8:12 pm

smartplane wrote:
Unfortunately, Boeing tinkered with a perfectly stable, satisfactory STS, and retrospectively added MCAS-related functionality.

If we believe all the reports that have been released, they added MCAS before flight test because it was needed, whether aero or rules is another question, but it was also reported that during actual flight testing they had to increase the authority.
 
XRAYretired
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 8:30 pm

par13del wrote:
XRAYretired wrote:
Which bit (no pun intended) do you still not get?
Ray

...the part where folks are going around saying the NG is proven safe yet it is vulnerable to the same cosmic ray bit flip error and the FAA has not grounded the NG nor so far required Boeing to enact the same bit flip protections for the NG.

In case you are not following the point I was trying to address, the bit flip failure that the FAA used to fail MCAS in my opinon was not a real world test scenario.
https://www.seattletimes.com/business/b ... -controls/

If they wanted to make the MAX safer, unless restricted by grandfathering, there were better ways to get the dual computer use other than using MCAS which already had the loss of almost 400 lives on its plate.

From the beginning. Bit Flip is a standing requirement and has been for decades. The problem has arisen because of the discovery of catastrophic failure modes introduced by MCAS that were not present previously on NG and not recognised on MAX originally. It is likely that both NG and original MAX were assessed in this regard but, in the absence of those failure modes, achieved a pass (possibly without a test being required).

If there is no change to the categorisation of the failure modes in NG, then the original pass result may be upheld, for NG.

Just to throw it in to the mix, Mr Markos leaked email implies that they now recognise catastrophic failure modes associated with Speed Trim. Assuming this is so and they relate to the FCC, the NG assessment would have been re-visited to assess if a combination of 5 bits could result in the scenario of the revised category failure modes, and if so a test may have been performed. In any case, there is a ready made solution in the FCC X-Check developed for the MAX problem that is reported to apply to MCAS, Speed Trim and Mach Trim. Either way, Boeing/FAA may elect to apply the X-Check for Speed Trim and Mach Trim to NG.

Ray

Edit: Sorry, second part of you post. Bit flip is developed from measurement based science including much time and effort by NASA (including scientific test flights). Go tell em they got it wrong. I see nothing that suggests any subterfuge or alteria motives in the application of standing requirements.
Last edited by XRAYretired on Wed Jan 15, 2020 8:47 pm, edited 1 time in total.
 
XRAYretired
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 8:35 pm

smartplane wrote:
PW100 wrote:
phollingsworth wrote:
. . .
Because of the history of the 737 the STS uses only one Pitot-Static system at a time (I believe there are two computers that can be switched between). Therefore if the pitot-static system fails to a low speed the system will runaway nose down. Boeing use the now infamous 'bad maths' to show that this STS was ok. It turns out that it probably is, even if the pilots are less likely to diagnose it properly and quick as the failure rate of the system is much lower than the original estimates.


Thanks for your excellent and well layed out posts. It's really enjoyable to read them amongst some of the garbage being trown around here.

Just want to add that apart from the low STS failure rate, another consideration is that for most STS failure modes control column inputs override/stops STS (unlike MCAS, as that would defeat its primary purpose).

Unfortunately, Boeing tinkered with a perfectly stable, satisfactory STS, and retrospectively added MCAS-related functionality.

Hopefully the joint FAA/EASA tests will include exploring the flying characteristics with MCAS off, and then MCAS and STS off.

Speed Trim was not touched NG to MAX. Referring to MCAS as part of Speed Trim was a device employed by Boeing to 'hide' MCAS from scrutiny ref. the recent batch of internal emails that makes is abundantly clear to be the case.

Ray
 
kalvado
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 8:50 pm

par13del wrote:
kalvado wrote:
Well, bit flip is not punitive.

Remind us again on the risk factor of the bit flip analysis that the FAA tested, and please don't leave out any of the zero's in the numbers, since safety and risk factors are at the heart of this grounding.

People are quoting "those exact bits" scenario, which is indeed low probability. I consider this as a proxy for all possible computer errors, though, so probabilities add up...
 
MartijnNL
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Wed Jan 15, 2020 9:09 pm

https://www.volkskrant.nl/nieuws-achter ... ~bdb9f623/

Great article about the 737 Max. Easy to understand for everyone.

If you can read Dutch, I must add. ;) But you can copy past the article (in a few parts) in Google Translate.

Unbelievable some members continue to blame the pilots and defend Boeing. I think most aviation fans, including me, love Boeing, but nobody can deny the Max story has developed into a really bad dream.
 
acechip
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 2:16 am

airhansa wrote:
I think it's ridiculous to suggest that designing an airplane that is not able to aerodynamically fly without the input of automated controls (MCAS) is acceptable. Boeing's mistake was not that the automated system was malfunctioning, but rather than the plane itself cannot fly as it is expected to by nearly everyone.

You have hit the nail on the head. Almost none of the commercial airplanes are designed to be inherently unstable with automated corrections. This is rather different from envelope protection, where by the system prevents erroneous inputs that will lead to an out-of-control situation. Here, the situation that goes out-of-control by manual input is expected to be auto-corrected by software. In the sense, you are already causing a problem in the first place because of inadequate design.
 
LMP737
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 2:24 am

Revelation wrote:
Indeed, and it's hard to see someone with Calhoun's background leading the revolution.


Guys like Calhoun are part of the problem. Remember, he was on the BOD while all this was going on. It's going to be more of the same, just with a different name on the corner office.
Never take financial advice from co-workers.
 
planecane
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 3:06 am

par13del wrote:
smartplane wrote:
Unfortunately, Boeing tinkered with a perfectly stable, satisfactory STS, and retrospectively added MCAS-related functionality.

If we believe all the reports that have been released, they added MCAS before flight test because it was needed, whether aero or rules is another question, but it was also reported that during actual flight testing they had to increase the authority.

Not exactly. During flight test they discovered the problem that was expected at high speed also happened at low speed and that MCAS was needed at low speed. Authority was increased because control surfaces need to move more at low speed, not because the predicted high speed issue was worse.
 
planecane
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 3:10 am

XRAYretired wrote:
par13del wrote:
XRAYretired wrote:
Which bit (no pun intended) do you still not get?
Ray

...the part where folks are going around saying the NG is proven safe yet it is vulnerable to the same cosmic ray bit flip error and the FAA has not grounded the NG nor so far required Boeing to enact the same bit flip protections for the NG.

In case you are not following the point I was trying to address, the bit flip failure that the FAA used to fail MCAS in my opinon was not a real world test scenario.
https://www.seattletimes.com/business/b ... -controls/

If they wanted to make the MAX safer, unless restricted by grandfathering, there were better ways to get the dual computer use other than using MCAS which already had the loss of almost 400 lives on its plate.

From the beginning. Bit Flip is a standing requirement and has been for decades. The problem has arisen because of the discovery of catastrophic failure modes introduced by MCAS that were not present previously on NG and not recognised on MAX originally. It is likely that both NG and original MAX were assessed in this regard but, in the absence of those failure modes, achieved a pass (possibly without a test being required).

If there is no change to the categorisation of the failure modes in NG, then the original pass result may be upheld, for NG.

Just to throw it in to the mix, Mr Markos leaked email implies that they now recognise catastrophic failure modes associated with Speed Trim. Assuming this is so and they relate to the FCC, the NG assessment would have been re-visited to assess if a combination of 5 bits could result in the scenario of the revised category failure modes, and if so a test may have been performed. In any case, there is a ready made solution in the FCC X-Check developed for the MAX problem that is reported to apply to MCAS, Speed Trim and Mach Trim. Either way, Boeing/FAA may elect to apply the X-Check for Speed Trim and Mach Trim to NG.

Ray

Edit: Sorry, second part of you post. Bit flip is developed from measurement based science including much time and effort by NASA (including scientific test flights). Go tell em they got it wrong. I see nothing that suggests any subterfuge or alteria motives in the application of standing requirements.


I think but flip is an issue on MAX and not NG because MAX has MCAS. The bit flip can cause an MCAS runaway. On the NG it would only produce runaways that are easily contracted with opposite column force.
 
smartplane
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 3:27 am

XRAYretired wrote:
smartplane wrote:
PW100 wrote:

Thanks for your excellent and well layed out posts. It's really enjoyable to read them amongst some of the garbage being trown around here.

Just want to add that apart from the low STS failure rate, another consideration is that for most STS failure modes control column inputs override/stops STS (unlike MCAS, as that would defeat its primary purpose).

Unfortunately, Boeing tinkered with a perfectly stable, satisfactory STS, and retrospectively added MCAS-related functionality.

Hopefully the joint FAA/EASA tests will include exploring the flying characteristics with MCAS off, and then MCAS and STS off.

Speed Trim was not touched NG to MAX. Referring to MCAS as part of Speed Trim was a device employed by Boeing to 'hide' MCAS from scrutiny ref. the recent batch of internal emails that makes is abundantly clear to be the case.

MCAS was described under the STS umbrella, to reduce the risk of scrutiny.

Excluding the so-called MCAS sub-system, the core STS system (code and parameters) was modified between NG and MAX, with some specific MCAS-style capability buried within to defeat scrutiny. Later NG deliveries may have included the later STS / MCAS code, though without MCAS proper, it was window dressing to fool the authorities.

If the FAA compare early NG and MAX STS there will be obvious differences.

When we read of the illusions Boeing was creating with the airworthiness authorities and customers, concealing specific MCAS functionality within STS was considered easy, low hanging fruit, nowhere even close to Jedi mind tricks.
 
speedbird52
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 3:45 am

I made a short argument trying to debunk the idea that it is better for Boeing to give up on the MAX. Can anyone with more knowledge than me critique?

The idea that Boeing can afford to ditch the 737 MAX program is perhaps the stupidest assertion I have ever read. I decided to do a little math based off of the little public information availible:

The development cost of the 737 MAX program was 1.8 billion dollars. Boeing have sold 387, all at a list price of between 100 million-134 million. But airplanes seldom sell for list price, in fact they sell far below list price. Let’s assume that each 737 MAX sold for around 50 million based off of this article from 2009 showing that American paid roughly half price for 737 NGs:

https://blog.seattlepi.com/aerospace/20 ... 2-million/

The result is that Boeing has made 19.5 Billion dollars on the program from airlines. Do you think airlines are going to let that money go if Boeing cancels the MAX program? Absolutely not. They will want every penny back, PLUS additional compensation. Boeing also has 400 737 MAX’s sitting around in Arizona, Renton, Seattle and Everett. None have been delivered, so none have made Boeing any money. I have no access to data with regards to how much it costs to build a 737 MAX, but I am going to assume it is around 10 million dollars. 787*50 million is roughly 19.3 billion dollars. Every penny sent straight to the scrapyard.

I know what you are thinking: “Well, airlines could just order the NSA, they will sure be happy to pick it up”. There is absolutely no way Boeing can develop the NSA in less than 5 years. Don’t forget how much money the program is going to cost to develop. And since the 737, Boeing’s main cash revenue, is gone, Boeing won’t have much money to develop the NSA. Guestimating from the 787 development cost, being EXTREMELY optimistic, I would guess NSA development cost would be at least 15 billion dollars. Add that to the debt pile. Also, the FAA is going to be in no hurry to certify the NSA, seeing as the last time they rushed the process, hundreds died. In all the time it will take Boeing to develop the NSA, the airlines aren’t going to sit on their hands and wait: There are a lot of 737 NGs needing replacement. You know what product can replace them? Bombardier C-Series and A320NEOs of course! Airlines aren’t going to do Boeing a favor: Airbus will likely provide great pricing, and airlines will have a brand new type in their fleet. And just like that, most of the 737NG replacement market (The main market for the 737 MAX and NSA by the way) is gone.

Well, it’s here: The Boeing 797 NSA. Now Boeing can take back some market share, since Airbus will have been doing nothing in the last five years.

Oh right. Airbus WILL be doing something if Boeing is developing an NSA. Maybe we will see an A220-500 and A220-700 stretch for the lower range market, and a rewinged A320 and A321 for the mid to long range market. Speaking of which: There goes the market case for an NMA assuming the A320 is still efficient enough. But why go down the path Boeing went on? Why not make a new aircraft entirely. And just like that, Airbus is now competitive with Boeing, and what has already become a drastically shrunk market for Boeing just shrunk in half.

Did I mention that the 737 is where most of BCAs profits are? So in the 5 years Boeing has had no narrowbody aircraft, their stock price would have been in freefall. Which means less money to fund NMA.

Lets do some math now: 19.3 billion dollars spent physically building the aircraft. 19.5 Billion refunded to airlines. 15 Billion spent on developing a new aircraft. Boeing has now lost 34 BILLION DOLLARS. This does not factor loss of stock price, loss of investors, loss of consumer trust, reimbursement costs, and loss of market size.

At the moment, assuming the 737 MAX flies again this year, Boeing are set to loose 9.2 billion.

https://qz.com/1734220/the-737-max-has- ... -counting/
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 6:35 am

LMP737 wrote:
Revelation wrote:
Indeed, and it's hard to see someone with Calhoun's background leading the revolution.


Guys like Calhoun are part of the problem. Remember, he was on the BOD while all this was going on. It's going to be more of the same, just with a different name on the corner office.


Unless Calhoun changes the policy!

People love hitting Boeing a lot even when things was going better for them. It is not normal that already in 2004 the 7E7 / 787 at the time was not a good idea... :roll:

Many argue without obvious evidence. Changing a working method is feasible it is the responsibility of any large company.

No company was, no company is, and no company will immune to error! :)
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Do it! "...
 
RickNRoll
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 7:51 am

PixelFlight wrote:
patrickjp93 wrote:
PixelFlight wrote:
Ok, you are now maybe ready to understand that Boeing did no want any sim training for the 737 MAX, not because of the regulators, but because Boeing promised to the operators that there will not require sim training time to the pilots flying the MAX and, even agree to pay 1M$ per aircraft in case that promise will be broken. Really, it's not the regulator that enforced that impossible goal, but Boeing itself !

The goal WAS possible, if the FAA hadn't been incompetent rubes with shoddy regulatory frameworks. There's no scientifically/mathematically sound reason the 737 MAX shouldn't be grandfathered just because of a new redundant sensor system or because it gets tall enough to not need it.

It was Boeing impossible promise to the operators. The regulator did not force Boeing to set that goal.


Exactly. Boeing had painted themselves into a corner. They knew exactly where the corner was all along.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 8:59 am

kalvado wrote:
DenverTed wrote:
phollingsworth wrote:

The original Speed/Mach trim function was created to account for the shift in neutral point as the Mach number increases and decreases, e.g. avoid Mach Tuck (which can kill you). However, over the years is have been expanded to provide a number of additional functions, primarily around speed stability. This is critical at low speed, high AoA where if the aircraft gets behind the power curve and is negatively stable in speed you can rapidly approach stall. STS does this by actively trimming the aircraft to a higher speed than it is currently flying at (eg more nose down). When the 737NG was certified in the mid 1990s the FAA was fine with the handling and stall ID characteristics of the aircraft. However, the JAA baulked at it. The JAA, mainly from the legacy of the UK CAA is very stall averse. As such they required an augmentation system to help identify stall (different than avoidance). This is because the 737 does not have hard envelope limits. Since the NGs behaviour around stall only needs the pilot to let go of the yoke, Boeing got away without using a stick pusher. Instead they made STS even more aggressive as Vs is approached, making it even harder to hold AoA. This effectively makes the aircraft behave like it has a clean stall before it even stalls. Boeing even did a press release about this in 1997. Because of the history of the 737 the STS uses only one Pitot-Static system at a time (I believe there are two computers that can be switched between). Therefore if the pitot-static system fails to a low speed the system will runaway nose down. Boeing use the now infamous 'bad maths' to show that this STS was ok. It turns out that it probably is, even if the pilots are less likely to diagnose it properly and quick as the failure rate of the system is much lower than the original estimates.

Do you think the bit flip issue could be found on the NG as well? I am surprised it is only an issue on the MAX. Not that I think they need to fix it on the NG, but they should just say that the NG is grandfathered in at a lower safety factor if that is the case.

with NG, service history is a big reason to keep things untouched.
Otherwise STS logic failure / trim runaway and response to that should be reviewed. General narrative we hear so far is that software runaway in NG is stopped by column switches and, as such, can be quickly arrested by pilot's first responce action. Which may or may not be the second good reason to keep NG as-is.


I agree that history is a important with assessing the safety of STS on the NG. It can also help us assess the safety of MCAS, if used appropriately. In addition to the history of 'reasonable' responses to STS runaways, the question of bit flip is something people should look at in general. For STS, and many other systems on the NG the bit-flip is inherently a non issue. The reason for this is that all of these systems are purely simplex in nature. Any redundancy, fail-over, etc is performed by the flight crew. There are no comparators in the logic between more than one sensor that could be affected by a bit-flip. This means that you have divergent failure modes between the two key parts of the system, eg the computer and the human.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 9:13 am

PW100 wrote:
phollingsworth wrote:
. . .
Because of the history of the 737 the STS uses only one Pitot-Static system at a time (I believe there are two computers that can be switched between). Therefore if the pitot-static system fails to a low speed the system will runaway nose down. Boeing use the now infamous 'bad maths' to show that this STS was ok. It turns out that it probably is, even if the pilots are less likely to diagnose it properly and quick as the failure rate of the system is much lower than the original estimates.


Thanks for your excellent and well layed out posts. It's really enjoyable to read them amongst some of the garbage being trown around here.

Just want to add that apart from the low STS failure rate, another consideration is that for most STS failure modes control column inputs override/stops STS (unlike MCAS, as that would defeat its primary purpose).


The aft column cutout question is one that has me scratching my head. Mainly because I run into the end of my knowledge of the specifics of the user needs for STS in the stall ID function. The original STS function, of creating speed stability through the flight envelope is well served with fore and aft column cutouts. However, for stall ID the aft column cutout could easily defeat the purpose of the system existing in the first place.

If, for a minute, we assume that the column cutout function on STS is implemented in a non-self defeating way then the presence of the aft column cutout on an AoA driven stall ID function isn't an issue. Were it would be an issue is to meet stick force requirements, in fact it would make the whole situation worse. Instead of a stick force that degrades smoothly it would have a hard jump when the system cuts-out.

This brings us back to the training regime around MCAS, for both vs 1.0 and 2.0. The removal of the aft column cutout to stop trim runaways changes the behaviour of the aircraft. Given that most pilots are taught to 'yank' the column first it delays response to any runway. To make maters worse on the MAX is column cutout still works for some failures but not for others. This is a recipe for bad outcomes. Couple this will the fact that the failure mode presentation has changed, from continuous to intermittent and things get bad fast. As a system operator changes in behaviour over what is expected/trained are critical. Even if an intermittent runaway failure is theoretically possible on the NG STS it doesn't look like it is really trained for/ever happens.

What I would like to see is if we train all 737 pilots to counter unexpected trim solely with the trim switches as a first response do we get acceptable failure responses. This is very different from taking a bunch of competent pilots and telling them not to use the column cutout. If we do, then stop training the use of column movement to stop an STS failure. This will make flight-crew response better overall as there is less to commit to memory.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 9:22 am

XRAYretired wrote:
smartplane wrote:
PW100 wrote:

Thanks for your excellent and well layed out posts. It's really enjoyable to read them amongst some of the garbage being trown around here.

Just want to add that apart from the low STS failure rate, another consideration is that for most STS failure modes control column inputs override/stops STS (unlike MCAS, as that would defeat its primary purpose).

Unfortunately, Boeing tinkered with a perfectly stable, satisfactory STS, and retrospectively added MCAS-related functionality.

Hopefully the joint FAA/EASA tests will include exploring the flying characteristics with MCAS off, and then MCAS and STS off.

Speed Trim was not touched NG to MAX. Referring to MCAS as part of Speed Trim was a device employed by Boeing to 'hide' MCAS from scrutiny ref. the recent batch of internal emails that makes is abundantly clear to be the case.

Ray


You are correct in that the STS as driven by airspeed alone was not touched. What Boeing did do was add AoA to the stall ID portion of the 'speed' trim system. MCAS in all variants is grafted onto the existing STS. The fact is that, in hindsight, using airspeed alone to drive a stall ID system is a bit of kludge. Stall is not a first order function of airspeed. Using AoA would have been far better from the start. However, Boeing has always tried to minimise changes from one version of the 737 to the next, for good or bad. So logically adding authority to the STS system was the preferred method back in the 1990s.

With the MAX Boeing realised you could no longer rely on a secondary measurement for sensing and going to and AoA based system was required, this is for the stick feel in MCAS0.9. Logically, it then makes sense to use it in a more aggressive stall ID function. However, this plus the means of application opened up a whole other can of worms.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 9:25 am

If the pilots detect a runaway trim within 3 seconds (like Boeing intended), they would disable MCAS manually and it would not function as intended. One must be honest and accept that safety was not a driving factor for the development, it was costs and keeping fleet commonality. The rest was trying to smuggle the system past the regulator.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 12:02 pm

Ertro wrote:
SomebodyInTLS wrote:
The point was that everyone in those discussions realised that trying to squeeze the new engines into the available space *was* going to compromise the aero and other dynamics. Boeing seemed to think the performance hit would be worth it over the additional time and costs associated with changing the gear


Boeing and a.net might have had different opinions on the matter. Boeing could have been more optimistic and a.net more pessimistic. I don't buy that Boeing foresaw all the problems and brick wall and went for it anyway. To me more sensible theory is that Boeing thought they can make it work and that means without extending the landing gear since that is what was part of the grand plan. Boeing clearly disagreed with A.net since if they would have agreed then they would not have started the MAX project in the way that they did.


I'm not sure why you talk about disagreement since "they thought they can make it work" is what I was saying.

Both Boeing and A.net were aware of the compromises the decision would entail. If we go back to the comment I was responding to, you said that no-one was aware of the potential difficulties the MAX design choices resulted in until it bit Boeing on the backside. That's just not true.

The need for MCAS coming as a surprise to Boeing is pretty clear? Is it not? [...] They thought they can tweak the aerodynamics with some metal pieces and to avoid high speed MCAS. [...] They thought they can make it work with original landing gear and without MCAS at all


That's your assumption - I and probably others were not surprised that some tuning of the controls might also be required. How Boeing handled that, however...
"As with most things related to aircraft design, it's all about the trade-offs and much more nuanced than A.net likes to make out."
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 1:40 pm

889091 wrote:
As the saying goes, "It takes two to tango."

By that I mean - how could a certifying authority be hoodwinked to the level that the FAA was, when it signed off on the MAX.

We've seen heads roll/more will roll(?) at Boeing. Are we to expect the same from the FAA side of things?


An edgy distinction might be that manager talk and speak qualitatively while engineers have to talk and speak quantitatively. So when it comes to 'blaming' it is easy to pin numbers and blame on an engineer, but much harder to do the same to a manager. They are better able to cover their *ss when things go wrong. To some degree both at Boeing and at the FAA engineers and technicians were overruled/finessed* by managers. Someone with experience in the field might use better words than I have.

*My worry was addressed by Mark Twain, “The difference between the almost right word and the right word is really a large matter. 'tis the difference between the lightning bug and the lightning.”
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 1:52 pm

planecane wrote:
par13del wrote:
smartplane wrote:
Unfortunately, Boeing tinkered with a perfectly stable, satisfactory STS, and retrospectively added MCAS-related functionality.

If we believe all the reports that have been released, they added MCAS before flight test because it was needed, whether aero or rules is another question, but it was also reported that during actual flight testing they had to increase the authority.

Not exactly. During flight test they discovered the problem that was expected at high speed also happened at low speed and that MCAS was needed at low speed. Authority was increased because control surfaces need to move more at low speed, not because the predicted high speed issue was worse.


The high and low speed problems are actually slightly different. The high-speed problem is a control force gradient per g in a manoeuvre. In actuality this will exist at all speeds, unless the wing-body-nacelle interactions change drastically with speed. The reason for this is the control force gradient per g is proportional to static margin, wing-loading and general size of the controls. It is basically independent of speed. So a the force to go from 1.5g to 1.5g+ shouldn't change much with airspeed. What seems to be a problem for MAX here is that the force changes rapidly as the g loading changes, ie CL increases, and may go to or through zero at some point. By moving the stab nose-down this puts the trim speed at a much higher point and effectively moves the trim load factor below 1g for the given airspeed, this means that pilot will have to first overcome the negative loading (see FAR 25.143(g)). By adding MCAS you would get and artificially increasing control force just to maintain a given g loading, when the next increment would still be near 0. This is what MCAS has to keep firing. You effectively have to make it harder and harder to stay at the given g loading let alone increase it.

The 'low-speed' problem is a alpha/speed stability at 1g and/or stall identification issue. Pilots fly speed, aircraft know alpha (AoA). Looking at FAR 25.201 & .203 and CS-25.201 & .203 are two areas where a problem could arise. In .203 we worry about the gradient must remain positive at all times and the average gradient should not be less than 1 lbf per 6 knots (25.173(c)). If we are worried about stick force gradients here trimming nose-down won't always help us. The reason for this is the gradient with respect to speed is less for higher trim speeds. So moving the stab nose down only moves us closer to the 25.173(c) problem. It also doesn't really change the flattening of the curve as your velocity decreases (stick force gradient goes to 0 as velocity goes to zero). The gradient is directly proportional to static margin. So as the static margin decreases, eg aft cg or shifting wing aerodynamic centre, nose down stabiliser makes the average gradient worse. What it will do is increase the force required to hold the aircraft nose-up, and at some point make it impossible for the nose not to fall. Which strangely enough is 25.203(d)(1)'s identification of a stall.

For more on this I can recommend Etkin & Reid, Dynamics of Flight: Stability and Control, Chs 2-3.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 2:24 pm

Checklist787 wrote:
LMP737 wrote:
Revelation wrote:
Indeed, and it's hard to see someone with Calhoun's background leading the revolution.

Guys like Calhoun are part of the problem. Remember, he was on the BOD while all this was going on. It's going to be more of the same, just with a different name on the corner office.

Unless Calhoun changes the policy!

People love hitting Boeing a lot even when things was going better for them. It is not normal that already in 2004 the 7E7 / 787 at the time was not a good idea... :roll:

Many argue without obvious evidence. Changing a working method is feasible it is the responsibility of any large company.

No company was, no company is, and no company will immune to error! :)

I don't think the word 'error' covers what happened with MAX. It was a deliberate strategy to avoid cost (sim training cost that Boeing agreed to pay) by making compromises on safety ('jedi mind tricks' to bundle MCAS in with STS and not mention it anywhere in the training material, classification of MCAS as 'major' rather than 'hazardous' to avoid costly analysis and testing which could have discovered MCAS's flaws before the two crashes). At best it was an error of commision (i.e. something done deliberately) rather than an error of omission (something done by mistake or by laziness).

Conspiracy is a loaded term, but it covers the case where more than one person is involved which is clearly the case here. It's pretty clear from the message dump that the conspiracy reached the Chief Technical Pilot and only a fool would believe that he was acting on his own initiative so it's sure that it reaches further up the chain of command. The stuff we've seen so far is pretty tepid, IMO. Odds are high that someone higher up had just as poor regard for electronic communication pitfalls. DoJ/FBI is really good at their jobs. If I were a Boeing executive, I'd be pretty nervous right now.

seahawk wrote:
If the pilots detect a runaway trim within 3 seconds (like Boeing intended), they would disable MCAS manually and it would not function as intended. One must be honest and accept that safety was not a driving factor for the development, it was costs and keeping fleet commonality. The rest was trying to smuggle the system past the regulator.

Sad but true. Well said.
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 2:42 pm

phollingsworth wrote:
The high and low speed problems are actually slightly different. The high-speed problem is a control force gradient per g in a manoeuvre. In actuality this will exist at all speeds, unless the wing-body-nacelle interactions change drastically with speed. The reason for this is the control force gradient per g is proportional to static margin, wing-loading and general size of the controls. It is basically independent of speed. So a the force to go from 1.5g to 1.5g+ shouldn't change much with airspeed. What seems to be a problem for MAX here is that the force changes rapidly as the g loading changes, ie CL increases, and may go to or through zero at some point. By moving the stab nose-down this puts the trim speed at a much higher point and effectively moves the trim load factor below 1g for the given airspeed, this means that pilot will have to first overcome the negative loading (see FAR 25.143(g)). By adding MCAS you would get and artificially increasing control force just to maintain a given g loading, when the next increment would still be near 0. This is what MCAS has to keep firing. You effectively have to make it harder and harder to stay at the given g loading let alone increase it.

The 'low-speed' problem is a alpha/speed stability at 1g and/or stall identification issue. Pilots fly speed, aircraft know alpha (AoA). Looking at FAR 25.201 & .203 and CS-25.201 & .203 are two areas where a problem could arise. In .203 we worry about the gradient must remain positive at all times and the average gradient should not be less than 1 lbf per 6 knots (25.173(c)). If we are worried about stick force gradients here trimming nose-down won't always help us. The reason for this is the gradient with respect to speed is less for higher trim speeds. So moving the stab nose down only moves us closer to the 25.173(c) problem. It also doesn't really change the flattening of the curve as your velocity decreases (stick force gradient goes to 0 as velocity goes to zero). The gradient is directly proportional to static margin. So as the static margin decreases, eg aft cg or shifting wing aerodynamic centre, nose down stabiliser makes the average gradient worse. What it will do is increase the force required to hold the aircraft nose-up, and at some point make it impossible for the nose not to fall. Which strangely enough is 25.203(d)(1)'s identification of a stall.

For more on this I can recommend Etkin & Reid, Dynamics of Flight: Stability and Control, Chs 2-3.

Many thanks for your very detailed explanations.
Did you get this first hand from 737-8/9 MAX facts, or it's your own analysis ?
:stirthepot: 737-8 MAX: "For all speeds higher than 220 Kts and trim set at a value of 2.5 units, the difficulity level of turning the manual trim wheel was level A (trim wheel not movable)." :stirthepot:
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 3:43 pm

phollingsworth wrote:
The high and low speed problems are actually slightly different. The high-speed problem is a control force gradient per g in a manoeuvre. In actuality this will exist at all speeds, unless the wing-body-nacelle interactions change drastically with speed. The reason for this is the control force gradient per g is proportional to static margin, wing-loading and general size of the controls. It is basically independent of speed. So a the force to go from 1.5g to 1.5g+ shouldn't change much with airspeed. What seems to be a problem for MAX here is that the force changes rapidly as the g loading changes, ie CL increases, and may go to or through zero at some point. By moving the stab nose-down this puts the trim speed at a much higher point and effectively moves the trim load factor below 1g for the given airspeed, this means that pilot will have to first overcome the negative loading (see FAR 25.143(g)). By adding MCAS you would get and artificially increasing control force just to maintain a given g loading, when the next increment would still be near 0. This is what MCAS has to keep firing. You effectively have to make it harder and harder to stay at the given g loading let alone increase it.

The 'low-speed' problem is a alpha/speed stability at 1g and/or stall identification issue. Pilots fly speed, aircraft know alpha (AoA). Looking at FAR 25.201 & .203 and CS-25.201 & .203 are two areas where a problem could arise. In .203 we worry about the gradient must remain positive at all times and the average gradient should not be less than 1 lbf per 6 knots (25.173(c)). If we are worried about stick force gradients here trimming nose-down won't always help us. The reason for this is the gradient with respect to speed is less for higher trim speeds. So moving the stab nose down only moves us closer to the 25.173(c) problem. It also doesn't really change the flattening of the curve as your velocity decreases (stick force gradient goes to 0 as velocity goes to zero). The gradient is directly proportional to static margin. So as the static margin decreases, eg aft cg or shifting wing aerodynamic centre, nose down stabiliser makes the average gradient worse. What it will do is increase the force required to hold the aircraft nose-up, and at some point make it impossible for the nose not to fall. Which strangely enough is 25.203(d)(1)'s identification of a stall.

For more on this I can recommend Etkin & Reid, Dynamics of Flight: Stability and Control, Chs 2-3.

I agree, this is a very informative post.

What I don't understand is why MAX is that different from NG at lower speeds. The high speed stuff is more understandable, the bigger nacelle is adding more lift so stick force lightens. At slow speed nacelle lift should be less of a factor, no? So is it more about the shift in CG and/or aerodynamic centre that your post mentions? Given that Boeing is moving forward with MCAS 2.0, can we suggest that it is a solvable problem?
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 4:00 pm

I heard that at slow speeds and higher AoA the engine and wing leading edge work like slotted flaps.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 4:10 pm

seahawk wrote:
I heard that at slow speeds and higher AoA the engine and wing leading edge work like slotted flaps.


That's what I understand as well.

Leading edge slats/flaps will provide lift, but pitch/AoA will become negative/pitch down. However, at higher speeds and positive AoA the stick lightens too much and it will pitch up. If that's the case, there's a delta in between these two parts of the flight where it's just right.

To me it sounds like this problem wasn't very well understood until after the design was set.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 4:39 pm

Revelation wrote:
phollingsworth wrote:
The high and low speed problems are actually slightly different. The high-speed problem is a control force gradient per g in a manoeuvre. In actuality this will exist at all speeds, unless the wing-body-nacelle interactions change drastically with speed. The reason for this is the control force gradient per g is proportional to static margin, wing-loading and general size of the controls. It is basically independent of speed. So a the force to go from 1.5g to 1.5g+ shouldn't change much with airspeed. What seems to be a problem for MAX here is that the force changes rapidly as the g loading changes, ie CL increases, and may go to or through zero at some point. By moving the stab nose-down this puts the trim speed at a much higher point and effectively moves the trim load factor below 1g for the given airspeed, this means that pilot will have to first overcome the negative loading (see FAR 25.143(g)). By adding MCAS you would get and artificially increasing control force just to maintain a given g loading, when the next increment would still be near 0. This is what MCAS has to keep firing. You effectively have to make it harder and harder to stay at the given g loading let alone increase it.

The 'low-speed' problem is a alpha/speed stability at 1g and/or stall identification issue. Pilots fly speed, aircraft know alpha (AoA). Looking at FAR 25.201 & .203 and CS-25.201 & .203 are two areas where a problem could arise. In .203 we worry about the gradient must remain positive at all times and the average gradient should not be less than 1 lbf per 6 knots (25.173(c)). If we are worried about stick force gradients here trimming nose-down won't always help us. The reason for this is the gradient with respect to speed is less for higher trim speeds. So moving the stab nose down only moves us closer to the 25.173(c) problem. It also doesn't really change the flattening of the curve as your velocity decreases (stick force gradient goes to 0 as velocity goes to zero). The gradient is directly proportional to static margin. So as the static margin decreases, eg aft cg or shifting wing aerodynamic centre, nose down stabiliser makes the average gradient worse. What it will do is increase the force required to hold the aircraft nose-up, and at some point make it impossible for the nose not to fall. Which strangely enough is 25.203(d)(1)'s identification of a stall.

For more on this I can recommend Etkin & Reid, Dynamics of Flight: Stability and Control, Chs 2-3.

I agree, this is a very informative post.

What I don't understand is why MAX is that different from NG at lower speeds. The high speed stuff is more understandable, the bigger nacelle is adding more lift so stick force lightens. At slow speed nacelle lift should be less of a factor, no? So is it more about the shift in CG and/or aerodynamic centre that your post mentions? Given that Boeing is moving forward with MCAS 2.0, can we suggest that it is a solvable problem?


The nacelle will shift the aerodynamic centre of the wing-nacelle combination forward. This is a function of the location of the nacelle aerodynamic centre and the planform area of the nacelle relative to the wing. However, this would be a constant effect, at lower angles of attack, for the range of speeds. Though the aerodynamic centre will move aft as Mach number increases. Note: stick force is actually a function of V squared, so it naturally increases as speed increases. The fact is the deflection needed to trim the aircraft is a first order function of CL and not separately of V. What is a bigger change is the way the nacelle-wing interaction effects the shift in aerodynamic centre as the angle of attack increases, e.g. it moves forward as the tips 'unload'. This reduces static margin and reduces stick force. It will also, at high AoAs effect when and how the wing root stalls. This will muddle stall identification.

Keep in mind, a 1g and 2g stall are not much different, save the greater tendency for the wing-tip to drop in a turn. However, in a wind up turn, where the stick force per g drops fast, or goes neutral you can quickly take an aircraft from 2g to something much higher, which would be very bad. In wings level 1g flight approaching stall bending the airframe is less of an issue; however, not recognising that you have stalled the aircraft and crashing into the ground or ocean is a real concern.

Is MCAS 2.0 sufficient. I would think and hope so. The basic principles of using stabiliser trim to provide stall identification and speed and g force stability are well documented. The question is on the specifics to the 737MAX and its flight control implementation.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 4:40 pm

seahawk wrote:
I heard that at slow speeds and higher AoA the engine and wing leading edge work like slotted flaps.

True, but wing leading edge didn't change from NG and engine is not all that much bigger than NG's engine, no?
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 4:50 pm

In the vertical it is a lot higher: https://leehamnews.com/2018/11/14/boein ... he-pilots/
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 6:02 pm

Revelation wrote:
Given that Boeing is moving forward with MCAS 2.0, can we suggest that it is a solvable problem?

Boeing already moved forward with MCAS 2.0 and submitted it to the FAA, as we know the FAA initiated a bit flip scenario maybe to test how MCAS 2.0 performed, based on what was released, the computers froze up and they required Boeing to update the computers.
So far no additional word on how MCAS 2.0 performed, so either they are going to fail it in a spectacular way with terrible wording, or MCAS 2.0 is now a non-event and they have moved on to other things while not clearing it in the public's eyes.

Let's hope the test flights if and when they happen cause some dissemination of information from folks other than Boeing...
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 6:23 pm

Weren't flight tests by the FAA suppose to begin this week?
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 6:24 pm

Any news on the EASA non-MCAS flights? Has anybody seen anything on flightradar or similar. Last thing i heard they were talking around Jan 15, and here we are.
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 6:52 pm

acechip wrote:
airhansa wrote:
I think it's ridiculous to suggest that designing an airplane that is not able to aerodynamically fly without the input of automated controls (MCAS) is acceptable. Boeing's mistake was not that the automated system was malfunctioning, but rather than the plane itself cannot fly as it is expected to by nearly everyone.

You have hit the nail on the head. Almost none of the commercial airplanes are designed to be inherently unstable with automated corrections. This is rather different from envelope protection, where by the system prevents erroneous inputs that will lead to an out-of-control situation. Here, the situation that goes out-of-control by manual input is expected to be auto-corrected by software. In the sense, you are already causing a problem in the first place because of inadequate design.


When did we get to the idea that the 737 is inherently unstable? MCAS kicks in when the plane is approaching a stall condition. The plane doens't go into a stall condition by itself. It was put in because the plane is light when the pilots are pulling back in manual flight. I have seen no evidence from anyone saying the MAX can't fly without intervention, only that MCAS was added so that the MAX flew the same as the NG to maintain the type certificate.
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 6:58 pm

Many people that contribute regular to this topic speculated that the Max would return quickly after the grounding, I was one of the few spektics that thought it would take longer but I never imagined that it would take this long! Reports recently released from various airlines seem to indicate a return to service mid summer at the earliest and that's if all goes well between now and then! I haven't done all the additions on the cost of this grounding but there is no way Boeing is ever gonna make up the difference now. Too many Billions are being spent in various ways with costs going up drastically with the manufacturing shutdown of the line.

The cash cow that the 737 once was has turned very quickly to a white elephant bleeding Boeing precariously close to bankruptcy! There is no two ways about this... this Max grounding will impact Boeing negatively for decades to come...
 
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 7:09 pm

MrBretz wrote:
Weren't flight tests by the FAA suppose to begin this week?

Palop wrote:
Any news on the EASA non-MCAS flights? Has anybody seen anything on flightradar or similar. Last thing i heard they were talking around Jan 15, and here we are.

Who knows?

The sources that suggested this were not what I would call mainstream aviation media sources.

We do know from the 777x production/test thread that KPAE conditions aren't very good today.

So either the sources were inaccurate, or weather is a factor.
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Re: Boeing 737MAX Grounding, General Discussion Thread, January 2020

Thu Jan 16, 2020 8:28 pm

Checklist787 wrote:
I think Calhoun will reverse the machine. Since he is good at communication, he is the man for the situation. The Board has understood.

If a member of A.net understands it, I think a man of his stature knows it. Anyone who claims the opposite actually wants to continue eating Popcorn in 2020 ...

Look even Leeham is attacking the 777X to maintain doubt when the most difficult thing at Boeing has already passed ...


Changing the ingrained culture at a company like Boeing is like trying to turn a fully-laden supertanker. It happens very slowly.

If you think the most "difficult thing" at Boeing has already passed, then, IMHO, you're very optimistic. Let's see what 29th brings.
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