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Boeing 737 Max Recertification Testing - Finally.

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Boeing 737 Max Recertification Testing - Finally.

Old 7th Aug 2020, 23:58
  #221 (permalink)  
 
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Originally Posted by Bend alot View Post
More the point why did it's authority need to be increased during initial testing?

I guess a valid question to the FAA as a feedback question.
If my memory serves, MCAS was designed to address an issue in high-speed high-g turns at high AOA, based on wind tunnel tests and simulations. When they did the flight tests with the real aircraft they found similar issue at low-speed and high AOA (and with no g-dependency), which had not been found in wind tunnel or simulation. So they extended MCAS to work in larger area of flight envelope and removed the g threshold so it triggered solely on AOA. Because speed was lower in the new target area, the maximum authority was increased to have the same effect.

Note that:
1. I am not sure if there was also an altitude dependency (which was then removed), some reports mention altitude but they may be mistaken or it may be that the original target area of flight envelope (or target manoeuvre) would only occur at high altitude.
2. I am not definitely sure if authority was increased across the board or only in the new lower-speed areas - I suspect the latter (i.e. the original authority for the original problem was not changed)
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Old 8th Aug 2020, 01:11
  #222 (permalink)  
 
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Yes that is similar to what I recall.

Given the FDR MCAS activation of the crashed aircraft it seems the authority (or motor speed increase) did not change as the aircraft speed increased.

One would assume that the FAA will be having a good look at the low speed high AoA flight envelope area for this certification.

The new MCAS has been given a bit more thought than the original band-aid version, I am sure.
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Old 8th Aug 2020, 08:46
  #223 (permalink)  
 
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Originally Posted by Bend alot View Post
One would assume that the FAA will be having a good look at the low speed high AoA flight envelope area for this certification.
In the FAA paperwork submitted, there's a somewhat wishy-washy response to this part of the JATR recommendation. I'm sure this will be commented on in the spirit of their commitment to transparency.
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Old 8th Aug 2020, 09:59
  #224 (permalink)  
 
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It would be some what embarrassing for the FAA should they not.

JATR regulators may be a little flexible for re-certification, one country my go above and beyond to do evaluation tests - Tick Tock.

That could then expose the other regulators cozy relationships or interpretations of regulations for a back scratch later.

High stakes on this - and we see very little to no transparency to date on "the fix" from Boeing or the FAA.
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Old 8th Aug 2020, 10:51
  #225 (permalink)  
 
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Infrequent 789,
You appear to misunderstand the effects of 'g' and AoA; or is it memory

The need to change the magnitude of MCAS trim would more likely be an effect of Mach - difficult to replicate in a wind tunnel.
Although described as a trim increase, this could have been a larger value, or an earlier trigger point with an extended duration or change of trim speed.

The aerodynamic intrigue with MCAS is why AoA was required at all. Existing technologies could manage low speed situations ( speed trim ), and Mach trim at higher speed. The Max modifications appear to be similar to the military 'Leading Edge Extension' which involves AoA and vortex management, but implemented with full FBY technology.

The initial, single MCAS trim input is variable; modified system: -

"The MCAS flight control law becomes active when the airplane exceeds an AOA threshold that varies depending on Mach (airspeed). If activated by a high AOA, MCAS moves the horizontal stabilizer at a rate of 0.27 degrees per second, which is the same trim rate as Speed Trim with flaps down. The magnitude of the MCAS command is a function of Mach. At higher airspeeds, flight control surfaces are more effective than at lower airspeeds. Therefore, a smaller MCAS command at higher airspeeds has the same effect as a larger MCAS command at lower speeds. At low Mach, when the stabilizer has lower effectiveness, the MCAS command can be up to the maximum of 2.5 degrees of incremental stabilizer movement. The MCAS flight control law is reset after the AOA falls sufficiently below the AOA threshold." Preliminary Summary Page 23

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Old 8th Aug 2020, 14:19
  #226 (permalink)  
 
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Originally Posted by safetypee View Post
Infrequent 789,
You appear to misunderstand the effects of 'g' and AoA; or is it memory
Oh it may well be memory, my aero engineering was a long time ago, my career ended up going in a different direction. I am pretty sure however that there was a (minimum) g threshold for MCAS to activate in the first version, which was later removed to allow MCAS to address the "low-speed high-AOA" problem area discovered in flight tests.

The need to change the magnitude of MCAS trim would more likely be an effect of Mach - difficult to replicate in a wind tunnel.
Although described as a trim increase, this could have been a larger value, or an earlier trigger point with an extended duration or change of trim speed.
Magnitude of trim needed is definitely going to be Mach-dependent, but again there was a change to this (probably to data in a lookup table) as they needed MCAS at lower speeds. My understanding is that it was a trim authority increase by increasing the trim run time, and trim speed didn't change (there are only two speeds for auto and I'm pretty sure it was already using the faster one - that is a hardware change and I don't think they made an MCAS hardware change post-flight-test).

The aerodynamic intrigue with MCAS is why AoA was required at all. Existing technologies could manage low speed situations ( speed trim ), and Mach trim at higher speed. The Max modifications appear to be similar to the military 'Leading Edge Extension' which involves AoA and vortex management, but implemented with full FBY technology.
I think the lift from the engine nacelles is the problem, with pitching moment even worse because the engines are further forward. That lift is probably non-linear with AOA. Not sure why the effect wasn't found in wind tunnel or computer modelling given that they found it at high mach but not low - I doubt my aerodynamics was ever good enough to work that out. I'm also not sure why it isn't needed with flaps when it appears to be needed at same speed without flaps (since it seems it can kick in the minute flaps go up) - again, that may be my aerodynamics lacking.
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Old 8th Aug 2020, 14:42
  #227 (permalink)  
 
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I think the lift from the engine nacelles is the problem, with pitching moment even worse because the engines are further forward.
I would also think that stab downforce needed to be increased to counteract the moment arm produced by the fwd position of the engines, but since the flight deck stab trim indication is similar to the NG this must have been done by a hidden re calibration of the stab incidence. This would contribute to a change in pitch characteristics due to the required increased downforce.
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Old 8th Aug 2020, 15:46
  #228 (permalink)  
 
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Originally Posted by infrequentflyer789 View Post
That lift is probably non-linear with AOA.
FYI: it's my understanding, the only non-linear issue the MCAS was designed to correct applied to tactile stick force when the aircraft was at extreme angles of attack at the upper edges of the control envelope. With the engines mounted more forward and up, the aircraft tended to go into rocket mode faster and unload the control forces. As the MCAS was not needed in other flight attitudes it was automatically disabled as with flaps down, etc. There were specific Part 25 certification requirements that required positive linear stick force as the control stick was pulled back up to and throughout stall angles. The MCAS provided increased stick force by moving the stabilizer nose down. There are much better explanations in other threads.

And just as a side note, while it might be just semantics, the proposed AD will only correct known safety related problems and will not "re-certify" the MAX. All MAXs today retain their original certification and remain certified per Part 25. Even if there are new Part 25 certification changes via a separate NPRM, they will not apply backwards to any existing aircraft and will only apply to new model aircraft certifications regardless of manufacturer.
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Old 9th Aug 2020, 17:28
  #229 (permalink)  
 
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The NPRM is online now and open for commenting
https://www.regulations.gov/document...2020-0686-0001

(And I filed a comment)
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Old 9th Aug 2020, 18:48
  #230 (permalink)  
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Originally Posted by infrequentflyer789 View Post
Not sure why the effect wasn't found in wind tunnel or computer modelling given that they found it at high mach but not low - I doubt my aerodynamics was ever good enough to work that out. I'm also not sure why it isn't needed with flaps when it appears to be needed at same speed without flaps (since it seems it can kick in the minute flaps go up) - again, that may be my aerodynamics lacking.
This could be an artifact of the wind tunnels they use. I haven't been involved in wind tunnel testing in decades - so this may be obsolete information - but at least when I was involved, Boeing didn't really have a good low speed wind tunnel. The primary wind tunnel was the "BTWT" - Boeing Transonic Wind Tunnel - which was really good from about Mach 0.6 to nearly 1.0. The have a real low speed wind tunnel in Philly - with moving ground plane capabilities (at least when I was involved it was used for reverser testing) - but it's only good up to maybe 180 knots. That left a big gap in capability between ~Mach 0.3 and 0.6. When we were doing the 767 way back when, they rented the University of Washington tunnel to cover that gap - but even then there were complaints that UW equipment wasn't completely up to snuff...
As noted, it's been a long time since I was involved so this could be completely obsolete - but if not it might explain why they didn't see the low speed effects on the MAX until they got to flight test.
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Old 9th Aug 2020, 21:18
  #231 (permalink)  
 
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Originally Posted by infrequentflyer789 View Post
... I'm also not sure why it isn't needed with flaps when it appears to be needed at same speed without flaps (since it seems it can kick in the minute flaps go up) - again, that may be my aerodynamics lacking
I remember reading that using the flaps extended the slats which changed the airflow at the front of the wing ...

Gotit:
In https://leehamnews.com/2019/09/27/bj...-wire-part-10/
MCAS is not active on the 737 MAX when flaps are deployed. This is because when flaps are out the slats are out as well and these
diminish the disturbance to the pitch moment curve from the larger and further forward-higher slung engine nacelles.


Last edited by Peter H; 9th Aug 2020 at 21:48. Reason: added gotit
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Old 11th Aug 2020, 04:21
  #232 (permalink)  
 
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The Max modifications appear to be similar to the military 'Leading Edge Extension' which involves AoA and vortex management, but implemented with full FBY technology.
Excuse me, but I do not think the MCAS does anything close to what the LEX configurations did for the fighters since the 70's. Nor was it intended to do so. Good item for the Tech Log.

Secondly, the fighter LEX configurations provide good directional stability and keep the main wing providing useable lift at higher AoA than previous configurations WITHOUT FBW FLIGHT CONTROL SYSTEMS. Gasp! I am constantly amused by how many folks still have trouble uderstanding what FBW does and the various implementations - e.g. full FBW with zero mechanical or hydraulic anything like the Viper and Raptor and Stubbie, or hybrid systems as the early Hornets had and likely even the latest Super Bugs. Dunno about the Tiffie or Russian planes.

From everything I read about MCAS, and its evolution, the primary independent variables were AoA and mach and flaps/gear position. Upon flight tests, several aspects of the kludge were changed. But the biggest sin was implementing such a system without informing the pilots, and not even the reliance upon a single sensor and the repeated activation a few seconds after using the trim switches on the yoke.
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Old 11th Aug 2020, 05:29
  #233 (permalink)  
 
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Oh dear....

‘Don’t rock the boat with Boeing.’ In survey, FAA employees who monitor airplane makers report feeling pressured.
https://www.chicagotribune.com/busin...outputType=amp
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Old 11th Aug 2020, 06:13
  #234 (permalink)  
 
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Ali is still doing fine @FAA. What would you expect.
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Old 11th Aug 2020, 06:49
  #235 (permalink)  
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TD,
Always appreciate your valid comments.
Many believe that the Boeing Company will provide world class R&D for new designs etc.
If the wind tunnel tests were made available/rented at the UW, then this indicates that R&D wasn't a part of current budget, hardly a "world class" Company?
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Old 11th Aug 2020, 09:15
  #236 (permalink)  
 
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Think Boeing has always used outside tunnels as well as their own, tunnels are pretty widely shared between industry and academia, as well as internationally.
Their role has been taken in part as well by numerical simulation, so overall tunnel use is probably well down from even 20 years ago.
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Old 11th Aug 2020, 09:40
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I would like to see all congress people, committees and industry executives that pressured the FAA to reform and streamline on aircraft certification projects in the 2010-2019 period to demonstrate accountability. .

Boeing took the lead on embedding the FAA:
https://thehill.com/policy/transport...cation-process

and the rest of the industry was very supportive:
https://docs.google.com/viewerng/vie...7_v3.pdf&hl=en

Looking back, everybody sees oversight was lost. Boeing was self certifying and changed product rules (aggressive grandfathering of design & requirements was back on the table) after being removed by the FAA earlier on. https://www.flightglobal.com/faa-rul.../32615.article. But apparently they changed their minds after the 787 Dreamliner development. and certification.

My point is everybody was pushing & supporting delegation, kpi driven streamlining and keeping EASA at arms length. To improve competitiveness of the national airspace industry. Which proved driven by short term profit maximization (stock value, buy backs, dividends, executive bonuses) .

Google logged everything for us over the last 10 years. People changed their mind, but it is clear what happened.
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Old 11th Aug 2020, 10:06
  #238 (permalink)  
 
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Boeing used and uses Farnborough for low speed plus others. The point is to use wind tunnels over and over again to get to know their characteristics. Over time you can compare the results between your other programs. It doesn't matter if a company owns everything. Low speed is the most complex part and - like noise - one of the hardest to simulate in the computer. Big manufacturers test globally in wind tunnels.
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Old 11th Aug 2020, 13:23
  #239 (permalink)  
 
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' I do not think the MCAS does anything close to what the LEX configurations did for the fighters since the 70's. Nor was it intended to do so.'

'LEX' similarity was used to describe the problem; an anticipated outcome of the design, but which warranted a 'military style' solution.
In previous 737 variants the aerodynamic changes were managed with simple speed and Mach corrections (STS, Mach trim).
However, the Max engine / nacelles resulted in aerodynamic effects (similar to LEX) which required mitigation involving several parameters; - speed, Mach, and AoA, where the combination, and apparent non-linearity warranted high integrity computation, detailed system assessment, and validated assumptions.
Boeing chose to use single sensor, single path computation, and simplified certification; - hindsight defines history, and that which must be learnt.

The Tech Log plea is noted; however the subject is part of a complex system of aerodynamics, design, engineering, certification, communication, etc. If individual aspects are discussed in isolation, influencing interconnections could be overlooked.
In this area, dual monitored AoA as part of the modification to inhibit the effects of failures might be easy to understand and certificate. However, the additional computation which can apparently identify the better AoA input of a 'failed' dual input is harder to understand without more detailed description ("valid erroneous" data - 6.1.3). Such computation might not be not perfect, but good enough; all of this requires further technical explanation, test results, proof of concept, and certification acceptability.

Yes LEX chines interact with directional stab; some early Russian aircraft changed nose cones until they found one that did not depart - slice (SETP conference - SU 27 briefing, question re 'cobra' manoeuvre for airshows), nowadays FBW is much better.
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Old 21st Aug 2020, 16:00
  #240 (permalink)  
 
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Transport Canada test flight scheduled.

https://www.reuters.com/article/us-b...-idUSKBN25G223
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