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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Old 5th Jun 2019, 11:34
  #181 (permalink)  
fdr
 
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Smythe, you have a point,;to a carpenter the fix is a hammer.

The aero around the engine/pylon/wing is pretty complex, and modelling is anywhere from rough to detailed, but the full detail takes enormous amounts of CPU hours even for a single solution such as RANS. To get unsteady solutions that tend to better approximate the real world is a whole bunch of magnitude greater in computational overhead. URANS, DES/LES or other sorts of unsteady solutions will hint at the outcome, but then they are also limited, unless the underlying model that is used to develop the mesh of the structure is able to move appropriately from the loads applied, as an aeroelastic model. That will dim the lights across the oceans to undertake, so the best guesses in the wind tunnel models which have their own problems go out in sheet metal or carbon, and get flown. The wings are not rigid structures, and flow occasionally goes where you expect, as often as not it does weird stuff. That is the world we live in, and that makes design an art form. The 787 was a surprise to see VGs on the outer sections, that is for quite specific reasons and that would have been annoying to the guys at TBC to encounter.

Flow control by VGs makes some sense on some issues, there is only a slim likelihood that alteration to the strakes could remove the issue on the Max, VGs around the wing won't do anything much. VGs on the elevator, and T's, Ls, wedges etc would alter the control power of the elevator, which may assist. Doing that always makes for a review of aeroelastics and also PIO susceptibility.

Repurposing the MCAS was obviously seen as an elegant and expedient solution, which is regrettable in hindsight, however the reason why that was done is now of interest, as if it was related to stall prevention, then there is a whole lot of issues that needed to be covered in the design and that seems to be pretty darn quiet.

In the end, the Max will be a good plane, it is a painful experience for everyone concerned, particularly those torn up by the accident directly. If the OEM grows a conscience, they will learn from this and be better for it in the future. The industry needs both of the major OEMs, so they need to get this right.


Icarus: "If you don't like software "nudging the stab" up or down then I guess you hate Tailstrke Avoidance inputs right? Or Airbus Alpha floor? Or Embraer stall avoidance?". There is nothing wrong with having devices, when they have been appropriately assessed for failure modes. The expedient action of the repurposing missed opportunities to get the risks sorted, and to make the system compliant with the existing regs however safe their protection may be. A stick pusher without an override function is effectively what MCAS had morphed to, from being a limited authority SAS. That is a big change and the devil is still in the details which remain obscured at this point in the public arena.
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Old 5th Jun 2019, 11:36
  #182 (permalink)  
 
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Originally Posted by Icarus2001 View Post
Once again, that part of MCAS was not the issue from what we know so far. It was that ONE FAILED AOA vane could cause MCAS to activate. THAT was the issue. A software fix is fine, the fact that it could operate on erroneous data from a single source is not.

If you don't like software "nudging the stab" up or down then I guess you hate Tailstrke Avoidance inputs right? Or Airbus Alpha floor? Or Embraer stall avoidance?
Well, I dont know which Embraer aircraft you have in mind and wheter it is fully FBW.
Speaking of Airbus aircraft a major difference compared to the 737 it that the Airbus (and Boeing from the 777 up) is fully FBW with triple redundancy or more of everything. To name a few on the 320, triple AOA sensors, triple FCC with two separate processors in each for a combined six processors running in parallell. Triple electric motors and power supply and additional hydraulic motor for the stab. This is done to decrease the risk of erroneous activation of the flight control surfaces, as well as giving backup should any one system fail to activate.

Compare this with the 737 stab where most components is single. Single stab motor, single power supply to that motor, only two processors in total in those two FCC so if one give erroneous data there is no way to know which one is correct and which one is at fault.
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Old 5th Jun 2019, 15:19
  #183 (permalink)  
 
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There is no problem with a properly designed fly by wire system because from the very start the designers know how catastrophic sensor and computer failures can be if not planned for. It is hard to argue that the 737 MAX was designed that way, or if it was designed at all. It is like opening the electrical box of a 60 year old house that has been remodeled by several generations of "weekend warriors" (guilty of that myself). You can't really say that such an electrical system is "designed", stuff gets patched in as you go and you are always thinking "if I could rip out the walls I really should add a separate circuit for the microwave but oh look, here is a wire that is only being used for the smoke alarms, so I'll tap into that circuit...." and you end up with a house where if the microwave short circuits the smoke alarms are disabled...

I have found it difficult to find any decision that Boeing made in this debacle where they put safety first. I'm sure there must be some place where they thought "oh, this would be an easy fix but it is not the safest way to do it so we will do something harder/more expensive" but it is not apparent.
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Old 5th Jun 2019, 15:45
  #184 (permalink)  
 
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The A320 can be dispatched and hence flown for revenue with a single failure on most triple redundant systems.
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Old 5th Jun 2019, 17:15
  #185 (permalink)  
 
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Salute!

Thanks, FDR. Sound like a pilot with advanced degree, maybe even a test pilot. But don't know due to no public background on these forums. So credibility is not 100%.. It;'s good, but only based upon good technical arguments and explanations.
++++++++++++++++
I shall stand by initial assertion that the "problem" was aero, and that Boeing chose to use a kludge versus an aero solution just as they did with the STS. Apparently, the basic plane had been modified enuf thru the ages that it was too "slippery" and longitudinal stability needed "help" to tell the pilot that the plane was going faster than the last "trimmed airspeed". But most here know that it is the AoA change that increases induced drag and then actual CAS for the parasite drag and such, so it ain't just speed. It's AoA and the change in Cl that uses rho vee squared and surface area, and so on.

The "single point/sensor" failure would not have played a role if the basic aero problem was solved with basic aero engineering. Nevertheless, the MCAS was implemented with warts and all. Shoddy fault tree analysis, maybe even lack of flight tests to look at the slow and low envelope versus a medium altitude, turning, banked maneuver. I am glad I was not a line pilot that had not been informed of the system and how it worked.

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Last edited by gums; 5th Jun 2019 at 18:08.
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Old 5th Jun 2019, 17:42
  #186 (permalink)  
 
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VGs' are not certainly the solution in this case, there is a fair bit of CFD needed to sort out the underlying issue, and then a whole bunch of playing with the subsequent control forces to ascertain the real extent and solution of the problem, if, and only if it is considered that having a stab that cannot be controlled due to excessive airloads in manual mode is a bad thing
.

What concerns me is that the initial CFD modelling showed at 0.6 degrees would be enough to mitigate the issues. On flight test, that was found not to be sufficient, and somewhere, the 0.6 turned in 2.5 degrees.
That is an exponential difference, and I would really like to see the modelling and assumptions that supported either numbers...

There is also the NYT report where the test pilot stated MCAS orginally used G force, and that was dropped somehwere along the line...now what was that used for? It seems that the high G manuevers the pilot was talking about such as wake turbulence or avoidance (that caused stall?)

Again, the CFD assumptions that required MCAS or where mitigated by MCAS would be very, very interesting to look at.

Vortex tabs to prove laminar flow over the wings should not required at this point in design evolution. I was really surprised when they showed up on the 787....will see on the 777-W
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Old 5th Jun 2019, 18:03
  #187 (permalink)  
 
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Originally Posted by Smythe View Post
.

What concerns me is that the initial CFD modelling showed at 0.6 degrees would be enough to mitigate the issues. On flight test, that was found not to be sufficient, and somewhere, the 0.6 turned in 2.5 degrees.
That is an exponential difference, and I would really like to see the modelling and assumptions that supported either numbers...

There is also the NYT report where the test pilot stated MCAS orginally used G force, and that was dropped somehwere along the line...now what was that used for? It seems that the high G manuevers the pilot was talking about such as wake turbulence or avoidance (that caused stall?)

Again, the CFD assumptions that required MCAS or where mitigated by MCAS would be very, very interesting to look at.
I’ve seen no mention of CFD modeling, so I don’t know if they used CFD or actual flight tests to determine the design parameters.

According to the article you reference, MCAS was originally designed to mitigate undesirable handling characteristics during a high-speed accelerated stall. The g-meter input was used along with the AOA as a sanity check on the accelerated stall condition. At the higher speeds, the 0.6 unit input was probably enough.

Later on, it was decided to expand MCAS’s role to correct control feel issues during low-speed, 1-g stalls. This required removing the g-meter from the loop and increasing the input to 2.5 degrees because at the lower speed a greater stab movement was needed. At this point someone should have noticed that 1) there was no redundancy to prevent the single-point failure, and 2) there was no test for high-speed (0.6 input) versus low-speed (2.5) stab input.

Too many chefs, to little communications, and no one providing a thorough top to bottom review of the final design.
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Old 5th Jun 2019, 18:47
  #188 (permalink)  
 
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https://www.seattletimes.com/busines...al-regulators/

Boeing on hold as it awaits 737 MAX verdict from global regulators
By Dominic Gates
Seattle Times aerospace reporter

Top Boeing executive Greg Smith said Wednesday the jetmaker is in a holding pattern, waiting for the verdict from aviation regulators worldwide on its fix to the 737 MAX before it can move ahead with getting the plane back in service.

Separately, he raised the possibility that production of the second version of the forthcoming 777X may be delayed due to lack of near-term demand.

Smith, chief financial officer and executive vice president for strategy, said Boeing has turned over to the regulators its software fix for the 737 MAX flight-control system, as well as recommendations for additional pilot training and is now answering detailed technical questions from all corners of the globe every day.

The fix should prevent the erroneous malfunction of the system that contributed to the fatal crashes of two MAX jets in Indonesia and Ethiopia, killing 346 people. The plane has been grounded worldwide for more than 11 weeks.

“We’ve completed the software and training and passed that over,” said Smith. “Now we’ve got regulators from around the world coming in with more questions before we go to the next milestone.”

“We’ve pulled out all the stops as far as what anyone needs in any part of the globe and being responsive,” he said.

His remarks make clear that Boeing is working to achieve at least a measure of consensus among safety regulators round the world before moving to have the MAX fly passengers again. And one reason for that, he said, is concern that the brands of both the 737 MAX and Boeing itself have been damaged and need careful repair.

Smith said Boeing is looking outside the company for image and reputation support, “hiring folks on the brand management side, or on crisis management, getting some experts in there to help us manage that.”

“We’re putting whatever resources are required, inside and outside the company, to help us restore the MAX brand and work on the company brand,” he said. “We need to bring the best and brightest to do that, to understand where we are globally with the brand and what do we need to do in the U.S. versus in China and by airline.”

Speaking at the UBS Global Industrials conference in New York, Smith said that even after the regulators give clearance to end the grounding of the MAX, the process of returning the fleet to service will be an enormous logistical challenge that will take weeks or months depending on the needs of each airline.

He said Boeing teams are doing detailed planning for when that moment comes, negotiating with each customer airline and getting ready to deploy teams of technicians to take planes out of storage, install the software fix and provide the necessary pilot-training resources.

Meanwhile, Boeing is coordinating with its 737 suppliers, many of which are struggling to maintain a production rate to supply parts for 52 jets per month even though Boeing itself has cut back to rolling out 42 finished planes a month.

Separately, Smith also hinted at some issues with the new widebody 777X jet program in Everett. Two of the new airplanes with giant carbon composite wings have already rolled out of the factory, with two more in final assembly, and first flight is expected soon.

Smith said that GE has had “challenges” in producing the huge GE-9X engines for the airplane. “They are having to do some re-testing,” he said.

The plane is still expected “to fly this year and enter service in 2020,” said Smith.

In addition, demand for the 777X, and in particular for the smaller 777-8X version, has been soft and recent sales have been sparse.

Boeing will build the 777-9X first and was expected to deliver the -8X model perhaps a year later. Smith said Boeing is “looking at the timing and demand for the -8 to see if that still makes sense and do we want to push that out?”

Dominic Gates: 206-464-2963 or [email protected]; on Twitter: @dominicgates.
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Old 5th Jun 2019, 20:36
  #189 (permalink)  
fdr
 
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Originally Posted by Smythe View Post
.

What concerns me is that the initial CFD modelling showed at 0.6 degrees would be enough to mitigate the issues. On flight test, that was found not to be sufficient, and somewhere, the 0.6 turned in 2.5 degrees.
That is an exponential difference, and I would really like to see the modelling and assumptions that supported either numbers...

There is also the NYT report where the test pilot stated MCAS orginally used G force, and that was dropped somehwere along the line...now what was that used for? It seems that the high G manuevers the pilot was talking about such as wake turbulence or avoidance (that caused stall?)

Again, the CFD assumptions that required MCAS or where mitigated by MCAS would be very, very interesting to look at.

Vortex tabs to prove laminar flow over the wings should not required at this point in design evolution. I was really surprised when they showed up on the 787....will see on the 777-W
Hi. The NYT article gives the background that really answers those questions.

Originally the MCAS is conceived and accepted as a high g & high AOA triggered SAS device, which doesn't need a great range of motion to achieve it's outcome, the 0.6 value is born. Along the way, the device gets repurposed to being a low speed "anti-stall" device, and as the issue is a low speed event, the g trigger is removed, and the motion rage gets increased markedly. Ooops. In doing so, the two trigger AND logic that protected from single point failures was removed, and the response rate had increased by more than 4 times. doing this set up JT and ET accidents. The requirement in here was likely a need to comply with §25.203(a), "No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls." § 25.145 Longitudinal control may also have triggered the repurpose, § 25.145(a) It must be possible, at any point between the trim speed prescribed in § 25.103(b)(6) and stall identification (as defined in § 25.201(d)), to pitch the nose downward so that the acceleration to this selected trim speed is prompt with(1) The airplane trimmed at the trim speed prescribed in § 25.103(b)(6);
(2) The landing gear extended; (3) The wing flaps (i) retracted and (ii) extended; and (4) Power (i) off and (ii) at maximum continuous power on the engines.

§ 25.103 Stall speed (b)(6) The airplane trimmed for straight flight at a speed selected by the applicant, but not less than 1.13VSR and not greater than 1.3VSR.

So, it is possible that the early flight tests showed less than desirable nose down pitch authority near or in the stall with the stab trimmed at 1.13VSR to 1.3VSR. That this was found in early flight test wouldn't have needed a large error in CFD modelling to occur. It is why testing is done.

A simple fix would have been to add trim at that point, which it appears is what the repurposed design would now do. But doing so, the removal of the g trigger of the original accepted high speed, high g design now opened up a single failure point design which could trigger a very powerful trim change authority at any time.

Underneath this the testing issue would range from, an unacceptable under any case pitch up tendency, to a "less than prompt" response near or in the stall when set up under the rules for demonstration. I would guess to the pitch up tendency, but both are possible. In any event, while focusing on one problem, a simple fix probably set up the latent conditions for lack of fault tolerance. The design at this stage would probably be a no go item for an AOA probe failure of any form, I understand it alternates use between the single sensors, but is not user selectable to the other side in a failure case. As an SAS system that was deemed necessary to meet the § 25 subpart B requirements, it would have needed a warning system to the crew of a failure, and that puts the design as built into likely conflict with the rules. Sad day all round. The repurpose suggests that the low speed model was slightly different to expectation, the high speed case may have been found not to occur, or may still have existed, conclusion can't be drawn i\on the high speed case from the info provided. The decision makers probably didn't think that there was any consequential risk in removing a trigger requirement, it's not like adding more stuff, they removed a criteria that could have faults as well, but in so doing, made an intolerant design without failure alerting, alerting that had already been missed in the first set of design acceptance review. The process to end up at this point doesn't need overt action by any players, it just needs assumptions and lack of imagination or curiosity, and in a project under commercially derived time constraints and a policy driven by the customer through commercial to avoid changes necessitating differences training, well, the engineers are human, and the system has room for improvement.

Organisationally, the QA whistleblower & KC scandals pointed towards a broken high level institution in need of some navel contemplation, and such activity if still occurring in this case would be pretty ugly in the impending proceedings. Whether the ethics issues have been fixed will be on display in the wash up.

Last edited by fdr; 5th Jun 2019 at 20:46. Reason: grammer & spelling
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Old 5th Jun 2019, 21:10
  #190 (permalink)  
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Originally Posted by oggers View Post
Where is the thrust line compared to the NG? And with the more powerful engine can you cite any data that exists about how the handling would be affected had there simply been enough ground clearance to put the thrust line in the same place as the NG?

‘Large areas of the flight envelope’ is rather subjective. Can you cite the relationship between stab trim and flight envelope parameters of velocity, load factor and altitude, that formed the basis of MCAS as it was certified?

The Max engine center line is near the same as the NG. the moment has changed as the thrust can be higher. Neither design has the engine orthogonally aligned, so the forward position of the Max would alter the arm of the thrustline unless the incidence angle of the engine was adjusted. The original angle is a compromise for performance and handling.

The geometry problem is ground clearance for installing a larger diameter fan. Moving the engine forward permits bringing the engine core upwards from where it would have been if the gear had been extended only, reducing the extent of moment change that would occur just from the thrust line. That then added the aero moment which MCAS was expected to fix elegantly in the first instance.
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Old 6th Jun 2019, 00:40
  #191 (permalink)  
 
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A stick pusher without an override function is effectively what MCAS had morphed to, from being a limited authority SAS.
Except that it is not.

It can be overridden by the trim switch on the control column.

It can be switched off by using the switches on the centre console.

How is that "not capable of being overridden"?
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Old 6th Jun 2019, 02:04
  #192 (permalink)  
fdr
 
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Originally Posted by Icarus2001 View Post
Except that it is not.

It can be overridden by the trim switch on the control column.

It can be switched off by using the switches on the centre console.

How is that "not capable of being overridden"?

Icarus, fair point. it was then just a stick pusher system added without the awareness of the crew, and led to 2 crews planting planes into the turf. Without that information the crew are left in the dark in a single AOA failure in flight, that they have lost more than just an indicator or input to the ADC etc, they have lost something that affects the control of the aircraft, and is not annunciated or reported as such. MMEL permits despatch with heaters inop for the AOA in certain situations, without the crew being aware that an inadvertent icing encounter now may affect a system that they were not aware of being available. The MMEL also permits despatch with one of the two control yoke trim switches inop, there is always a manual trim and the cutout as you indicate. If the fault occurs on departure, there would be a pilot with a pickle switch.. If it happens during a comfort stop, then the pilot in the cockpit has to use what he can reach to sort it out and that comes to using the cutouts.

I stand corrected, but it remains a lousy architecture that does not appear to meet the advisory requirements of certification of a SAS system.
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Old 6th Jun 2019, 02:20
  #193 (permalink)  
 
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The Max engine center line is near the same as the NG. the moment has changed as the thrust can be higher. Neither design has the engine orthogonally aligned, so the forward position of the Max would alter the arm of the thrustline unless the incidence angle of the engine was adjusted. The original angle is a compromise for performance and handling.
I really doubt they would change the incidence angle of the engine to compensate for the increase moment arm due to the location being further forward and upwards.

Decreasing the incidence angle of the engine would have profound consequences in cruise. The decoupling of the wing angle and engine angle would send the flow computations out of control. Boeing has enough problems with laminar flow over the wings as it is.

Here is the logic. SMYD 1 is used for primary yaw damping and is connected to the both ADIRUs and left AOA Sensor for inputs.




A single AoA sensor is connected. 2 sensors are not connected to look at a disagree.

SMYD 2 is used to match SMYD 1 primary yaw damper commands, and is available as a backup under certain conditions when SMYD 1 is not available. SMYD 2 uses both ADIRUs and the right AOA sensor for inputs.

Only one FCC can control the mach trim actuator at a time. The IFSAU receives the FCC select signal from FCC B. This signal controls a relay in the IFSAU to find which FCC will give the mach trim actuator signals. The IFSAU sends the mach trim select status signal to the FCCs to show which FCC is in control. The IFSAU then sends mach trim power and motor drive signals to the mach trim actuator.

Only one FCC at a time supplies the speed trim signal to the stabilizer trim electric actuator. When the FCCs get electrical power, FCC A supplies the speed trim signals. If power remains on the FCCs, the on ground signal from the proximity switch electronics unit (PSEU) switches the FCC which supplies the speed trim signals. If one FCC fails, the other FCC automatically supplies the speed trim signal.


For Mach trim, Speed trim, (and MCAS?); the single active FCC CPU#1 command is made regardless of CPU#2, and regardless of the non-active FCC.

more at https://www.satcom.guru/2018/11/737-...n-command.html

Last edited by Smythe; 6th Jun 2019 at 02:45.
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Old 6th Jun 2019, 02:52
  #194 (permalink)  
 
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Originally Posted by infrequentflyer789 View Post
They had to extend the nose gear for all Max and design and certify a magic folding link thing on the main gear for the -10, I'm not sure why they didn't just do that job properly and give -8/-9 and -10 long enough legs to put the engines same place as the NG so that it flew like the NG, with no need for crazy software bodges.
I understand that this, which was really the start of the whole issue, was due to the basic 1960s 737-100 frame structure of the aircraft not having sufficient space between the ribs and spars for a landing gear of the new required length to be retracted into. Sorting that out would have been a complete frame redesign.
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Old 6th Jun 2019, 04:56
  #195 (permalink)  
 
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The maximum thrust of the engine is limited on the 737 by the ability to counteract the thrust induced pitch-up moment - hence the max thrust on the 737-8 is not a whole lot different than the max thrust on the 737-800. It might appear higher if you only look at the Sea Level Static rating, but big turbofan thrust lapses (decreases) with forward speed - the higher the bypass ration, the greater the lapse with forward speed. So, while the SLS thrust might be higher for the MAX, the thrust at ~120 knots will be pretty similar to the equivalent NG.
BTW, this is nothing new - limiting the max thrust goes back to the 737-3/4/500 'classic' - the only difference being on the 'classic' the -300 and -500 had throttle blockers to prevent more thrust than the tail could handle, on the NG and MAX it's done with a rating plug.
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Old 6th Jun 2019, 09:43
  #196 (permalink)  
 
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Taking a tiny fragment from a large post.
Originally Posted by fdr View Post
A stick pusher without an override function is effectively what MCAS had morphed to ...
[SLF] Would a "real" stick pusher have been an adequate solution to the handling/certification problems MCAS was intended to solve?

1a) Would a stick pusher meet the "control column feel" certification requirements that MCAS is intended to address?
1b) Would it introduce any problematic certification issues?
1c) Would any false triggering be acceptably benign and infrequent?
1d) Are there any additional sim training obligations for certification?
1e) What have I forgot? e.g. any hi/low speed issues; is more than control column feel needed for approach-to-stall avoidance?

2) What additional benefits to handling/airworthiness would a bug/feature free implementation of MCAS provide?
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Old 6th Jun 2019, 09:49
  #197 (permalink)  
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[QUOTE=Smythe;10487440]I really doubt they would change the incidence angle of the engine to compensate for the increase moment arm due to the location being further forward and upwards.

Decreasing the incidence angle of the engine would have profound consequences in cruise. The decoupling of the wing angle and engine angle would send the flow computations out of control. Boeing has enough problems with laminar flow over the wings as it is.

/QUOTE]

Don't disagree. Back in the last century a well known airline based around the south china sea evaluated various trim angles for the engines on their 4 engine jets, and gained a benefit from doing so from how the planes were being delivered. There wasn't much difference in handling but there was a difference in fuel burns.
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Old 6th Jun 2019, 10:16
  #198 (permalink)  
 
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Originally Posted by fdr View Post
it was then just a stick pusher system added without the awareness of the crew, and led to 2 crews planting planes into the turf.
Except that a stick pusher acts on the elevetor, not the stabilizer, though.
And so is never beyond the stick authority.

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Old 6th Jun 2019, 12:36
  #199 (permalink)  
 
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Originally Posted by tdracer View Post
The maximum thrust of the engine is limited on the 737 by the ability to counteract the thrust induced pitch-up moment - hence the max thrust on the 737-8 is not a whole lot different than the max thrust on the 737-800. It might appear higher if you only look at the Sea Level Static rating, but big turbofan thrust lapses (decreases) with forward speed - the higher the bypass ration, the greater the lapse with forward speed. So, while the SLS thrust might be higher for the MAX, the thrust at ~120 knots will be pretty similar to the equivalent NG.
BTW, this is nothing new - limiting the max thrust goes back to the 737-3/4/500 'classic' - the only difference being on the 'classic' the -300 and -500 had throttle blockers to prevent more thrust than the tail could handle, on the NG and MAX it's done with a rating plug.
The 737NG is rarely thrust limited, so there's not a compelling reason to provide much additional thrust in the MAX. The new LEAP engines were added primarily for increased fuel efficiency and not increased thrust.
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Old 6th Jun 2019, 13:34
  #200 (permalink)  
 
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​​​​​​Inside the Effort to Fix the Troubled Boeing 737 MAX
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