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Was MCAS needed?

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Was MCAS needed?

Old 12th Jan 2021, 08:52
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Was MCAS needed?

Perhaps MCAS was not needed to all on the 737Max. Patrick Ky of EASA has said that "they pushed the aircraft to its limits during stall tests, assessed the behaviour of the aircraft in failure scenarios, and confirmed that the aircraft is stable and has no tendency to pitch up even without MCAS."

https://theaircurrent.com/aircraft-d...37-max-at-all/
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Old 12th Jan 2021, 11:11
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I obviously don't know all the ins and outs of the MAX design process, but since the beginning I have wondered why: if the longer engines caused uncommanded additional pitch-up at high AoAs, then why not just modify the artificial feel in pitch and leave the control surfaces alone? Or have the stick shaker start at a slightly lower AoA to warn of the impending pitch-up?
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Old 12th Jan 2021, 13:56
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Salute!

Good point, Bergerie.
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Old 12th Jan 2021, 18:18
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I don't know why* Boeing didn't bring in FBW on the B737. Even if they did so only in pitch, it would have brought benefits such as auto pitch trim, no pitch/power couple, turning without needing to pull back; and MCAS would not have been needed - just a change of some variables in the software.

* well, of course, I do.
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Old 12th Jan 2021, 18:24
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Making of MAX was a ruthlessly comercial act by Boeing. They weren't going to spend a penny extra if it could have been saved. If it could be certified they would have sold the dummy without MCAS, May be with some luck they would have gotten away. Afterall it was a false activation that exposed them.
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Old 12th Jan 2021, 18:28
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I realize that the Boeing Company has a diminished reputation, but there is one thing it is noted for. It is extremely hard to add a new “feature” to a Boeing design because in Boeing’s philosophy, “it has to buy its way on”. A lot of good ideas have been rejected over the years. Not saying MCAS was a good idea, but Boeing had to have thought it essential or they would not have expended the resources to add it. The need would have been identified by Boeing engineers. So I don't doubt there was a technical need, even if the solution leaves something to be desired.
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Old 12th Jan 2021, 18:47
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Originally Posted by Uplinker View Post
I don't know why* Boeing didn't bring in FBW on the B737. Even if they did so only in pitch...
It would just be yet another big patch for a very old airplane, with questionable benefits. They were better off pushing the airplane out as it was (in hindsight, with better MCAS redundancy), and rather focus on developing a 737 successor from scratch.
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Old 12th Jan 2021, 21:45
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Originally Posted by Uplinker View Post
I don't know why* Boeing didn't bring in FBW on the B737. Even if they did so only in pitch, it would have brought benefits such as auto pitch trim, no pitch/power couple, turning without needing to pull back; and MCAS would not have been needed - just a change of some variables in the software.

* well, of course, I do.
Didn't they add "FBW" for the spoilers in the MAX? Anything else probably would have required a re-design of the impacted flight control system that simply was too expensive, and would have taken too much time, always worth remembering that Boeing was in quite a desperate game of catching up to the order advantage Airbus had.

That said, what puzzled me at the time was the difference in testing program, although Boeing had to change quite a lot more on its new variant than Airbus had, Airbus did more than twice the number of test flight hours, which in itself does not say all that much of course, but it was a point i noted in passing.
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Old 12th Jan 2021, 22:01
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According to the FAA it was needed.
The 737 MAX was designed to handle and feel the same to the pilot as the 737 NG. Without the MCAS function, in some small areas of the flight envelope — such as approaching a stall and during higher g-force maneuvering — the new engines contribute to the control column feeling lighter in the 737 MAX than the regulations allow. These are not areas of the flight envelope in which the airplane normally operates. However, FAA regulations, specifically 14 CFR 25.143, 25.201, 25.203, 25.251, and 25.255, still require the control column to have a higher pull-force feel in these flight regimes than would exist on the 737 MAX without the added stability from the STS and MCAS function.

For the FAA to certify the 737 MAX, the original design had to meet FAA requirements for control force feel when maneuvering or deviating significantly from trimmed equilibrium. The aircraft manufacturer is responsible for making design decisions and showing compliance to applicable regulations. In this case, Boeing elected to include an additional flight control law in the STS, which is part of the flight control software that provides required control force feel to the pilot.

This flight control law, or MCAS, enhances the feel of the column forces in manual flight, and is only operative with flaps up. MCAS provides signals to move the horizontal stabilizer at elevated angles of attack to compensate for the aerodynamic effects of the 737 MAX’s larger and more forward-located engines, resulting in the required column feel to the pilot.
Source https://www.faa.gov/news/media/attac...ummary-v-1.pdf (page 10)
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Old 12th Jan 2021, 22:12
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Stick force gradient needed to be the same as the older 737's to keep the common type and simulator time down to 0. The big selling point with the Max was that no simulator training would be needed to fly it with a previous 737 type rating saving airlines lots of money.

Lots of people falsely believe it to be an anti stall feature due to the pitch up moment but it is not true. Boeing came out after the crash and said it was for handling characteristics.
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Old 12th Jan 2021, 22:17
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Stick force gradient must not become lighter towards a stall. But this is what supposedly happened.

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Old 12th Jan 2021, 22:28
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As #9 MCAS was required to meet stick force requirements; it was not directly related to the stalling characteristics. The comments about stalling could relate to acceptability if MCAS had been inhibited; thence a comparison with the NG stall, a certification consideration for the failure case.

MCAS was also driven by the Boeing focus of type commonality, minimum differences training; thus a stick force tweak to achieve a 'NG like' feel (#10).

FBW might not have resolved the issue(#4). Electrical signalling was not required; the associated computation and sensor redundancy could not be justified - why not add third AoA to the existing 'mechanical' system.
Also, the Boeing FBW philosophy of keeping the pilot in the loop - stick force feel - negates the auto trim follow up. The C*U algorithm is based on trim for speed, which would not provide an additional stick force change to rectify the deficiency in manual flight. Hence the need for separate computation to drive the trim directly - MCAS.
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Old 12th Jan 2021, 23:00
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Salute!

Awww man, this statement bugs me:

MCAS was also driven by the Boeing focus of type commonality, minimum differences training; thus a stick force tweak to achieve a 'NG like' feel (#10).
The MCAS implementation was not some gentle input to the existing column "feel". Good grief. It put in gobs of nose down trim and did it for "x" bumps and then did it again if the trigger parameters still existed. It was not well thought out and was poorly implemented, at that.

If the MCAS could make the Max "feel" like the NG, then I guess the STS in the NG is very harsh, as that system seems to be another kludge to make the pilot do something that should be part of basic trim action by the pilot as the plane increases or decreases speed. GASP!

We need to stop making excuses and simply admit the MCAS was a poor solution to an aero design that changed the stick force gradient so much that the jet would not meet the criteria.

Gums sends...

P.S. FBW computer solution is not the answer without a sh!!!tload of waivers from the certifying authorities. The 'bus is a very stable plane, as the AF crew demonstrated. They didn't even recognize that they were stalled! The military planes using FBW use it for variour reasons, and not all are to correct for a poor aerodynamic characteristic. As with the 'bus, FBW computer flight control systems can reduce workload and smooth things out (things we old farts did without HAL doing a lot except autopilot funtions when in level flight)
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Old 13th Jan 2021, 01:34
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Originally Posted by Uplinker View Post
I don't know why* Boeing didn't bring in FBW on the B737. Even if they did so only in pitch, it would have brought benefits such as auto pitch trim, no pitch/power couple, turning without needing to pull back; and MCAS would not have been needed - just a change of some variables in the software.

* well, of course, I do.
For an aircraft that is only manually flown for the best part of 30 seconds per sector, is there really a justifiable need for FBW?
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Old 13th Jan 2021, 10:07
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Originally Posted by CessNah View Post
For an aircraft that is only manually flown for the best part of 30 seconds per sector, is there really a justifiable need for FBW?
Would you not say the opposite is true?
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Old 13th Jan 2021, 10:31
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Originally Posted by CessNah View Post
For an aircraft that is only manually flown for the best part of 30 seconds per sector, is there really a justifiable need for FBW?
I fly a 737 manually for a lot longer than 30 seconds every sector...
And FD and AP are MEL items.

Back to your Cessna 150.
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Old 13th Jan 2021, 11:15
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Swatting bugs around a cowpat

Yo gums, #13 Let's swat some 'bugs'
The design concept of MCAS was fine. It met the objectives - as misguided as they appear to be with hindsight.
However, it was the engineering implementation that was flawed, together with weak and failed processes in testing, airworthiness, and certification; all of the holes in the Swiss Cheese lined up - they were already there just waiting for MCAS (it could have been something else).

The system operation which you describe was a failure case, not normal operation.

Yes, in hindsight, MCAS was not an ideal solution, but alternatively if the system has been well engineered, tested, certified, etc, then we would not be having this discussion - MCAS could have been hailed as a neat low-cost fix for an old design pushed too far.

Whilst the rule-makers and regulators fix, patch, kluge, the holes in regulation - swatting bugs around their cowpat, the greater concern is that they might not see the 'cowpat' which they are still standing in; that should 'bug' us.

Hindsight; 'sometimes things go wrong, so that we will know when they go right'.
Aviation assumes that the wrong things will be minor, manageable by limited capability humans, but sometimes those assumptions are wrong.
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Old 13th Jan 2021, 17:17
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John Leahy ex CCO of Airbus said A380 was undone because when they launched in 2000 the engine manufacturers offered them engines which according to them was the most economical and anything better was atleast ten years away. But in three years the engine manufacturers turned out new engines for B787 which was 12% better in SFC. B737 was also undone by new engine technology in a different way. The 1960s low wing aircraft was unsuitable for high bypass engine era. Had Boeing anticipated this they should not have made 800 series itself but built a new aeroplane which being later generation aircraft may be even outlasted Airbus neo. Unfortunately they had gone for widebody 787 and by the time it turned profitable Boeing was left with no gumption to invest in another new aircraft. They hurriedly did another plastic surgery on the old lady which turned out very nasty and apart from financial damage has even destroyed their credibility and reputation.
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Old 13th Jan 2021, 17:32
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Salute!

Far being for me to debate a certified test pilot with all the degrees and actual diplomas from the "schools".

I cannot find the "#13" ref document.... I gave up looking for technical, non-political, non-management and so forth once PPRuNe closed discussion here on the Tech Log. Whatever document had "#13" ? It must have been buried in all the "return to service or certification" threads that were not worth the time to look at due to legal and non-pilot posts. I tried to get tech stuff on Tech Log to no avail.

I do not take completely agree with:

The design concept of MCAS was fine. It met the objectives - as misguided as they appear to be with hindsight.
I agree that moving the stab would change the control input gradient in order to require more aft stick for more AoA or whatever. It would not necessarily require more pounds per stick movement unless it influenced other systems in the control implementation. I do not agree that it was an acceptable concept to make the plane "feel" like the NG model. So meeting the "objective" of the Part "x" that FAA required seems to me to be the biggest bone of contention I have. How would moving the stab angle of incidence and then remain there at other parts of the envelope? "Oh, Gums, we just return to some other angle when ?" What angle?
I appreciate the implementation of moving the stab "gradually" to help with "elevator" pitch authority/movement. But my experience was with three jets that did not have "elevators" (and one that was a delta with elevators/elevons). We moved the stab for pitch control! In the "neutral" position, we followed basic FAA stability and control requirements. On autopilot, they trimmed the slab very slightly to maintain either mach hold or altitude. When we pulled hard it got harder approaching higher AoA, except for the last few degrees in the VooDoo. In the VooDoo we definitely got a "light stick" in the end game, and I wish you could have flown one to see what I mean.
----
and then I completely agree with:

However, it was the engineering implementation that was flawed, together with weak and failed processes in testing, airworthiness, and certification; all of the holes in the Swiss Cheese lined up - they were already there just waiting for MCAS (it could have been something else).
You nailed it there, PEI. And do not forget some management issues that Big B is now paying for.

Gums sends...









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Old 13th Jan 2021, 19:46
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Originally Posted by vilas View Post
John Leahy ex CCO of Airbus said A380 was undone because when they launched in 2000 the engine manufacturers offered them engines which according to them was the most economical and anything better was atleast ten years away. But in three years the engine manufacturers turned out new engines for B787 which was 12% better in SFC. B737 was also undone by new engine technology in a different way. The 1960s low wing aircraft was unsuitable for high bypass engine era. Had Boeing anticipated this they should not have made 800 series itself but built a new aeroplane which being later generation aircraft may be even outlasted Airbus neo. Unfortunately they had gone for widebody 787 and by the time it turned profitable Boeing was left with no gumption to invest in another new aircraft. They hurriedly did another plastic surgery on the old lady which turned out very nasty and apart from financial damage has even destroyed their credibility and reputation.
Actually, Boeing was planning a completely new aircraft to replace the 737 (part of, IIRC, was called 'project 2016'). Problem was when Airbus announced the A320 NEO, it caught Boeing completely off guard. Not only where they resource limited (both 787 and 747-8 still a year away from type cert/EIS), if Boeing had launched the new aircraft immediately, it would have been five to six years before initial cert. Worse, both Boeing and Airbus were pushing 737s and A320s out the door at ~50/month - it would take another four to five years to get a brand new aircraft up to that sort of production rate. It would have meant giving the A320 NEO a near monopoly and the market for about five years of production - roughly 3,000 aircraft. That was too bitter of a pill to swallow, so Boeing cobbled together the MAX - which kept the A320 NEO advantage down to around 12 months.
With the benefit of 20-20 hindsight, doing a completely new aircraft looks much better than what happened on the MAX, but no one could foresee that 10 years ago...

BTW, the A380 had much bigger problems than it's engines (although they didn't help) - even now Leahy is unwilling to admit he screwed the pooch when they launched the A380 just as the market was moving to big, long range twins.

I've posted this before, but the problems with MCAS all go back to two related assumptions. First, that an MCAS malfunction was no worse than a stab trim runaway, and two that the crew could recognize and accommodate such a failure in four seconds (this second assumption pre-dating the MAX, possibly going back to the original 737-100/200). Based on these assumptions, an MCAS malfunction was judged as no worse than "Major" - same as a routine engine shutdown - and single failures are acceptable.
Turns out that both assumptions were wrong - four seconds to recognized and correct for a stab trim runaway was too optimistic (at least for some pilots), and it was not recognized as a Stab Trim malfunction. Had MCAS been originally identified as a Critical system (as it is now), it never would have been designed the way it was.
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