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Was MCAS needed?

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Was MCAS needed?

Old 16th Jan 2021, 11:27
  #61 (permalink)  
 
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Gums I enjoy your posts and as FlightDetent says this is probably not the place for flight control discussions except for MACS but....
1. A320 has spring feel for artificial feel as in F86 without the bob weight.
2. Longitudinal control is manoeuvre demand, eg stick force per G or per pitch rate, or a blend of the two depending on speed/alpha.
3. Beyond the autotrim envelope a pull force is required for lower speeds, a push force is required for higher speeds.
4. In Direct Law - stick to surface, the aircraft is conventionally stable and requires manual trim operation to trim out forces when the speed is changed. Stick force per G is conventional.
5. My first aircraft with an electronically signalled flight control mode was the Mirage111 in Autocommand, circa 1969 plenty of Mirage pilots earlier.
6. My first FBW airliner was Airbuse's A300 FBW F-BAUD March 1986.
7. There were literally years of negotiations/ discussions on what FBW characteristics would be considered as acceptable as "Equivalent Safety" to the then current stability and control standards standards US FAA/ UK CAA/ French DGAC.
Links may help - the sidestick link is from a serious flight simmer with an OEM sidestick which he stripped for repairs.
https://sim-on-a320.com/blog/2016/08...al-sidesticks/
https://davi.ws/avionics/TheAvionicsHandbook_Cap_12.pdf
Thank you for your interesting posts - and your service.
Cheers mate.
Oh https://sim-on-a320.com/blog/2017/03...prings-repair/


Last edited by zzuf; 16th Jan 2021 at 11:46.
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Old 16th Jan 2021, 13:52
  #62 (permalink)  
 
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Thanks a bunch, ZZ you too, Detent.
I am remiss by not going back to all the AF447 data and such to see the gradients.
My main point about discussing FBW is too many folks do not understand exactly what it is and how it can be implemented differently in different planes. I really liked the Canadian approach concerning the Max, and seems to me that the 727 had to use a pusher or similar implementation due to its characteristics at high AoA. Ditto for other commercial planes.
=====
Off topic, but I would have loved to have had a hop in the F-86. Our sqd checked out the last two U.S. Air National Guard units that were flying the thing, but USAF would not let us try it, heh heh. Ditto for the Mirage, and my first fighter was the F-102, so would have been neat to compare the deltas.
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Old 16th Jan 2021, 13:55
  #63 (permalink)  
 
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gums, Detent, et al, caution with specific comparisons.
Following on from zzuf, good refs; - associating the A320 with feel and bob wts could be misleading.

Airbus chose the combination of electrical signalling, digital computation, and side stick as their FBW system. All of these challenged the certification status quo; this required adaptation, interpretation, and new regulations. Airbus designed the future, rewrote the regulations.
Conversely, Boeing, using the improved technologies where possible, chose to follow history. Moving sticks, trim based force-feel, fleet commonality.

The piloting differences matter little (avoiding endless opinionated debate); these aircraft meet the safety requirements, and are flown as any aircraft can be.

Airbus chose a manoeuvre demand algorithm C*, a combination of pitch rate and 'g', with rate dominating low speed, 'g' higher. The trim followup restores a stable condition and minimises drag. (Military systems may favour 'g' more than rate)

Boeing is similar except the trim followup had to maintain the speed-dependent force-feel; this is the basis of the C*U algorithm, where U is the speed aspect.

The 737 Max with MCAS extended previous trimming adjustments (STS), to compensate for aircraft weaknesses; in no way is this FBW (electrical signalling at best). Post modification, MCAS fails safe, no trim demand or incremental runaway (although separately, the stab still could).

Relating this to AF447; the A330 did exactly what the pilot demanded, for the situation, as the crew interpreted it - UAS - pitch/ power. The technical / icing cause had be been identified from previous incidents and modification was in hand. The safety lessons stem from why this one accident differed from several previous incidents with the same trigger - go to previous threads for debate.

In the 737 Max accidents the aircraft did its own thing; crews were in recovery mode without explanation or guidance.

Even after modification, MCAS and the revised checklists could still lead to an AF447 type of incident. The checklist for AoA disagree requires action for unreliable airspeed; this biases the crew's mind to UAS, requiring pitch/power, bypassing wider situational aspects such as cross referring standby instruments which could clarify the situation, not requiring any manoeuvre.
This suggests that Boeing's operational approach to MCAS, alerting and drills, are still out of sync; perhaps not really appreciating if (why) MCAS is required - certification or training.
Old aircraft, even with new mods still require old style situational awareness to help crews understand, without ambiguity, and not presuming any failure, cause, or action. It is difficult to impose new technical philosophies on an old aircraft; similarly for operational aspects.
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Old 16th Jan 2021, 16:26
  #64 (permalink)  
 
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Thanks, PEI.

My view is we are experiencing an epoch of old tech mixed with new tech, but still have to deal with basic, fundamental laws of physics and aerodynamics.

Many of the younger folks seem to believe we can do anything with sfwe, and a few older folks of the "macho, Yeager wannabes" revert to attitude, pitch, power solutions. I am a old fart, but matured during the transition of pure mechanical flight controls and actual feedback to zero feedback and eventually, electical commands to servoactuators that moved the rudder, ailerons, elevators, flaps, and the beat goes on.

My views are here and I shall check in on ocassion, but basically lurk.
======

The MCAS debacle represents an attempt to use technology to compensate for a basic design that did not satisfy the "rules" and the company wanted to sell the plane with no additional training for pilots flying the basic plane. So save a few bucks here, but then spend billions down the road, and then add the personal losses by a few hundred folks.

I sure hope all manufacturers look at this debacle and look in the mirror.

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Old 17th Jan 2021, 02:09
  #65 (permalink)  
 
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Gums, F102, I think I have your pic!! High over Thailand 6Jul1964, probably 100 miles or so north of Korat.
Courtesy RAAF Avon Sabre and A4 radar gunsight. Concede a gun splash??? Ha Ha good old days.
Truly sorry about the thread drift, won't do it again.
Best regards.



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Old 17th Jan 2021, 02:30
  #66 (permalink)  
 
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@ zzuf,

YOU SWINE!!!
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Old 17th Jan 2021, 02:53
  #67 (permalink)  
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I ask a question that may well not be for this thread or indeed any thread:

In the whole MCAS affair, Boeing has received (and will continue to) all the justifiable anger of humans and the law. However, South West Airlines who wrote the $1m per airframe penalty clause appear not to have received even a glancing blow?

Whilst it can be said that Boeing were big enough to make their own mistakes (and did) would they have been as likely to commit to MCAS if not for such a high penalty being set by their customer?

Of course, it might just have been 'good old capitalism' at work ...
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Old 17th Jan 2021, 03:25
  #68 (permalink)  
 
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Originally Posted by PAXboy View Post
I ask a question that may well not be for this thread or indeed any thread:

In the whole MCAS affair, Boeing has received (and will continue to) all the justifiable anger of humans and the law. However, South West Airlines who wrote the $1m per airframe penalty clause appear not to have received even a glancing blow?

Whilst it can be said that Boeing were big enough to make their own mistakes (and did) would they have been as likely to commit to MCAS if not for such a high penalty being set by their customer?

Of course, it might just have been 'good old capitalism' at work ...
Boeing was selling an aeroplane not cocaine. They are solely responsible for the product and it's aftermath. By this absurd logic why not blame the Airbus Neo or the bigger fan engines fitted on them? Without which the MAX would never have been made?
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Old 17th Jan 2021, 07:11
  #69 (permalink)  
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Truly sorry about the thread drift, won't do it again.

Unless it is appropriately infrequent ... in which case, it will not only be tolerated, but encouraged, aided, and abetted.
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Old 17th Jan 2021, 14:41
  #70 (permalink)  
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No vilas that is not a valid comparison but I shall let the matter go.
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Old 17th Jan 2021, 14:51
  #71 (permalink)  
 
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Gums and TD thanks. The penalties of, and that they did indeed try to put alloy sticky-outs on to get what they wanted first is a relief. As to Airbus, yuk. I've tried my whole career to avoid flying one. I like feel and feedback, thank you very muchly.

I always thought the answer for FBW sidesticks was to mount the whole stick and force sensing assembly on a moving base that mimicked the actual movement of the control surfaces. That way you'd get both moving, and more softer felt motion at slower speeds, plus the benefit of feeling any interference from the computer limits. It's a Win-Win-Win option. Not sure if they do that at Gulfsteam or on the C17? It was certainly a trick Airbus missed.

Last edited by RVF750; 5th Mar 2021 at 16:32.
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Old 17th Jan 2021, 22:03
  #72 (permalink)  
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Perhaps that level of sophistication was not available when they started out their FBW design. When it was - they already had so many machines in service, and pilots used to their side stick, that it would have caused more problems to uprate? Gosh they might even have had carriers specifying that the change must not compel any SIM time by force of financial penalty...

Seriously, if they had a working system making a fundamental change would be diffcult. Might be good for the longer term but Bus were still climbing the market at the time and such major changes could have been tricky.
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Old 17th Jan 2021, 22:35
  #73 (permalink)  
 
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However, South West Airlines who wrote the $1m per airframe penalty clause appear not to have received even a glancing blow?
Southwest was just a customer seeking a deal on a product, Boeing said yes we can do that, so the airline bought in. The longitudinal handling issues required them to do something to alleviate the problem and after trying aerodynamic fixes, which didn't work, Boeing came up with MCAS. SW are not in the aircraft design business, they just say as a customer what they would like, it's up to the manufacturer to tell them what's possible. SW made an offer, Boeing accepted, same as when you walk into a car dealer.
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Old 19th Jan 2021, 09:21
  #74 (permalink)  
 
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What puzzles me is the difference in the stick force criteria and the real world!
My UK instructors Bible circa 1969 BLAC flight instructors manual has no mention of increasing back pressure or stick position in the stalling exercises nor theory.
Likewise my British Gliding Association instructor manual from 2005.
Never during heavy metal simulator stall exercises has any one mentioned stick pressure.
The only times Iíve had the death rattle go was on approach in turbulence on the Trident..whether incorrect load and genuine or the system I was never told but there wasnít any ďohh the stick force warned meĒ moments.
Iíve flown several gliders where it doesnít change although a few have rudimentary springs used for trimming which do have increased force purely due to the stretch.
Model aircraft controllers..the most difficult aircraft Iíve flown..have simple centring springs iirc with a parallel mechanism.
The only increasing force requirement in my real world is on low performance paragliders which is intended to stop low airtime pilots stalling inadvertently...say he who has accidentally stalled and spun them.
Off hand I can remember five occasions where colleagues got stall warnings or stalled.
Two were with incorrect droop selection, one at low speed which ended up in a field, the second extended at two high a Mach number and went through several levels in a holding stack before control was regained.
The third was a mate who was too busy talking during an intermediate level off having forgotten to engage auto throttle.
The final pair were both on visual approach into Nice on the then new Fokker 100 which supposedly had an all talking autothrottle and stall protection system which it didnít.
Stick force didnít come into it.
IMHO whoever drew up the requirements should have been an experienced pilot rather than a bean counter.


Last edited by blind pew; 19th Jan 2021 at 09:36.
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Old 19th Jan 2021, 10:50
  #75 (permalink)  
 
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Originally Posted by RVF750 View Post
..............As to Airbus, yuk. I've tried my whole career to avoid theying one. I like feel and feedback, thank you very muchly.

I always thought the answer for FBW sidesticks was to mount the whole stick and force sensing assembly on a moving base............. It was certainly a trick Airbus missed.
I am interested to know if you have actually been type rated onto Airbus FBW? I have flown them for 13 years and personally don't think the FBW side-sticks need an "answer"; they work very well.

Having previously been type rated on four different makes of conventional aircraft; I took to the Airbus FBW like a duck to water. You either see and feel your control limits through a yoke the old fashioned way, or you feel that you are at neutral or have reached the limit of the side-stick quadrant on the Airbus. That and the PFD is all the feedback I need.

The Airbus side-stick and FBW relationship is very poorly taught, I grant you. No TREs I asked ever properly explained to me how to use the side-stick, and eventually I taught myself. Initial training is hampered by not having linked controls - I learned a lot in my early days on turbo-props when Captains would invite me to "follow them through on the controls.
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Old 19th Jan 2021, 13:53
  #76 (permalink)  
 
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The airbus sidestick is fine. The lack of feedback from the other side is a bit of a problem.

The non-moving thrust levers, compared to say the Boeing, are the thing that I would change. Having said that - it's the way of the world now.
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Old 31st Jan 2021, 06:30
  #77 (permalink)  
fdr
 
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Originally Posted by PEI_3721 View Post
gums, Detent, et al, caution with specific comparisons.
Following on from zzuf, good refs; - associating the A320 with feel and bob wts could be misleading.

Airbus chose the combination of electrical signalling, digital computation, and side stick as their FBW system. All of these challenged the certification status quo; this required adaptation, interpretation, and new regulations. Airbus designed the future, rewrote the regulations.
Conversely, Boeing, using the improved technologies where possible, chose to follow history. Moving sticks, trim based force-feel, fleet commonality.

The piloting differences matter little (avoiding endless opinionated debate); these aircraft meet the safety requirements, and are flown as any aircraft can be.

Airbus chose a manoeuvre demand algorithm C*, a combination of pitch rate and 'g', with rate dominating low speed, 'g' higher. The trim followup restores a stable condition and minimises drag. (Military systems may favour 'g' more than rate)

Boeing is similar except the trim followup had to maintain the speed-dependent force-feel; this is the basis of the C*U algorithm, where U is the speed aspect.

The 737 Max with MCAS extended previous trimming adjustments (STS), to compensate for aircraft weaknesses; in no way is this FBW (electrical signalling at best). Post modification, MCAS fails safe, no trim demand or incremental runaway (although separately, the stab still could).

Relating this to AF447; the A330 did exactly what the pilot demanded, for the situation, as the crew interpreted it - UAS - pitch/ power. The technical / icing cause had be been identified from previous incidents and modification was in hand. The safety lessons stem from why this one accident differed from several previous incidents with the same trigger - go to previous threads for debate.

In the 737 Max accidents the aircraft did its own thing; crews were in recovery mode without explanation or guidance.

Even after modification, MCAS and the revised checklists could still lead to an AF447 type of incident. The checklist for AoA disagree requires action for unreliable airspeed; this biases the crew's mind to UAS, requiring pitch/power, bypassing wider situational aspects such as cross referring standby instruments which could clarify the situation, not requiring any manoeuvre.
This suggests that Boeing's operational approach to MCAS, alerting and drills, are still out of sync; perhaps not really appreciating if (why) MCAS is required - certification or training.
Old aircraft, even with new mods still require old style situational awareness to help crews understand, without ambiguity, and not presuming any failure, cause, or action. It is difficult to impose new technical philosophies on an old aircraft; similarly for operational aspects.

C* works nicely however it leads to the atrophy of procedures in the event of a law change, and that has led to a few rapid dissemble events. It is nice to fly though when working rite/write/right... reversion needs practice.
C*U makes you appreciate C*.

The aerodynamic issues of MAX (incidentally a popular name for dogs, and cats) are able to be resolved aerodynamically. The fact that there is a lifty surface poking out before the wing is not a problem, the non-linearity in what it does though is. At the same time, the bits that are added to the BRT to cure previous non-linearities are kept.... and no one got out an angle grinder? On NTRS, there's about 1000 papers related to high alpha lifting body flow control, and, surprise, the bits on the pained side of the cowl work on those factors, and got rid of the exact opposite issue that existed which led to the implementation of the sticky-outy stuff, which ended up assisting the non-linearity that was then the needy bit for the guys to redo the "lets run the trim nose down" bit of MCAS... whew. one breff.

Getting out the hacksaw or angle grinder would affect one thing, however, it would increase VS1g by a bit. But putting a LET on the mid-span TE Flap would do more than compensate for that, and even the inner flap of the Yehudi area would function to increase CL, and drag the CP aft mitigating what otherwise would be a Cm reducing shift of spanwise lift distribution inwards and therefore forwards... it's a swept wing.

There are a few pretty neat papers with NTRS and with AIAA ARC, DLR, DTIC, Delft, TsAGI, RAeS etc on the matter of low aspect VGs on nacelles and flow interaction with a wing, they are easy reading, and how and why are well described. Slender body flow control gets to the same point, and John C Lin, Carranto, Bruce Storms, Li, and the rest of the mob of researchers covered the issues pretty well, following on Bob Liebecks works, however, the subject has always been missing a simple point, one group of researchers looked at low-speed stuff and just that area. in a cell next door, literally, was a bunch of guys looking at high-speed effects of another device, and apparently, there wasn't any time for two to compare notes over coffee, as the flow structures are the same... the effects are different the mechanisms are the same. The fringe benefit is you also get the real world supercritical flow outcomes that do not exist except on paper due to the point that details matter.

Anyway, a hacksaw or angle grinder would have removed the issues of stick force gradient and would have been readily offset by simple mods to the TE of the flap that would have reduced VS1g, and therefore retained or improved V speeds, while nicely reducing drag in transonic flight. There is a modest increase in trim drag in cruise due to the wing actually doing what it is supposed to do, but that is around 1% of the magnitude of the drag reduction on the wing (which is still only a little of the total lift, etc... ) some 16M USD later, we did prove to non-believers that, surprise, lift and drag are orthogonal, because they are, well, orthoganol. The negligible trim change was recorded, it was measurably less than the trim change resulting from shifting 1 x 67kg SLF from 18C to 1B. For the 737 particularly, the flap track design hates vibration, and the mods to the flap reduce that considerably because the reason for the vibration, instability of the Kutta condition at the TE, is resolved.

just sayin' MCAS was a hammer fix for a typo by a builder.

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Old 31st Jan 2021, 10:20
  #78 (permalink)  
 
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Sounds good fdr,
BUT MCAS does not operate with flaps extended as I understand things. So then, do what you like with the flappery, it matters not.
A simple old engineer like me sees the easiest and best fix as small strakes on the aft fuselage set to zero incidence in the CRZ case. Now they cost a bit of wetted area and maybe a bit of extra drag on CLB coz the strakes do not align with the streamlines. However it is a simple once for all fix with no failure case. Never mind this software stuff, KISS.
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Old 31st Jan 2021, 10:32
  #79 (permalink)  
 
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fdr, thank you for the extensive background.

Re 'angle grinder' fixes. One of the first western public conferences where TsAGI presented a paper on high alpha aerodynamics. - In answer to a question on their approach to strakes re nose slice, why some aircraft could perform 'cobra' manoeuvres (SU27) vs those requiring extensive theoretical development (F18); 'if a particular aircraft had a problem then change the nose cone until one was found which did not roll off'. A lesson in practicality.

EASA neatly concludes the certification need for MCAS:-
"MCAS has been established to play only a limited role in augmenting the stability and stall characteristics of the aircraft in certain conditions. Ö needed to ensure the stability margins that make the aircraft fully compliant to the applicable regulations on stall demonstration and pitch control characteristics. This explains its inclusion in the original 737 MAX design.
These stability margins are required by regulation in order to support the flight crew handling of the aircraft during certain manoeuvres such as approach to stall Ö
MCAS was needed to provide full compliance but also that the loss of this function does not preclude the safe flight and landing of the aircraft; i.e. the 737 MAX remains stable following the loss of the MCAS function."

Boeing 737 MAX Return to Service Report
https://www.easa.europa.eu/sites/def...ice_Report.pdf
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Old 1st Feb 2021, 16:46
  #80 (permalink)  
 
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Salute!

Outstanding discussion and analysis by fdr. love it

Other than many posts concerning the aerodynamic solutions other than the kludge MCAS, I still need someone to explain how the 'bus got certified with no control feedback related to AoA or mach or....

I can unnerstan that a certain command from the yoke or stick should result in some aerodynamic response, but we have the gee command in the 'bus folks, then hybrid commands in the B777 and then the laws I flew with some of the systems years ago along with the Concorde.

My opinion is the FAA and other certification agencies need to revise the requirements. The MCAS was implemented to satisfy a relationship on a chart or graph that would be meaningless in the AB 320 and later. When I looked it up, it looked like something for the Aeronica Champ I flew in 1960.

Oh well, I hope we move forward and avoid a chain of decisions and implementations that resulted in the MAX tragedies.

Gums sends...
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