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Was MCAS needed?

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Was MCAS needed?

Old 2nd Feb 2021, 22:26
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Bergerie, ' a better solution'
We might assume that a STS type of fix was considered / tested.
STS as I understand is only a single trim input, also like the 747 example it uses speed as the sensor; loss of sensor was not critical.
MCAS 'downfall' involved the dynamics of the situations; the need of AoA as a sensor and more than one trim input. Introducing these was critical to the MAX deficiency (type similarity / certification), which enabled the opportunity, with corporate pressures, to poorly engineer MCAS where loss of sensor was critical.
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Old 3rd Feb 2021, 03:06
  #102 (permalink)  
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Originally Posted by Bergerie1 View Post
fdr,

Thank you (and all you other knowledgeable posters here too). As you say fdr, the 747 stalls very nicely, I did many on the -100s and -200s during CofA check flights. When the 747 was put on the British register, D. P Davies required a stick nudger (not a pusher) to restore longitudinal stability shortly before the clean stall at aft CG. I quote his words from his book, Handlng the Big Jets:-

"With trim speed of 1.3Vs, after a small elevator force to start the speed reduction, the stick force falls to zero while the aeroplane quietly progresses all the way to the stall on its own .... It is common knowledge that the UK attitude to stalling is quite firm and this degree of instability, although slight, was declared unacceptable..... ."

Here was a simple cheap and reliable fix which had already been used on Boeing aircraft. It would have required no training with only a mention in the manuals. Would this not have been a better solution?
K, for those wanting to stall any of these aircraft, a note first up, there is a maintenance procedure for inspections after stall buffet encounter on most if not all of these aircraft. The amount of buffet that occurs varies from type to type and with configuration. Clean is usually quite light buffet and associated loads on the tail, but heavy buffet entry in the MD11 in the cruise, at high Mach pretty mush shredded the elevators on both sides, and came close to two ripples on the pond. A fully configured stall buffet can be quite impressive, as are the loads to the tail and flap tracks.

25.173 requires 1lb/6kts average gradient over the desired window being assessed, which is within the flight envelope from stall to upper limits. There are two main methods of determining compliance, but basically, taking the load on the control, coming off the trimmed speed, (1.23 or 1.3 depending on the rules applied earlier in the Part, the year etc... ) and slowing down as well as speeding up around that datum. For a stall speed around 100kts, the minimum trim point is Vref... so the loads being measured total are around 4lbs to 6 lbs of elevator force, which is quite light. The requirement is for the average to be that, and the guidance material in AC25.07 shows clearly that a reduction in force can occur towards an extreme of the test points, and still be acceptable. That is open to interpretation in so far as a zero gradient for the last 10kts could be fully compliant, but considered objectionable from an HQ view by the testing driver. My recollection is that the stick gradient in the mid-trim condition was way higher than that, and never encountered reversal, which is where a rationale for a SAS system would arise. D.P. Davies did good work, and perhaps at aft CG limit -0.5% at light weight, high thrust, he encountered an undesirable that Boeing didn't. It is possible. Is it likely? TBC did a comprehensive testing program on the 747 under the gaze of Joe Sutter and coming in the wake of the Trident, and some 727 funnies, they were not without awareness of what was needed. A spring balance to the controls could detect that level of force in the day, today, some of the FBW systems record the forces applied, or force sensing gloves can be applied and calibrated to give the info. (make your own, go grab some Phidgets... ).


Last edited by fdr; 3rd Feb 2021 at 07:19.
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Old 3rd Feb 2021, 03:28
  #103 (permalink)  
 
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Gums, I joined this forum about 3 years before I retired (basically right after the Asiana 777 crash-landing at SFO). Several co-workers knew of my presence, but I was very, very careful about what I posted to avoid anything that might be considered sensitive, proprietary, or anything that would otherwise get me crossed up with management. More than once, someone posted something that I knew was total BS, but couldn't respond because my knowledge was considered to be Boeing proprietary (one example that comes to mind was a discussion regarding 747-8 fuel burn - of which I had first hand knowledge but couldn't talk about).
Since retirement, I don't need to worry as much but still need to be careful about posting info that could still be considered sensitive. Much of what I know about MCAS is based on an off-the-record discussion I had with a Boeing test pilot friend shortly after the Ethiopian crash. I felt obligated not to post any of that information as not to betray his trust. Most of it has since been reported in the Seattle Times, so I'm confident discussing it - but a few things I still need to keep under wraps...
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Old 3rd Feb 2021, 17:42
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"Most of it has since been reported in the Seattle Times"

I knew people who worked for Airbus who greatly enjoyed Byron Acohido's pieces in the Seattle Times.
I often wonder whether Boeing would have preferred to have him silenced (in a friendly, painlesss, way).
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Old 4th Feb 2021, 18:19
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Lecture on the 737 MCAS story by Chris Brady kicks off in just over 10 mins: https://www.solent-raes.org.uk/lectures
Free to join (I'm just another listener, nothing to do with it so I hope it's allright to post this).
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Old 4th Feb 2021, 18:27
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Thanks the harp plays already.
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Old 4th Feb 2021, 19:45
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PPRuNe got mentioned!
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Old 4th Feb 2021, 22:26
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It was worth it. Thanks for telling us right on time.
Thanks to RAeS for sharing this event. Most appreciated.
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Old 5th Feb 2021, 07:52
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Originally Posted by Jhieminga View Post
Lecture on the 737 MCAS story by Chris Brady kicks off in just over 10 mins: https://www.solent-raes.org.uk/lectures
Free to join (I'm just another listener, nothing to do with it so I hope it's allright to post this).
Recording still available on the above link. Lecture starts at around the 15:55 mark.
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Old 6th Feb 2021, 12:13
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Re
Recording still available on the above link
That link will disappear soon (if not already) but it will remain available at this link:
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Old 6th Feb 2021, 15:18
  #111 (permalink)  
 
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https://leehamnews.com/2021/02/02/th...in-the-making/
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Old 6th Feb 2021, 15:34
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Great presentation by Chris Brady. Many thanks to him for an informative, comprehensive and unbiased lecture.

Most particularly, how MCAS can look like normal THS operation and therefore not necessarily lead the pilots to Runaway Stab Trim.
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Old 5th Mar 2021, 14:33
  #113 (permalink)  
 
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Follow-up presentation just posted by Chris taking a more detailed technical look at MCAS.
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Old 18th Mar 2021, 21:30
  #114 (permalink)  
 
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it was legally required AFAIK, Airbus ran into similar problems when desigining the A321 through of course its esier when you have FBW
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Old 21st Sep 2022, 03:51
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As suggested previously, when encountering the longitudinal stability issue, Boeing could have completed aerodynamic mods to the design to remove the anomaly, which resulted from the nacelle adding lift to the wing/nacelle structure that was more than anticipated. That alone would have suggested that an angle grinder should be taken to the strakes to trim their nose hair a shade. Instead, they came up with a neat trick to change a high speed design that had questionable redundancy on trigger events to being low speed too by removing one of the two trigger conditions which gave a single point of failure as a matter of certainty. As AOA probes have a fairly modest MTBF in use, that wasn't a great concept.

Here is a set of charts that show the effect of having strakes or not, which would have been a relatively minor change to the aircraft. As the engines are inboard, it is a matter of certainty that reducing the section CLmax proximate to the nacelle would have ended up in an improvement in the stick force/g. Being judicious, the effect to Vs1g would have been quite modest, and surely, please surely the OEM noted that the stall speed was curiously lower with their design than expected, otherwise, they need a serious boot in the bottom of their trousers for being myopic.


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