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737 Max Solution, Please Explain

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737 Max Solution, Please Explain

Old 17th Oct 2020, 14:38
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737 Max Solution, Please Explain

As Boeing always offered a triplex alpha sensor array why do they not simply make that the standard?
Why, given the opprobrium and distrust they've generated over the origingal system, have they decided on a duplex system that at first glance looks like a cheapskate half- measure to be followed by an artifcial third electronic comparator, but not yet.

What is the additional cost of a triplex system over the simplex one? Surely not a deal-breaker on a $60m aircraft? I'd haave thought the development work on the new solution would far outweigh simply reverting to triplex and wrapping the problem up.

Anyone able to explain the reality?
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Old 17th Oct 2020, 15:40
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meleagertoo,
First you should confirm if the Boeing option for three AoA applies to the 737. I suspect not, because as you argue this would have been the simpler option.

Second, the modifications to the Max appear contain the adverse effects of AoA and MCAS with computation using the existing two AoA sensors, thus changing to three would be a big issue particularly if not required for return to service.

EASA's position has to be confirmed; their preference was for three. We wait with interest if the proposals based on two are sufficient or not. I suspect that they will be because the MCAS argument is about the required level of safety for an existing certification. However, future certifications - MAX 10 perhaps might be argued as requiring safety improvements re multiple failures, alerting, workload, confusion, etc.
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Old 17th Oct 2020, 20:45
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We were given to believe that the MAX was available with a triple sensor Alpha system. That is what so much of the argument gas been about.

I'm not interested in EASA. I'm asking why Boeing hasn't just gone to this pre-existing option as a way of overcoming the sh!tstorm over a single sensor.
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Old 17th Oct 2020, 22:16
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Originally Posted by meleagertoo
We were given to believe that the MAX was available with a triple sensor Alpha system. That is what so much of the argument gas been about.

I'm not interested in EASA. I'm asking why Boeing hasn't just gone to this pre-existing option as a way of overcoming the sh!tstorm over a single sensor.
I very much doubt that the MAX was offered with a triple AoA system, or that it was a "pre-existing option". Why would it have been?

What evidence do you have to support the assertion that it was?
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Old 18th Oct 2020, 01:44
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Originally Posted by DaveReidUK
I very much doubt that the MAX was offered with a triple AoA system, or that it was a "pre-existing option". Why would it have been?

What evidence do you have to support the assertion that it was?
Agreed - to the best of my knowledge no Boeing commercial aircraft have 3 mechanical AOA sensors - although a third synthetic AOA is available on some models.
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Old 18th Oct 2020, 10:16
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I can't help wondering why simple strakes were not fitted to the aft fuselage a la Lear 51 etc. These strakes can be aligned with the streamlines in crz and produce a trifling amount of drag but do their job at extreme attitudes. Very simple and no failure modes. What have I missed?
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Old 19th Oct 2020, 10:16
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Originally Posted by mustafagander
I can't help wondering why simple strakes were not fitted to the aft fuselage a la Lear 51 etc. These strakes can be aligned with the streamlines in crz and produce a trifling amount of drag but do their job at extreme attitudes. Very simple and no failure modes. What have I missed?
I don't think the Lear 51 made it into production.

Joking aside, AFAIK the strakes on the Lear 60 and those retrofittable on earlier models are there to provide stabiliity when the tail is blanked at a high AoA.

That's not the problem on the MAX.
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Old 20th Oct 2020, 09:53
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OK but there are other aircraft for which strakes provide an safe, effective and cost effective solution with no failure modes.
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Old 20th Oct 2020, 10:06
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Strakes, ventral fins, etc. aid directional stability at high AoA, where the vertical stabiliser and the rudder are shaded from airflow by the fuselage.

As said above, directional stability at high AoA is not an issue with 737 MAX.
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Old 20th Oct 2020, 17:17
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Originally Posted by mustafagander
OK but there are other aircraft for which strakes provide an safe, effective and cost effective solution with no failure modes.
What are these "other aircraft" that exhibit the characteristics of the MAX that MCAS was intended to mitigate?
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Old 21st Oct 2020, 09:33
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Hoo boy Dave. Stop being intentionally obtuse.
Probably in our first year Aerodynamics lectures we learnt that at high angles of attack aircraft mush along climbing, if at all, nowhere near the attitude at which the fuselage currently presents. This means that strakes on the lower fuselage, best as close as possible to the empenage for maximum leverage, generate a significant nose down force with no crew action and no failure modes. Hence a huge assist to unstall. Details I can't be bothered to find - after 50 years I've likely lost the paperwork. It is not difficult to design a system like this to counter the nacelle lift afflicting the B737 Max.
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Old 21st Oct 2020, 17:23
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Originally Posted by mustafagander
Hoo boy Dave. Stop being intentionally obtuse.
Probably in our first year Aerodynamics lectures we learnt that at high angles of attack aircraft mush along climbing, if at all, nowhere near the attitude at which the fuselage currently presents. This means that strakes on the lower fuselage, best as close as possible to the empenage for maximum leverage, generate a significant nose down force with no crew action and no failure modes. Hence a huge assist to unstall. Details I can't be bothered to find - after 50 years I've likely lost the paperwork. It is not difficult to design a system like this to counter the nacelle lift afflicting the B737 Max.
B737 itself had a problem at high AoA. That's why they deviced STS. When they further tilted the bigger engines to create space underneath it became critical. There were no soft options. So they took the pilot out of the loop and gave complete control to MCAS. But they did it without any redundancy and clandestinely. Airbus has triple redundancy and yet a few problems happened which they sorted out quickly. But Boeing planned the software upgrade in there own sweet time. Unfortunately the whole thing came apart very quickly.
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Old 21st Oct 2020, 18:25
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Originally Posted by mustafagander
Hoo boy Dave. Stop being intentionally obtuse.
Probably in our first year Aerodynamics lectures we learnt that at high angles of attack aircraft mush along climbing, if at all, nowhere near the attitude at which the fuselage currently presents. This means that strakes on the lower fuselage, best as close as possible to the empenage for maximum leverage, generate a significant nose down force with no crew action and no failure modes. Hence a huge assist to unstall. Details I can't be bothered to find - after 50 years I've likely lost the paperwork. It is not difficult to design a system like this to counter the nacelle lift afflicting the B737 Max.
Boeing did look at various aerodynamic mods to solve the high AOA issues - however none provided an adequate solution (and believe it or not, Boeing does have some pretty sharp aero types ). OTOH, MCAS did exactly what was needed (at least when it worked as intended).
This was all discussed in some detail in at least one of the many MCAS threads, if you want to look through a few thousand posts to find it...
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Old 21st Oct 2020, 18:25
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Originally Posted by mustafagander
It is not difficult to design a system like this to counter the nacelle lift afflicting the B737 Max.
So why, in your view, didn't Boeing adopt your apparently simple and obvious solution instead of inventing MCAS ?
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Old 21st Oct 2020, 19:22
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I assume weight gain, drag and associated costs in fuel burn are what nixed an aerodynamic fix, rather than the fact it couldn’t be done.
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Old 21st Oct 2020, 19:53
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Wouldn't it prove to the outside world that practical flight behavior IS different if one would install visible strakes and stuff to counter nose up behavior? This is exactly what they had wanted to keep quiet about and avoid unwanted attention from say the certifying authorities.
To be fair any modern computer aircraft would get the same modifications quietly coded into it's software so nobody would ever know about.
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Old 22nd Oct 2020, 06:30
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Or maybe Boeing just didn't have the expertise available that we have on PPRuNe.
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Old 23rd Oct 2020, 03:29
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Perhaps their aerodynamisists are not up to scratch Dave, seeing as they went down that road to no avail. Perhaps the Ppruners offering up their advice should have been offered a contract, or they could get together themselves, form a company like Raisbeck, solve the problem, and make a fortune modifying aircraft, or as Raisbeck did, have the mods come as standard fit off the production line as with Beechcraft for one. The rear strakes on the Lear etc are to solve a completely different issue to the MAX.
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Old 21st Sep 2022, 02:47
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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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Old 21st Sep 2022, 12:42
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What would the difference in development/testing time for an aerodynamic vs a software fix?

I am assuming (with zero knowledge of development processes) it would be significantly longer and more expensive to develop and test a hardware fix than a software fix incorrectly deemed to be of lower criticality?
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