737 Max Solution, Please Explain

[meleagertoo]

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?

[PEI_3721]

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.

[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.

[DaveReidUK]

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?

[tdracer]

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.

[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?

[DaveReidUK]

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. :O

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.

[mustafagander]

OK but there are other aircraft for which strakes provide an safe, effective and cost effective solution with no failure modes.

[FlyingStone]

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.

[DaveReidUK]

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?

[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.

[vilas]

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.

[tdracer]

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 :ugh:). 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...

[DaveReidUK]

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 ?

[Jwscud]

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.

[Less Hair]

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.

[DaveReidUK]

Or maybe Boeing just didn't have the expertise available that we have on PPRuNe. :O

[megan]

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.

[fdr]

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.


https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/2000x889/screen_shot_2022_09_21_at_12_46_48_pm_f500ad5dba6921606e7de7 f75d0ae27c26ff606a.png

[Jwscud]

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?

[Less Hair]

I'd expect the opposite. Some hardware fix is faster than software. AFAIK the MAX software is done externally (Rockwell Collins?), one of the reasons the modifications finally took so long. Unusual strakes at certain places might point to aero issues that some might prefer to not put on display if possible?

[safetypee]

The charts above do not indicate the nature of the longitudinal stability issue - seen by the pilot as change of stick force with speed.

The MAX problem was twofold.
First identify the nature of the stability problem, as required for certification, which might be fixed with a relatively easy ‘bent tin’ mod.
Second was to make the MAX handling (stick force vs speed) as close to that of previous variants so that there was minimum type difference training - no additional simulator time.

The optimisation task was further complicated by the -800 certification being marginal for these aspects with respect to the latest certification standards, avoided by ‘grandfather rights.’

So where a simple ‘bent tin’ mod could be certificated, new, variable - ‘auto reconfiguring tin’ with speed would be required for type similarity. As I recall there was previous discussion on this re tests of LE ‘flaps’, strakes, etc (#13 and previous threads)

[fdr]

The charts above do not indicate the nature of the longitudinal stability issue - seen by the pilot as change of stick force with speed.

The MAX problem was twofold.
First identify the nature of the stability problem, as required for certification, which might be fixed with a relatively easy ‘bent tin’ mod.
Second was to make the MAX handling (stick force vs speed) as close to that of previous variants so that there was minimum type difference training - no additional simulator time.

The optimisation task was further complicated by the -800 certification being marginal for these aspects with respect to the latest certification standards, avoided by ‘grandfather rights.’

So where a simple ‘bent tin’ mod could be certificated, new, variable - ‘auto reconfiguring tin’ with speed would be required for type similarity. As I recall there was previous discussion on this re tests of LE ‘flaps’, strakes, etc (#13 and previous threads)

Actually it does.

The chart refers to CL, and of less importance to Cd. The CL chart is everything of interest. From the CL, and reading the thesis alone, the pressure distribution on the wing can be determined, and that gives the Cm, and that is the point of interest. Strakes/Vanes/Chines whatever their name, act to mitigate the interference effect that occurs from the.... NACELLE. The effect is a high AOA effect, and the interference suppresses CL. So remove tghe chine and CL in that area of the wing reduces at high AOA, which can be tuned to match the effect of the component from the nacelle of the MAX that causes the longitudinal stability issue, which was Subpart B to Part 25, §25.173, arguably most subparas.

Why would the trimming of the chine affect static stability? It suppresses the peak of the CL/AoA (a-slope) for that part of the wing. As the wing is a swept wing, that area is forward and therefore the Cm of the whole wing will become more negative than previously, which is what is needed for the stability, along with of course a beneficial reduction in downwash to that tail that occurs due to the local high CL peak at high AOA that. otherwise exists, which happens to increase the tail downforce for a given stab trim (THS for bus drivers) which otherwise exacerbates the problem at the higher AOA's.

As a reduction in the chine will suppress the peak CL for that section, the total lift available at the Vs1g condition, (well, Vsr, §25.103) will be slightly reduced, and so the V will increase slightly, which affects:

Vr
V2
Vref

and those in turn affect slightly

TODR
SSL (speed is marginally higher, therefore to meet §25.115 the ROC is slightly higher, as in... less than the thinkness of the ASI needle...)

The TODR and SSL changes may affect the RTOW on some occasions, but are basically negligible.

The tested performance that was certified was presumably inadvertantly (please tell me it was inadvertent.... :ooh:) predicated on the additional "benefit" of the lift from the nacelle, which consequently gave the static stability problem and that then gave the rat cunning repurpose of a high speed system that required 2 trigger events, altered to give both a high speed (wind up condition event) and the low speed (the discovered surprise package in the middle of the testing) surprise, low speed triggering being achieved by removing the speed logic, making the system open to a single trigger event, from a single AOA probe sensor, and that gave us the carnival rides that ended in tears.

In all honesty, the planes stall just fine, and the average driver would not be getting into any real difficulty, however, while the -800 met the rules, the Max did not. Flying a plane with this static stability is actually quite nice, not for everyday IFR, but darned if it isn't what makes planes like the Pitts fun to fly, it does exactly what you ask it to do. Pilots have problems with stalls as we are not exposed to the extent that we should be, and that is not the problem of the plane, that is a training and competency matter. The response to an inadvertent stall by the average (is there such a thing?) crew range from the wrong input (confusion, instrument failures...etc) to nothing, sit back and watch as the plane does a wifferdill, to wild over reaction. All that is necessary is to ease off the g load at that time, by lowering the AOA, that's normally about 1-2 pounds of pressure on the stick, but we are white knuckled and freaked out as we are not trained correctly. Sims today are getting better aeromodels, they are variable, some are still shockers, but many give a good training point for drivers to become less freaked out by stalls. To get to a stall at low level, the crew have to be out to lunch, lights on, no one home. Recovery is simple, and prompt, and that plane talks to the driver. High altitude, the stall is very benign, but there will be a height loss in most cases, the excess thrust in minimal in most cases, and the plane is often on the backside of the drag curve. Still not a big deal, enjoy the scenery while speed builds up, and wave at the planes at the lower level as you pass them by.

One jet I fly has no stab trim change from takeoff, through 450Kts Indicated, to stall. (above 420 indicted there is a slight, noticeable push force, but nothing to bother grabbing the trim wheel for). That plane is beautiful to fly form, aerobatics or low fly in. (rather different to say the T-6 or T-28 where there is a fair amount of trim change with anything you do with power or speed, or any control).
§ 25.173 Static longitudinal stability. (https://www.ecfr.gov/current/title-14/section-25.173)Under the conditions specified in § 25.175 (https://www.ecfr.gov/current/title-14/section-25.175), the characteristics of the elevator control forces (including friction) must be as follows:

(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC, whichever is appropriate, or lower than the minimum speed for steady unstalled flight.

(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in § 25.175 (a) (https://www.ecfr.gov/current/title-14/section-25.175#p-25.175(a)), (c) (https://www.ecfr.gov/current/title-14/section-25.175#p-25.175(c)), and (d) (https://www.ecfr.gov/current/title-14/section-25.175#p-25.175(d)), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in § 25.175(b) (https://www.ecfr.gov/current/title-14/section-25.175#p-25.175(b)), when the control force is slowly released from any speed within the range specified in paragraph (a) (https://www.ecfr.gov/current/title-14/section-25.173#p-25.173(a)) of this section.

(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.

(d) Within the free return speed range specified in paragraph (b) (https://www.ecfr.gov/current/title-14/section-25.173#p-25.173(b)) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.

[Uplinker]

What are these "other aircraft" that exhibit the characteristics of the MAX that MCAS was intended to mitigate?

The Dash 8 Q400 has long fuselage strakes on the lower rear sides; below and forward of the empennage. Not sure what causes the issue they solve, however. The long thin fuselage perhaps?

[flypaddy]

I understand the ventral strakes on the Q400 are to straighten prop wash swirling around the fuselage. Pratt & Witney didn’t want to make a counter-rotating version of the engine for cost/parts reasons so the props turn the same direction. As a result, every change in thrust requires a rudder trim change.

[fdr]

Strakes can be applied in varying means for varying outcomes.

On a tail boom, a longitudinal strake on the opposite side to the anti-torque direction of thrust provides a local flow that gives lateral force. (vertical flow oriented VGs on the opposite side improve that outcome considerably)
On a chipmunk or tigermoth, they alter the spin characteristics when applied as extensions to the horizontal stab.
On the KingAir, Learjet etc they are applied so as to improve lateral directional stability. They can also reduce total drag, as they can cause reorientation of the flow that has considerable separation otherwise at the rear of the fuselage on most aircraft.
The Q-400 implementation gives flow cahnges around the body of the aircraft otherwise influenced by the wake of the propellers.
Strakes were added to the belly of the Harrier and reduced the thrust required to hover.
A series of strakes were trialled on the C-17 to reduce drag.
Strakes on various cargo aircraft are applied to control flow particularly when airdrop operations are being conducted.
Strakes were added to the DC-9-51 and other variants of the long beach cable companies toy. They are in an area of upwash around the lower forward fuselage, which increases the upwash at higher AOAs.
Strakes have been evaluated on various "slender bodies at high AOA" that happen to end up with asymmetric shedding of vortex structures coming off the nose cone, that cause nose slice and consequent roll off at very high AOA.

The downside with strakes is they can be overly effective in lateral directional stability, and that gets to be a problem with normal control authority. While they can reduce drag, particularly the lower angled types, for longitudinal stability they will mostly interact with the stabiliser when it is a variable stab, and can have an adverse drag count change.