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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Old 9th Jul 2019, 21:41
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Murphy, the bit about how to write something up does happen because some things can take more time and make take the jet out of service for awhile and that many mean cancelled flights, lost money, and pissed off managers. A common one is an overspeed event. Different inspections are required depending on amount of overspeed and duration. Shade the answer a few knots or a few seconds one way, and it makes the difference between and hour inspection and sign off versus a trip to the hangar for half a day or more. There’s also the possibility of some CYA going on. Writing down in the logbook that you killed your electric trim system, let the stick shaker run for the entire flight, but thought nothing of continuing to the scheduled destination might have brought some unwanted attention back on the crew. I know it would here. Feds periodically audit the logbooks for unusual goings on, and you might be amused to know the kind of violations that pilots have inadvertently confessed to with their write-ups. Keep in mind no one had a crystal ball to tell these guys that the plane was going to crash tomorrow. Now they have all the world looking at them. Hindsight is always 20/20 and sometimes 20/10.
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Old 9th Jul 2019, 22:54
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Although I believe a new thread dealing with Trim issues on MAX and probably NG- its doubtful the Moderators will agree. A thread this long becomes nearly impossible to stay on topic- or provide timely responses to a specific question or observation. perhaps Tech log would be better.

In any event- I thought a possible way to sort of reset what Boeing ** thought* was a good solution back in April- march time frame versus where they are now would help level set the discussion from here on. What I will post is a copy of a long article found from AV week - In april this year with the HOPE it will help to address most of the known issues to date. I did boldface-italicize on statement I believe is significant.


Fred George Aviation Week & Space Technology Apr 11, 2019

Boeing has demonstrated the old and new versions of the MAX’s Maneuvering Characteristics Augmentation System (MCAS) to pilots and regulators in its 737 MAX engineering cab simulator in Seattle. The MCAS is a new flight-control-computer (FCC) function added to the MAX to enable it to meet longitudinal stability requirements for certification.

However, the system is only needed to enhance stability with slats and flaps retracted at very light weights and full aft center of gravity (CG). The aircraft exhibits sufficient natural longitudinal stability in all other parts of the flight envelope without the MCAS to meet the rules. Boeing emphasizes that the MCAS is not an anti-stall or stall-prevention system, as it often has been portrayed in news reports.

The new software load [P12.1] has triple-redundant filters that prevent one or both angle-of-attack (AOA) systems from sending erroneous data to the FCCs that could falsely trigger the MCAS. It also has design protections that prevent runaway horizontal stabilizer trim from ever overpowering the elevators. Boeing showed pilots that they can always retain positive pitch control with the elevators, even if they don’t use the left and right manual trim wheels on the sides of the center console to trim out control pressures after turning off the trim cut-out switches.

Most important, the MCAS now uses both left and right AOA sensors for redundancy, instead of relying on just one. The FCC P12.1’s triple AOA validity checks include an average value reasonability filter, a catastrophic failure low-to-high transition filter and a left versus right AOA deviation filter. If any of these abnormal conditions are detected, the MCAS is inhibited.

Three secondary protections are built into the new software load. First, the MCAS cannot trim the stabilizer so that it overpowers elevator pitch control authority. The MCAS nose-down stab trim is limited so that the elevator always can provide at least 1.2g of nose-up pitch authority to enable the flight crew to recover from a nose-low attitude. Second, if the pilots make electric pitch trim inputs to counter the MCAS, it won’t reset after 5 sec. and repeat subsequent nose-down stab trim commands. And third, if the MCAS nose-down stab trim input exceeds limits programmed into the new FCC software, it triggers a maintenance message in the onboard diagnostics system.

According to a pilot who was shown the changes in a simulator session, the demonstration begins with the original MCAS software load. During a normal takeoff, at rotation, the left AOA indication moves to its maximum reading—as seen from the flight data recorder in the Ethiopian Airlines accident. Pilots currently do not experience this during initial or recurrent simulator training. The stickshaker fires continuously, using loud sound and control wheel vibration to focus the pilot’s attention on the critically high AOA indication.

The erroneous AOA reading also creates large-scale indicated airspeed (IAS) and altitude errors on the primary flight display (PFD) which can be both distracting and disorienting.

AOA is used by the aircraft’s air data computers to correct pitot and static pressure variations induced by changes in nose attitude in relation to the relative wind. Large errors in AOA can cause 20-40-kt. errors in IAS and 200-400-ft. errors in indicated altitude. This is accompanied by the illumination of annunciators on both PFDs that warn of disparities in the IAS and altitude between the left and right displays. As part of the MCAS redesign, Boeing also is upgrading the MAX with AOA dial indicator displays and AOA disagree warning annunciators on the PFDs.

After the high-AOA indication, pilots then follow the checklist for “airspeed unreliable,” which assures that auto-pilot, auto-throttles and flight directors are turned off. They then pull back power to 80% fan speed, set 10-deg. nose-up pitch attitude and climb to 1,000 ft. above ground level. At that point, they lower the nose, start accelerating and begin retracting slats and flaps at 210 kt. indicated airspeed. When the slats and flaps are fully retracted—the MCAS kicks in.

“It’s a good thing we knew what to expect. Otherwise tunnel vision from the ‘airspeed unreliable’ event could have blinded us to the subsequent MCAS nose-down trim input. When I noticed the trim wheels racing, I grabbed the left wheel. It was easy to stop the trim with hand pressure, but I knew in advance what was happening,” says the pilot flying. “We followed the checklist for runaway stabilizer, checking again for auto-pilot off and auto-throttle off. We turned off both trim cut-out switches and cranked the ‘frisbees’ [manual trim wheels on both sides of the center console] to relieve control pressures. We used manual trim for the remainder of the flight to landing touchdown and rollout. That was quite an eye-opener, as I had never been exposed to that during sim training,” he notes.

It is critical to follow the checklist memory items: Pull back thrust to 75% after retracting slats and flaps and set attitude at 4 deg., nose up. If speed builds up beyond 220-250 kt., controllability becomes increasingly difficult, he adds.

Pilots for three U.S. air carriers tell Aviation Week that during their sim training they had never been exposed to extreme and continuous AOA indication errors, they’ve not experienced AOA induced airspeed and altitude deviations on PFDs and have not had to deal with continuous stall-warning stickshaker distractions. They also note that they have never been required to fly the aircraft from the point at which a runaway stab trim incident occurred all the way to landing using only the manual trim wheels. “We’re just checking boxes for the FAA,” says one Seattle-based pilot.

A full aerodynamic stall with the MCAS inoperative is another exercise pilots experience in the MAX engineering cab simulator. “We reduced thrust at 5,000 ft. and slowed the aircraft at about 1 kt. per sec. We were at a midrange cg [center of gravity] with gear, slats and flats up. We trimmed until we reached 30% above stall speed and then just continued to ease back on the control wheel,” one of the pilots says.

“Pitch feel was natural, progressively increasing as airspeed decayed. Somewhere between the audible low airspeed warning and stickshaker, I felt the slightest lightening on control pressure in my fingertips. Quite candidly, if I had not been watching for it, I don’t think I would have noticed any difference between the MAX and the Next Gen [NG] models. I kept pulling back through stickshaker, then buffet, then elevator feel shift [a function that doubles the artificial control feel forces near stall] and finally until the yoke was buried in my lap. The nose just flopped down gently at the stall, and I initiated recovery as I would in most other airplanes I’ve flown,” he adds.

During design of the MAX, Boeing added two more leading-edge vortilons [generating vortices over the top of the wing at high AOA] in 2018, for a total of six per side and also lengthened and raised the inboard leading-edge stall strips to assure stall behavior would be as docile as that of the NG.

Repeating many of the same maneuvers in the engineering cab simulator with the new software load would have been academic at best, as the triple-redundant AOA validity checks all but assure that the MCAS will not be triggered by erroneous AOA inputs in the future. But, FCC P12.1 changes do not protect against erroneous AOA causing stickshaker or large-scale distortions in indicated airspeed and altitude values. Those malfunctions still can cause distraction and disorientation, especially when flying at night and/or in instrument conditions.

The new MCAS protections built into the P12.1 software load preserve its essential role in enhancing the MAX’s longitudinal stability, while virtually guaranteeing that it won’t be triggered by erroneous AOA. And when it does activate, its nose-down stabilizer trim command authority will be limited to assure the pilots always can control aircraft pitch with the elevators.

However, the FCC software upgrades are not the only critical changes needed to boost safety margins for operators. Pilots who underwent the demonstration also say the sessions underscored the need for additional simulator training for dealing with compound emergencies involving AOA and runaway trim failures.
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Old 9th Jul 2019, 23:15
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Well I am astounded that the issue of false alarms is not being pursued here
in what realistic scenario would a stick shaker operate continuously ?
how can an aircraft get certified in the case that stick shaker is operating erroneously?
why is this not a "fix ASAP" directive to the manufacturer?
how is it possible, given that the aircraft is certified with this possibility, that pilots cannot readily turn the bloody thing off!?
In my view false alarms and safety device activation's should get treated with EXACTLY the same regulatory response as a failure to activate.

Am I really the only person who views this matter as really serious? what am i not getting here?
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Old 9th Jul 2019, 23:23
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Originally Posted by hans brinker
I have read every post on every MCAS related thread ( including all the closed ones) so forgive me if I don't remember the posters names. I am absolutely positively sure there were several that said exactly that.
"Exactly that" being "pitch, power & attitude."

Yup, several posters said just that, repeatedly and emphatically. It happened often enough and conspicuously enough that it would be difficult for anyone paying reasonable attention to the threads to have missed it.

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Old 10th Jul 2019, 00:07
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A sideline observation.

Reading though some of the MAX thread(s) comments, there are numerous references to the offset thrust line's causing a pitching moment change with thrust changes but a general dismissing of the normal lift effects attributed to the nacelle inlet (or the similar normal prop disc force on high-powered prop aircraft).

I wonder if some folks have missed the point of references to nacelle lift ? A useful diagram at (lower) p14 of this lecture presentation

https://people.clarkson.edu/~pmarzoc...0/AE-430-4.pdf

may clarify the concept.

It follows from a consideration of the graphics that the problem can be significant at high AoA and high thrust settings, particularly if the engine (specifically, the nacelle inlet or prop position) is stuck well out in front of the CG for other reasons.

The momentum/force change associated with the airflow direction change increases and can provide a very noticeable decrease in longitudinal static stability. This can be of sufficient magnitude that some aircraft require add-on stability augmentation (cf the typical SAS systems in some piston to turboprop design conversions). I am aware of one such design conversion in which the effect will cause the static stability to become quite the reverse in the missed approach situation with an inoperative SAS and full thrust during the miss. One usually sees a power restriction in the event of a U/S SAS to keep the problem at bay ..

Without being privy to the backroom Boeing details, I suggest that the MCAS concern relates more to the normal force effects than the offset thrust line couple problem. One can find plenty of net articles on the topic should you wish to read up further.
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Old 10th Jul 2019, 01:39
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as yoko points out you should not hand over control to another pilot if the craft is in an unstable state
neither do you had over to the AP with the craft unstable

where then are the standards set for what aircraft state or failing that ! the DISPLAY state that needs to be set for the AP to hand control back to pilots

how would you feel if your co pilot handed you an upset craft and then started shouting in your ear and thumping the yoke!
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Old 10th Jul 2019, 02:15
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Originally Posted by phylosocopter
as yoko points out you should not hand over control to another pilot if the craft is in an unstable state
neither do you had over to the AP with the craft unstable
In real life, there are exceptions to most rules. One of the exceptions might be handing over an unstable aircraft when you are frantically -- and unsuccessfully -- trying to figure out why it is unstable and what to do to stabilize it, while being bombarded by warnings, the stick shaker, etc. And even folks who don't think there should be exceptions might find themselves handing off "improperly" in the throes of sensory and task saturation.
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Old 10th Jul 2019, 02:16
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Originally Posted by phylosocopter
as yoko points out you should not hand over control to another pilot if the craft is in an unstable state
neither do you had over to the AP with the craft unstable

where then are the standards set for what aircraft state or failing that ! the DISPLAY state that needs to be set for the AP to hand control back to pilots

how would you feel if your co pilot handed you an upset craft and then started shouting in your ear and thumping the yoke!
This has always been a problem with a lot of autopilots in that if you demand more out of them than they are willing to do then they will dump the whole thing back in the lap of the pilot with there own lights and alarms. Classic case of leaving it in vertical speed climb at high altitude it will try to do it while bleeeding off speed until it can’t and then disconnect. Same would probably happen with engine flameout. Pilots are expected to monitor at all times but sometimes they get distracted or are just staring out the window. Agree with you about stick shaker et al. As a previous poster once mentioned, pilots used to have discretions on pulling circuit breakers to take care of this but not so much anymore. I would do it in a heart beat myself but other pilots may not. Hope there is a reommended change to this coming out of these accidents.
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Old 10th Jul 2019, 06:37
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Originally Posted by Tomaski
This has always been a problem with a lot of autopilots in that if you demand more out of them than they are willing to do then they will dump the whole thing back in the lap of the pilot with there own lights and alarms. Classic case of leaving it in vertical speed climb at high altitude it will try to do it while bleeeding off speed until it can’t and then disconnect. Same would probably happen with engine flameout. Pilots are expected to monitor at all times but sometimes they get distracted or are just staring out the window. Agree with you about stick shaker et al. As a previous poster once mentioned, pilots used to have discretions on pulling circuit breakers to take care of this but not so much anymore. I would do it in a heart beat myself but other pilots may not. Hope there is a reommended change to this coming out of these accidents.
Yes the point I am trying to make is that there does not seem to be any code or standards around the state of display and alarms in case of AP loss of situational awareness. I believe that it should be mandatory in any case where the automatics have lost situational awareness to silence any situational alarms on handover back to the wetware! is there a section on AP disconnect state somewhere? if not why not!?

It would appear that with AF 447 the PF followed an erroneous flight director into the initial stall.after AP disconnect ? ??
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Old 10th Jul 2019, 09:52
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Originally Posted by phylosocopter
Yes the point I am trying to make is that there does not seem to be any code or standards around the state of display and alarms in case of AP loss of situational awareness. I believe that it should be mandatory in any case where the automatics have lost situational awareness to silence any situational alarms on handover back to the wetware! is there a section on AP disconnect state somewhere? if not why not!?
Also, when possible, it might be appropriate for HAL to [very infrequently] warn the pilots of the context of an impending disconnect.

As an example I always felt that there was a real possibility that AF447's HAL might have been able to give the pilots a little notice with a message such as:
- pitot tubes playing up, recall unreliable speed drill
- just lost a pitot tube, recall unreliable speed drill
Sort of HAL in F/E conversation mode.

PS I've no access to any details of pitot tube handling, but quite a lot of the wording of the AF447 threads kept suggesting to me something might have been possible.
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Old 10th Jul 2019, 09:58
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737 driver to name one was very assertive about just that. He made it sound so easy, too easy in my view.
So he is a 737 pilot and you are a refrigeration engineer, is that right? Just going by your profile.
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Old 10th Jul 2019, 11:01
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There are stories saying that EASA now wants five issues rectified? None of the stories seem to list those five issues.
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Old 10th Jul 2019, 11:39
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As SLF, I have to say that I have mixed opinions about this thread along with others. It's good to see the exchange of technical information and detail. There is evidently an enormous amount of expertise on display. This is reassuring.

Less so are the disagreements. I've once been in the unfortunate position of overhearing two medical consultants argue over a course of action the outcome of which held some relevance to my continued existence.

Yet. The board is for debate. There may come a time in your careers when you need to consider how you would react in situation X. If it isn't covered by documented procedures, I guess that some degree of highly informed guesswork must creep in.

So carry on. These moments at altitude are when you earn the sort of money you should be earning.
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Old 10th Jul 2019, 11:40
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Originally Posted by Icarus2001
So he is a 737 pilot and you are a refrigeration engineer, is that right? Just going by your profile.
That is correct.

There is several similarities between those two professions.

The control software of complicated refrigeration plants occasionally contains one or more serious flaws.
The control software of aircrafts seems to occasionally contain one or two serious flaw(s).

The ability of the engineers and management responsible for the (flawed) software seems to vary.
The ability of the operators (read pilots) to handle the consequences of those flaws also seems to vary.

In both of those professions it is clear that engineers and management can not rely on operators beeing able to reliably compensate for seriously flawed software. Insted the engineers and mangement need to make sure there is no serious flaw in the software and that the flaws that might be there is not serious enough to completely shut down the plant (read aircraft).
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Old 10th Jul 2019, 11:50
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Originally Posted by RickNRoll
EASA now wants five issues rectified? None of the stories seem to list those five issues.
From https://www.bloomberg.com/news/artic...-can-fly-again with my numbers
EASA’s checklist includes a number of issues that have been disclosed:
1 the potential difficulty pilots have in turning the jet’s manual trim wheel
2 the unreliability of the Max’s angle of attack sensors
3 inadequate training procedures
4 a software issue flagged just last week by the FAA pertaining to a lagging microprocessor
5 also listed a previously unreported concern: the autopilot failing to disengage in certain emergencies.

“Any of these could significantly affect the return to service, but we don’t know if they are actually going to become requirements or are they just
items for discussion,’’ said John Cox, a former 737 pilot who is president of the aviation consulting company Safety Operating Systems.
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Old 10th Jul 2019, 12:20
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phylosocopter, #1295
Re “code or standards around the state of display and alarms in case of AP
There are standards in CS 25 etc and ACs, however many are interpretive and do not cover grandfather rights per se.

For auto flight, my old 146 had a trim light (amber, no chime) which basically indicated that the AP auto trim function was having difficulty I keeping up - why there may be small deviations, or in preparedness for possible disconnection (red / audio) at a limiting condition, parameter excursion and/or trim value - but not an extreme limit.
The flap trim compensation system was independent of the AP, and if this failed - not available, there was an annunciation of loss of function; manual trim required.

The 737 trim issue relates to use of the trim system by an ‘external’ add-on computation when the AP is not engaged. Thus, add-on systems (STS, MCAS) are conceptually independent, but the use of computation within the AP ‘box’ probably enabled a grandfather certification without additional annunciation.
Also, as some of previous threads have discussed, how might failure of the add-on systems be detected - single / dual sensing; or even a separate trim runaway. Significant problems for situation awareness.
STS uses air-data thus is under the ‘umbrella’ of Airspeed Disagree. MCAS is not, but some engineering views suggest that it was mistakenly considered that was because of it incorporation with the AP; again grandfather thinking - certification did not check the detail as should have been done for a new system.
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Old 10th Jul 2019, 12:35
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Is item 1 in the list above limited to the max or all 737s?
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Old 10th Jul 2019, 13:41
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Originally Posted by aliwyatt
Is item 1 in the list above limited to the max or all 737s?
IMHO most probably includes the NG .A previous post a long time ago from an involved at the time engineer mention ed that the MAX trim wheel was smaller in diameter by a few inches and had to have a damper added to avoid springback. and that on a test bed- mockup, a well conditioned female pilot could barely turn the wheel. Much earlier - months ago- another pilot posted it was near impossible on the NG to turn wheel under real conditions- and over a decade ago at a reasonable flight speed- he manual explained ho to use the roller coaster or yo yo game to reduce loads if plane was much out of trim when switched to manual. Then that method was discarded and never mentioned for the last decade or so. Works ( yo yo ) IF one has several thousand feet of altitude AGL- otherwise vertical parking result
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Old 10th Jul 2019, 15:06
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Originally Posted by RickNRoll
There are stories saying that EASA now wants five issues rectified? None of the stories seem to list those five issues.
Her you go, from Bloomberg:

. . . EASA’s checklist includes a number of issues that have been disclosed: the potential difficulty pilots have in turning the jet’s manual trim wheel, the unreliability of the Max’s angle of attack sensors, inadequate training procedures, and a software issue flagged just last week by the FAA pertaining to a lagging microprocessor. But the agency also listed a previously unreported concern: the autopilot failing to disengage in certain emergencies.

https://www.bloomberg.com/news/artic...-can-fly-again
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Old 10th Jul 2019, 15:44
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#1291 MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Hello John, good to hear from you on the topic. Indeed, the media appears to be convinced that MCAS is a "stall prevention system" that uses the HS to push the nose down at high AoA's.

I agree with you on "longitudinal static stability". I believe the issue was the relative lightness (pull, in pounds) on the control column as the MAX approached high AoAs vs. in comparison with the forces that obtained for the earlier NGs undergoing the same testing, that indicated an NG certificiation compliance matter, and raised the question regarding "same NG type certificate". I believe the certification required a higher force to signal to the pilot the approach to the stall - enter MCAS.

That "lightness" was apparently due to the additional lift from the larger engine nacelles, attended by the slightly more forward-of-CofG mounting. It is reasonable to think that the slightly higher thrust line due pod-clearance issues would slightly reduce the nose-up effect of more-forward engines, within that very narrow (or rare), band of high-AoA & flight conditions.

I may be missing bits and I certainly don't wish to sound "engineer-ish" about this in any manner! - way above my pay grade and training - this is just my recollection from an early reading of some of those web articles.
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