MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
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Its a very long article with a lot of detail re the manual trim issue perhaps someone can post a link or make a few ' more ' significant extracts
mainly trying to give a rational perspective
Title Boeing’s Latest 737 MAX Concern: Pilots’ Physical Strength
Turning manual crank during emergency procedure may be too difficult for some people
partial extract from a separate article referenced in the above
FAA experts also aim to study how issues regarding pilot strength were dealt with during certification approvals of older versions of the 737, according to the people familiar with the specifics.
Simulator sessions and flight tests have measured the strength required to turn the crank in various flight conditions for pilots of both genders, according to two of the people briefed on the details.
In a flight-simulator test earlier this month, Journal columnist Scott McCartney and pilot Roddy Guthrie, fleet captain for the 737 at American Airlines, experienced troubles in turning the wheel. Capt. Guthrie couldn’t move the wheel until Mr. McCartney pitched the plane’s nose down, easing some of the pressure on the wheel.
Government and industry experts are considering possible operational, training and pilot-manual changes to resolve safety concerns about the procedure, according to the people familiar with the specifics. The results are expected to be part of a package of revised software and training mandates that the FAA is seen issuing later this summer.
Capt. Chesley “Sully” Sullenberger, the retired US Airways pilot celebrated for his 2009 “Miracle on the Hudson” landing, said Wednesday that pilots should be required to spend time in simulators before the MAX returns to service, not only to review the updates to the MCAS software but to practice situations where manually turning the crank would be more challenging. At higher airspeeds, turning the wheel could require two hands, the efforts of both pilots, or may not even be possible, he said.
“They need to develop a muscle memory of their experiences so that it will be immediately accessible to them in the future, even years from now, when they experience such a crisis,” he said Wednesday at a hearing of the House Committee on Transportation and Infrastructure’s aviation subcommittee.
Mr. Sullenberger told the committee that he recently experienced a recreation of the fatal MAX flights in a flight simulator. He came away from it understanding how crews could have been overwhelmed by alerts and warnings without enough time to fix the problem.
The former commercial pilot also expressed broader concern about how the MAX was designed and certified by regulators. “It is clear that the original version of MCAS was fatally flawed and should never have been approved,” Mr. Sullenberger said in written testimony.
“The 737 MAX was certified in accordance with the identical FAA requirements and processes that have governed certification of previous new airplanes and derivatives,” a Boeing spokesman said in a statement. He also said the company continues to work on training requirements with global regulators and airlines.
The pending software fix is intended to make it easier for pilots to override MCAS, which moves the horizontal stabilizer to point the nose down
Simulator sessions and flight tests have measured the strength required to turn the crank in various flight conditions for pilots of both genders, according to two of the people briefed on the details.
In a flight-simulator test earlier this month, Journal columnist Scott McCartney and pilot Roddy Guthrie, fleet captain for the 737 at American Airlines, experienced troubles in turning the wheel. Capt. Guthrie couldn’t move the wheel until Mr. McCartney pitched the plane’s nose down, easing some of the pressure on the wheel.
Government and industry experts are considering possible operational, training and pilot-manual changes to resolve safety concerns about the procedure, according to the people familiar with the specifics. The results are expected to be part of a package of revised software and training mandates that the FAA is seen issuing later this summer.
Capt. Chesley “Sully” Sullenberger, the retired US Airways pilot celebrated for his 2009 “Miracle on the Hudson” landing, said Wednesday that pilots should be required to spend time in simulators before the MAX returns to service, not only to review the updates to the MCAS software but to practice situations where manually turning the crank would be more challenging. At higher airspeeds, turning the wheel could require two hands, the efforts of both pilots, or may not even be possible, he said.
“They need to develop a muscle memory of their experiences so that it will be immediately accessible to them in the future, even years from now, when they experience such a crisis,” he said Wednesday at a hearing of the House Committee on Transportation and Infrastructure’s aviation subcommittee.
Mr. Sullenberger told the committee that he recently experienced a recreation of the fatal MAX flights in a flight simulator. He came away from it understanding how crews could have been overwhelmed by alerts and warnings without enough time to fix the problem.
The former commercial pilot also expressed broader concern about how the MAX was designed and certified by regulators. “It is clear that the original version of MCAS was fatally flawed and should never have been approved,” Mr. Sullenberger said in written testimony.
“The 737 MAX was certified in accordance with the identical FAA requirements and processes that have governed certification of previous new airplanes and derivatives,” a Boeing spokesman said in a statement. He also said the company continues to work on training requirements with global regulators and airlines.
The pending software fix is intended to make it easier for pilots to override MCAS, which moves the horizontal stabilizer to point the nose down
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Because the manufacturer doesn't want to certificate a new type?
Notwithstanding pilots knowing they could handle MCAS better and engineers knowing they could have designed it better, the fundamental question is how was a new model of the 737 certificated which is intrinsically more dangerous than its predecessors?
What changed was the establishment of “Organization Designation Authorization Program” by which FAA abdicates most of its certification function.
ODA is briefly explained in WSJ video at
In the clip acting Administrator Elwell claims that ODA saves FAA $1.8 billion. That’s not true. According to Final Rule at https://www.govinfo.gov/content/pkg/...f/05-20470.pdf ODA was predicted to save just over $12m.
FAA also claims ODA does not amount to self-certification. Again, not true.
ODA originated in a National Research Council paper “Improving the Continued Airworthiness of Civil Aircraft: A Strategy for the FAA's Aircraft Certification Service (1998)”. https://www.nap.edu/read/6265/chapter/1
A Committee comprised predominantly of retired engineers from major manufacturers unashamedly recommended that FAA should concentrate resources on continued airworthiness rather than certification.
From the Executive Summary
“It is critically important that the FAA and AIR conduct business in a new fashion with regard to aircraft certification and continued airworthiness. As an essential first step, AIR should revise its budget and manpower allocations to better reflect its mission priorities, which are as follows:
1. continued airworthiness and other activities related to continued operational safety
2. rulemaking and policy development
3. certification
The vast majority of aircraft that will operate during the next 10 years have either already been manufactured or will be manufactured to already certificated design specifications. Monitoring the safety of operating aircraft is essential to obtain a true picture of safety, to detect and resolve problems as soon as possible, and to validate airworthiness standards.Improvements in standards for initial type certification are typically based on lessons learned from the continued airworthiness process. Therefore, making the continued airworthiness process more effective is essential to improving safety in the near term and providing the foundation for long-term improvements. The primacy of this task is acknowledged in the FAA’s stated priorities. Currently, however, AIR’s type certification activities receive more resources than the other two areas combined.”
What could go wrong?
What changed was the establishment of “Organization Designation Authorization Program” by which FAA abdicates most of its certification function.
ODA is briefly explained in WSJ video at
In the clip acting Administrator Elwell claims that ODA saves FAA $1.8 billion. That’s not true. According to Final Rule at https://www.govinfo.gov/content/pkg/...f/05-20470.pdf ODA was predicted to save just over $12m.
FAA also claims ODA does not amount to self-certification. Again, not true.
ODA originated in a National Research Council paper “Improving the Continued Airworthiness of Civil Aircraft: A Strategy for the FAA's Aircraft Certification Service (1998)”. https://www.nap.edu/read/6265/chapter/1
A Committee comprised predominantly of retired engineers from major manufacturers unashamedly recommended that FAA should concentrate resources on continued airworthiness rather than certification.
From the Executive Summary
“It is critically important that the FAA and AIR conduct business in a new fashion with regard to aircraft certification and continued airworthiness. As an essential first step, AIR should revise its budget and manpower allocations to better reflect its mission priorities, which are as follows:
1. continued airworthiness and other activities related to continued operational safety
2. rulemaking and policy development
3. certification
The vast majority of aircraft that will operate during the next 10 years have either already been manufactured or will be manufactured to already certificated design specifications. Monitoring the safety of operating aircraft is essential to obtain a true picture of safety, to detect and resolve problems as soon as possible, and to validate airworthiness standards.Improvements in standards for initial type certification are typically based on lessons learned from the continued airworthiness process. Therefore, making the continued airworthiness process more effective is essential to improving safety in the near term and providing the foundation for long-term improvements. The primacy of this task is acknowledged in the FAA’s stated priorities. Currently, however, AIR’s type certification activities receive more resources than the other two areas combined.”
What could go wrong?
You’ve just got airborne in a 737 MAX and the trim starts running. What are you going to do? There are multiple reasons why the trim could activate: some benign, some not so; as above, what is the trigger for performing the runaway trim checklist? OK, so not enough information to make a rational decision at this point - it would have to be a guess, something professional pilots don’t really like to do. You could say do the checklist, just in case, but you’d be doing it on every flight. Might as well disconnect the trim *before* you take off, to be sure.
This situation is confusing enough on its own; now add in UAS, stick shake, flashing lights, noises and you have overloaded sensory channels making cognition exponentially harder. This is why Human Factors play an equal or greater role here than the purely technical ones.
Fundamentally, for some time after takeoff, abnormal operation of the trim system is virtually indistinguishable from normal operation because 737 pilots (and I have been one) get used to frequent intermittent trim operation by the aeroplane systems, to the point that you’d only notice it if it wasn’t happening, like living next to a main road.
I think that’s the core of the issue that divides the “pilot’s fault” crowd from the “aeroplane’s fault” one.
You’ve just got airborne in a 737 MAX and the trim starts running. What are you going to do? There are multiple reasons why the trim could activate: some benign, some not so; as above, what is the trigger for performing the runaway trim checklist? OK, so not enough information to make a rational decision at this point - it would have to be a guess, something professional pilots don’t really like to do. You could say do the checklist, just in case, but you’d be doing it on every flight. Might as well disconnect the trim *before* you take off, to be sure.
This situation is confusing enough on its own; now add in UAS, stick shake, flashing lights, noises and you have overloaded sensory channels making cognition exponentially harder. This is why Human Factors play an equal or greater role here than the purely technical ones.
Fundamentally, for some time after takeoff, abnormal operation of the trim system is virtually indistinguishable from normal operation because 737 pilots (and I have been one) get used to frequent intermittent trim operation by the aeroplane systems, to the point that you’d only notice it if it wasn’t happening, like living next to a main road.
You’ve just got airborne in a 737 MAX and the trim starts running. What are you going to do? There are multiple reasons why the trim could activate: some benign, some not so; as above, what is the trigger for performing the runaway trim checklist? OK, so not enough information to make a rational decision at this point - it would have to be a guess, something professional pilots don’t really like to do. You could say do the checklist, just in case, but you’d be doing it on every flight. Might as well disconnect the trim *before* you take off, to be sure.
This situation is confusing enough on its own; now add in UAS, stick shake, flashing lights, noises and you have overloaded sensory channels making cognition exponentially harder. This is why Human Factors play an equal or greater role here than the purely technical ones.
Fundamentally, for some time after takeoff, abnormal operation of the trim system is virtually indistinguishable from normal operation because 737 pilots (and I have been one) get used to frequent intermittent trim operation by the aeroplane systems, to the point that you’d only notice it if it wasn’t happening, like living next to a main road.
Quite so, however, you can effectively sit back and relax at this time, we have enough information to now understand the system;
Normal operations: you get airborne, trim runs automatically for SRS....for a short period, and then stops. No other noises.... No problem enjoy your coffee.
MCAS issue as per last 2 accidents. You get airborne, and at some point you get a stall warning, Shaker, IAS/ALT disagree, and stab trim running tout suite. Use trim switches to run trim back to an in-trim case, and then Shut off the Trim. For my money, you are now unairworthy, and are going to return or divert to nearest suitable and enjoy a coffee. The aural warning is going to keep running as will the stick shaker until you take action to shut it up. If you are uncomfortable flying around without a stick shaker, then enjoy the noise. If you are OK with the fact you have, GS, ATT x 3, 2 IAS, 2 ALTM other than the one that has gone bad, then enjoy the quietness, and don't pull past buffet unless you enjoy the ride.
Broken Record: The Lion and Ethiopian crews did not have the information to hand to sort our the wheat from the chaff, in the time allotted. That doesn't make them incompetent, it makes them star crossed on the day.
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There is pretty much unanimous consensus that MCAS was poorly designed and implemented. Behind that process, however, were numerous decisions made at numerous levels - including some made by people with completely benign intentions who are likely now horrified by whatever role they played in this tragedy. As much as some might like to find a clearly defined group of bad actors at whom we can point fingers, I just don't think that is possible. I'm not saying here that Boeing didn't produce an ill-thought design, but that's different than saying it was done with intent or malice. Despite Boeing's attempts to minimize their financial liability (as one might expect from the legal side), it does appear that there is a serious attempt to examine and change their internal processes. Time will tell.
Attempting to jump from Boeing's to the pilots' roles skips over an entire body of causal factors which ought to be examined as well. As I have said before, pilots do not create the hiring process, the training syllabus, the regulatory structure, the operating environment, or the corporate culture. However, their responses in the cockpit will reflect all of these factors. The discussion around the pilots' actions tends to revolve around the discrepancy between what they did as opposed to what they were expected to have done. In safety culture language, this gap is sometimes referred to "work as imagined" and "work as done." When looking at the flight crew responses, I think the objective should be to identify all those things that contributed to this gap and takes steps to mitigate those factors. I have seen nothing in the evidence that indicates that the pilots were incompetent. On the other hand, I see evidence that they were not provided with the proper tools and training to deal with such a complicated malfunction. It appears in this area there are also sincere attempts to identify and correct some of these deficiencies, but again, it will take some time to see if there is proper follow-through.
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As I understand the system, if the stab was truly jammed in place - either by aerodynamic loads or some physical malfunction - the electric trim motor would turn, the clutch would slip, and the stab would not move. That being said, I know of no actual case of this happening on a 737.
There will be an out of trim combination of HS AND position and aircraft speed which generates forces on the jack screw that are so large they can't be overcome by using the trim wheel.
In this condition the stab motor can't overcome these forces either. ( the design principle is rightly that the trim wheels have mechanical authority). Selecting the cutout switches back to normal would then be counterproductive as MCAS can then drive the stab motor in the AND direction actually assisted by the air loading forces it has itself created.
At this point the situation is not recoverable other than possibly by using the unloading manoeuvre. Great if you know about it, have practiced it and have the altitude to implement it.
The ball nut which runs on the jack screw is an excellent low friction mechanism for the trim actuator to move the HS but the converse also applies so the ball nut will produce a back turning force on the jack screw from the HS. ( I believe there is a braking mechanism in the stab gearbox to prevent 'creep back' even in normal flight conditions).
I may have mis-remembered but I think there was indication of a small uncommanded AND change of HS position towards the end of ET302 which would be indicative of HS creep under what was probably a very large air loading condition.
I think arguments above are correct in principle so may help to explain some of what happened in the cockpit of ET302. They do, of course, depend on actual data for the forces involved. We don't have these data and I have an uncomfortable feeling that Boeing didn't.
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A simple question which I’m not sure has been discussed.
If the original amount of MCAS authority was required for certification, then how can the revised software which gives MCAS a reduced authority, still fulfil certification requirements and if it does, why then was an excessively powerful system allowed on an aircraft when that power and authority was in excess of what was required for certification?
If the original amount of MCAS authority was required for certification, then how can the revised software which gives MCAS a reduced authority, still fulfil certification requirements and if it does, why then was an excessively powerful system allowed on an aircraft when that power and authority was in excess of what was required for certification?
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A simple question which I’m not sure has been discussed.
If the original amount of MCAS authority was required for certification, then how can the revised software which gives MCAS a reduced authority, still fulfil certification requirements and if it does, why then was an excessively powerful system allowed on an aircraft when that power and authority was in excess of what was required for certification?
If the original amount of MCAS authority was required for certification, then how can the revised software which gives MCAS a reduced authority, still fulfil certification requirements and if it does, why then was an excessively powerful system allowed on an aircraft when that power and authority was in excess of what was required for certification?
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A simple question which I’m not sure has been discussed.
If the original amount of MCAS authority was required for certification, then how can the revised software which gives MCAS a reduced authority, still fulfil certification requirements and if it does, why then was an excessively powerful system allowed on an aircraft when that power and authority was in excess of what was required for certification?
If the original amount of MCAS authority was required for certification, then how can the revised software which gives MCAS a reduced authority, still fulfil certification requirements and if it does, why then was an excessively powerful system allowed on an aircraft when that power and authority was in excess of what was required for certification?
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To expand a little :-
There will be an out of trim combination of HS AND position and aircraft speed which generates forces on the jack screw that are so large they can't be overcome by using the trim wheel.
In this condition the stab motor can't overcome these forces either. ( the design principle is rightly that the trim wheels have mechanical authority). Selecting the cutout switches back to normal would then be counterproductive as MCAS can then drive the stab motor in the AND direction actually assisted by the air loading forces it has itself created.
At this point the situation is not recoverable other than possibly by using the unloading manoeuvre. Great if you know about it, have practiced it and have the altitude to implement it.
The ball nut which runs on the jack screw is an excellent low friction mechanism for the trim actuator to move the HS but the converse also applies so the ball nut will produce a back turning force on the jack screw from the HS. ( I believe there is a braking mechanism in the stab gearbox to prevent 'creep back' even in normal flight conditions).
I may have mis-remembered but I think there was indication of a small uncommanded AND change of HS position towards the end of ET302 which would be indicative of HS creep under what was probably a very large air loading condition.
I think arguments above are correct in principle so may help to explain some of what happened in the cockpit of ET302. They do, of course, depend on actual data for the forces involved. We don't have these data and I have an uncomfortable feeling that Boeing didn't.
There will be an out of trim combination of HS AND position and aircraft speed which generates forces on the jack screw that are so large they can't be overcome by using the trim wheel.
In this condition the stab motor can't overcome these forces either. ( the design principle is rightly that the trim wheels have mechanical authority). Selecting the cutout switches back to normal would then be counterproductive as MCAS can then drive the stab motor in the AND direction actually assisted by the air loading forces it has itself created.
At this point the situation is not recoverable other than possibly by using the unloading manoeuvre. Great if you know about it, have practiced it and have the altitude to implement it.
The ball nut which runs on the jack screw is an excellent low friction mechanism for the trim actuator to move the HS but the converse also applies so the ball nut will produce a back turning force on the jack screw from the HS. ( I believe there is a braking mechanism in the stab gearbox to prevent 'creep back' even in normal flight conditions).
I may have mis-remembered but I think there was indication of a small uncommanded AND change of HS position towards the end of ET302 which would be indicative of HS creep under what was probably a very large air loading condition.
I think arguments above are correct in principle so may help to explain some of what happened in the cockpit of ET302. They do, of course, depend on actual data for the forces involved. We don't have these data and I have an uncomfortable feeling that Boeing didn't.
Also, no matter how low friction that ball nut is, it seems what's preventing the use of the trim wheels when severely out of trim is somehow friction related. For example in Mentour's video it was getting harder and harder to trim AND the more they trimmed AND. At some point, at around 2.5 units from full AND, it became impossible to trim more AND. Assuming that was not a quirk of the simulator, this indicates that manual trimming in both directions is affected when the stabilizer is out of trim, both AND and ANU changes become harder.
MCAS on the Tanker 767
There is a report in today WSJ that there is a version of MCAS on the 767 based USAF tankers. (KC-46?)
This preceded the 737 Max system. It seems the wing pods for the refuel gear added extra lift.
https://www.wsj.com/articles/before-737-max-boeings-flight-control-system-included-key-safeguards-
Most WSJ articles are behind a paywall. It sees the tanker version relied on multiple inputs and had a control column cutoff.
Also it only made single adjustments to trim
I might have missed it but I don't seem to remember any mention of this previous system in this thread.
20driver
This preceded the 737 Max system. It seems the wing pods for the refuel gear added extra lift.
https://www.wsj.com/articles/before-737-max-boeings-flight-control-system-included-key-safeguards-
Most WSJ articles are behind a paywall. It sees the tanker version relied on multiple inputs and had a control column cutoff.
Also it only made single adjustments to trim
I might have missed it but I don't seem to remember any mention of this previous system in this thread.
20driver
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There is a report in today WSJ that there is a version of MCAS on the 767 based USAF tankers. (KC-46?)
This preceded the 737 Max system. It seems the wing pods for the refuel gear added extra lift.
https://www.wsj.com/articles/before-737-max-boeings-flight-control-system-included-key-safeguards-
Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed
Most WSJ articles are behind a paywall. It sees the tanker version relied on multiple inputs and had a control column cutoff.
Also it only made single adjustments to trim
I might have missed it but I don't seem to remember any mention of this previous system in this thread.
20driver
This preceded the 737 Max system. It seems the wing pods for the refuel gear added extra lift.
https://www.wsj.com/articles/before-737-max-boeings-flight-control-system-included-key-safeguards-
Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed
Most WSJ articles are behind a paywall. It sees the tanker version relied on multiple inputs and had a control column cutoff.
Also it only made single adjustments to trim
I might have missed it but I don't seem to remember any mention of this previous system in this thread.
20driver
Edit: Earlier posts other threads:
737MAX Stab Trim architecture
Ethiopian airliner down in Africa
Boeing 737 Max Software Fixes Due to Lion Air Crash Delayed
Last edited by GordonR_Cape; 29th Sep 2019 at 17:48.
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Thread a six inch long bolt through the nut and keep turning. Now apply a side load to the bolt and repeat the exercise. The more side pressure you apply the more torque is needed to turn the bolt. This is because the side load is relieving pressure on one side of the threads at the top and opposite bottom side but is increasing it on the other two sides. If you keep applying the side load eventually you won’t be able to turn the bolt at all. The bolt is the stabiliser screw jack. The side load is the aerodynamic forces transferred from surface of the stabiliser to the jack screw. The point where it no longer turns is the point where the clutch disengages the electric trim motor or the trim wheel no longer turns without using the elevator to momentarily unload the aerodynamic pressure on the screw jack.
As I said in an earlier post, a redundant electric servo to assist/replace the manual trim wheel will not only need to be able to apply more torque than the existing electric trim motor but the clutch and screw jack themselves may need to be beefed up to cope with the increased stresses on those parts. Adding an electric motor and motor control to each of the 7,500 NGs is not something that I believe Boeing will relish. Redesigning and retrofitting an HS screw jack is something that they will relish even less.
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The Stabilizer Trim System (STS ... onto which MCAS was grafted) wanders against my commands, and every 737 pilot's commands, every single takeoff. Multiple times. As the aircraft accelerates to flaps up speed, the STS is, not continually, but periodically, trimming nose up to maintain the speed it was previously trimmed for. Every single takeoff generates multiple STS trims, in the opposite direction to that desired by the pilot. When the trim moves, driven by the STS, the pilot trims in the opposite direction...habitually.
When the trim moves nose down "by itself", the pilots will naturally respond by trimming nose up. Probably barely noticing it. It looks just like the STS they work against every single takeoff... because it IS the STS...except that it is moving in the opposite direction. So much so, that the surviving crew wrote it up that way. It does NOT look like the stab trim runaway we were (once, long ago) trained for. Boeing could not have masked the problem any more effectively if they had tried.
If I "perform[ed] a trim runaway memory item once pitch starts wandering against [my] commands", I would do so every single flight.
I'll do it tomorrow, just for the heck of it.
When the trim moves nose down "by itself", the pilots will naturally respond by trimming nose up. Probably barely noticing it. It looks just like the STS they work against every single takeoff... because it IS the STS...except that it is moving in the opposite direction. So much so, that the surviving crew wrote it up that way. It does NOT look like the stab trim runaway we were (once, long ago) trained for. Boeing could not have masked the problem any more effectively if they had tried.
If I "perform[ed] a trim runaway memory item once pitch starts wandering against [my] commands", I would do so every single flight.
I'll do it tomorrow, just for the heck of it.
Now add to that stick shaker and eventually an overspeed warning in your ears and unreliable air speeds on the screens in front of you. I'm sure you're starting to get the picture that this is not simply pilot error.
The Max was an aircraft built around it's engines. That was what the Wright Brothers and other early 20th century aircraft designers did. Boeing is just doing it in 2019.
Time to call this aircraft a new type of you ask me. Do it right. Not one more life should be lost.
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David
The NTSB is not buying 'pilot error'. On the contrary, they say the design assumptions and testing of MCAS were flawed:
https://www.ntsb.gov/investigations/...ts/ASR1901.pdf
https://www.ntsb.gov/investigations/...ts/ASR1901.pdf
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There is a report in today WSJ that there is a version of MCAS on the 767 based USAF tankers. (KC-46?)
This preceded the 737 Max system. It seems the wing pods for the refuel gear added extra lift.
https://www.wsj.com/articles/before-737-max-boeings-flight-control-system-included-key-safeguards-
Most WSJ articles are behind a paywall. It sees the tanker version relied on multiple inputs and had a control column cutoff.
Also it only made single adjustments to trim
I might have missed it but I don't seem to remember any mention of this previous system in this thread.
20driver
This preceded the 737 Max system. It seems the wing pods for the refuel gear added extra lift.
https://www.wsj.com/articles/before-737-max-boeings-flight-control-system-included-key-safeguards-
Most WSJ articles are behind a paywall. It sees the tanker version relied on multiple inputs and had a control column cutoff.
Also it only made single adjustments to trim
I might have missed it but I don't seem to remember any mention of this previous system in this thread.
20driver
http://www.airforcemag.com/Features/...AS-System.aspx