Boeing 737 Manual Stabiliser Trim - an Historical Fact
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Boeing 737 Manual Stabiliser Trim - an Historical Fact
The latest issue of Aviation Week & Commercial Aviation (Dec 23 2019 - January 12, 2020) includes an article Boeing 737 Pilots Focus on Modified Procedures.
One paragraph states: “The new 737 training modules emphasize that pilots may need to use two hands to crank the wheel during a runaway trim scenario. It also says “unloading” the stabilizer – attempting to reduce airspeed and take the counterintuitive step of not pulling back on the yoke even though the aircraft is trimmed nose down – may be necessary to move the trim wheel.”
I first flew the Boeing 737-200 in 1977 and the Pilot Training Manual for that aircraft explained quite clearly how to use the manual trim in event of a runaway electrically operated stabilizer trim. However, following the introduction of the Boeing 737-300 and subsequent models, Boeing, for some unknown reason, withdrew those directions and instead inserted an amended version which included the words “in extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming.” The amended directions did not say precisely how this was to be accomplished and it was left to pilots to interpret what Boeing meant. Pilots with English as second language would have little idea what Boeing meant.
The following extracts are from the Boeing 737-200 Pilot Training Manual dated 1982 page 04-80.31 and was included in all earlier 737-200 PTM. Under the heading of Abnormal/Emergency Procedures it stated:“If other methods fail to relieve the elevator load and control column force, use the roller-coaster technique. If nose up trim is required, raise the nose well above the horizon with elevator control. Then slowly relax the control pressure and manually trim nose up. Allow the nose to drop below the horizon while trimming.”
Contrary to the advice mentioned by Sean Broderick of “take the counterintuitive step of not pulling back on the yoke even though the aircraft is trimmed nose down,” you can see the Boeing advice is to initially raise the nose well above the horizon with elevator control.
Boeing Airliner magazine published in May 1961, discussed the roller coaster technique in detail as it also applied to the Boeing 707. It was also known as the YoYo technique.
If the crews of the two fatal MAX accidents had access to the above advice in their manuals it is possible they may have been able to control the runway trim. But their manuals did not include using the “roller-coaster” technique to manually regain trim control, because Boeing had elected to remove that advice from all Boeing 737 models.
It is not surprising that the FAA recently appears to have failed to notice that Boeing had changed the original wording of the 737-200 Pilot Training Manual. Most of their inspectors had not been born in 1961 when the roller coaster technique was first introduced on the Boeing 707.
Finally, here is an extract from the November 1975 edition of the Boeing 737 Instructor Pilot Handbook. Page 51 and titled Runaway and Manual Stabilizer.
Out-of-trim recovery method.
Use manual trim
Accelerate or decelerate
Use roller coaster (least desirable)
Bank aircraft for nose up out of trim.
These methods were used when practicing out of trim recovery while airborne.
The FAA are merely re-inventing the wheel if they think their latest advice on using manual stabiliser trim to recover from a runway trim is new. We knew all about the problem sixty years ago and so did Boeing.
One paragraph states: “The new 737 training modules emphasize that pilots may need to use two hands to crank the wheel during a runaway trim scenario. It also says “unloading” the stabilizer – attempting to reduce airspeed and take the counterintuitive step of not pulling back on the yoke even though the aircraft is trimmed nose down – may be necessary to move the trim wheel.”
I first flew the Boeing 737-200 in 1977 and the Pilot Training Manual for that aircraft explained quite clearly how to use the manual trim in event of a runaway electrically operated stabilizer trim. However, following the introduction of the Boeing 737-300 and subsequent models, Boeing, for some unknown reason, withdrew those directions and instead inserted an amended version which included the words “in extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming.” The amended directions did not say precisely how this was to be accomplished and it was left to pilots to interpret what Boeing meant. Pilots with English as second language would have little idea what Boeing meant.
The following extracts are from the Boeing 737-200 Pilot Training Manual dated 1982 page 04-80.31 and was included in all earlier 737-200 PTM. Under the heading of Abnormal/Emergency Procedures it stated:“If other methods fail to relieve the elevator load and control column force, use the roller-coaster technique. If nose up trim is required, raise the nose well above the horizon with elevator control. Then slowly relax the control pressure and manually trim nose up. Allow the nose to drop below the horizon while trimming.”
Contrary to the advice mentioned by Sean Broderick of “take the counterintuitive step of not pulling back on the yoke even though the aircraft is trimmed nose down,” you can see the Boeing advice is to initially raise the nose well above the horizon with elevator control.
Boeing Airliner magazine published in May 1961, discussed the roller coaster technique in detail as it also applied to the Boeing 707. It was also known as the YoYo technique.
If the crews of the two fatal MAX accidents had access to the above advice in their manuals it is possible they may have been able to control the runway trim. But their manuals did not include using the “roller-coaster” technique to manually regain trim control, because Boeing had elected to remove that advice from all Boeing 737 models.
It is not surprising that the FAA recently appears to have failed to notice that Boeing had changed the original wording of the 737-200 Pilot Training Manual. Most of their inspectors had not been born in 1961 when the roller coaster technique was first introduced on the Boeing 707.
Finally, here is an extract from the November 1975 edition of the Boeing 737 Instructor Pilot Handbook. Page 51 and titled Runaway and Manual Stabilizer.
Out-of-trim recovery method.
Use manual trim
Accelerate or decelerate
Use roller coaster (least desirable)
Bank aircraft for nose up out of trim.
These methods were used when practicing out of trim recovery while airborne.
The FAA are merely re-inventing the wheel if they think their latest advice on using manual stabiliser trim to recover from a runway trim is new. We knew all about the problem sixty years ago and so did Boeing.
From what I can gather of the Ethiopian crash, the crew did attempt this but the MCAS kicked in again. My understanding of the MCAS is that it operates faster than normal trim.
While your understanding is mostly correct, it is not complete. Yes, mcas will add AND faster than the pilots can trim using manual electric trim, but if the pilots trim mcas will stop till 5 seconds after the manual electric trim input has stopped. As long as the pilots blip the trim every 4 seconds there should be no mcas input.
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centarus posted this interesting bit
Seems to this SLF that in the extreme case with stabilizer flipping up to give AND, pulling the yoke which normally raises elevator to give nose up is likkely to make it harder to trim stabilizer nose up. ( assuming that elevator is not blanked re airflow over the extreme nose down stabilizer )
I believe this was well covered in satcom.guru and the following links
https://www.satcom.guru/2019/10/flaw...-disaster.html
https://leehamnews.com/2019/11/29/bj...-crash-part-5/
https://www.moonofalabama.org/2019/0...g-737-ngs.html
B737 Max threads
https://www.satcom.guru/2019/03/taki...aiting-on.html
and from an earlier post *** NOT MINE***
...Contrary to the advice mentioned by Sean Broderick of “take the counterintuitive step of not pulling back on the yoke even though the aircraft is trimmed nose down,” you can see the Boeing advice is to initially raise the nose well above the horizon with elevator control.
I believe this was well covered in satcom.guru and the following links
https://www.satcom.guru/2019/10/flaw...-disaster.html
https://leehamnews.com/2019/11/29/bj...-crash-part-5/
https://www.moonofalabama.org/2019/0...g-737-ngs.html
B737 Max threads
https://www.satcom.guru/2019/03/taki...aiting-on.html
and from an earlier post *** NOT MINE***
" I agree it's a flawed design. And I used to work there. I'm glad I don't now.
Regarding the trim wheels: When the NG was being introduced, I happened to be the Lead Engineer in charge of them and a whole lot of other stuff. There were some issues. The new display system created a pinch point between the dash and the wheel. We had to make the wheel smaller. And the new trim motor resulted in the wheel, which is directly connected to the stabilizer by a long cable, springing back when electric trim was used. It was an undamped mass on the end of a spring. We had to add a damper.
Result: Depending on the flight conditions, the force to manually trim can be extremely high. We set up a test rig and a very fit female pilot could barely move it.
As I said, I'm glad I'm no longer there."
Regarding the trim wheels: When the NG was being introduced, I happened to be the Lead Engineer in charge of them and a whole lot of other stuff. There were some issues. The new display system created a pinch point between the dash and the wheel. We had to make the wheel smaller. And the new trim motor resulted in the wheel, which is directly connected to the stabilizer by a long cable, springing back when electric trim was used. It was an undamped mass on the end of a spring. We had to add a damper.
Result: Depending on the flight conditions, the force to manually trim can be extremely high. We set up a test rig and a very fit female pilot could barely move it.
As I said, I'm glad I'm no longer there."
Last edited by Grebe; 2nd Jan 2020 at 22:11. Reason: clarification
While your understanding is mostly correct, it is not complete. Yes, mcas will add AND faster than the pilots can trim using manual electric trim, but if the pilots trim mcas will stop till 5 seconds after the manual electric trim input has stopped. As long as the pilots blip the trim every 4 seconds there should be no mcas input.
To clarify. Manual Electric Trim (MET) is a bit slower than MCAS trim however Manual Electric Trim stops MCAS trim which will only re-commence 5 seconds after the Manual Electric Trim is stopped. There are numbers somewhere but the Ethiopian preliminary report FDR chart shows that MET is about 1.5 or 2 times slower than MCAS trim. Hmmm. Maybe 1/2 to 2/3 the speed is clearer, take your pick:-)
The Ethiopian preliminary report FDR chart also clearly shows the MCAS trim being prematurely interrupted by MET in at least one instance. At 05:40:27.
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737 Summary jan 2 seattle times
Contains many links and references
starts
starts
Jan. 2, 2020 at 6:00 am Updated Jan. 2, 2020 at 1:11 pm
In a series of stories that led media coverage of the two deadly crashes of Boeing’s 737 MAX jet, The Seattle Times was the first to reveal how Boeing misinformed the Federal Aviation Administration and airlines about key features of the plane’s automated flight control system.
The weekend after the second crash, a Seattle Times front-page story — citing proprietary Boeing information submitted to the FAA — laid bare how the federal regulator was not fully informed as Boeing expanded the powers of its MCAS flight control system, the automated software whose malfunctioning killed 346 people.
The follow-up stories, using numerous internal Boeing and FAA documents obtained by our reporters, showed how the flawed design was approved by a flawed regulatory process as the FAA increasingly delegated responsibility for safety assessments to the manufacturer, and how management at both organizations pressed for shortcuts and money-saving solutions against the recommendations of both low-level FAA officials and Boeing safety experts.
The weekend after the second crash, a Seattle Times front-page story — citing proprietary Boeing information submitted to the FAA — laid bare how the federal regulator was not fully informed as Boeing expanded the powers of its MCAS flight control system, the automated software whose malfunctioning killed 346 people.
The follow-up stories, using numerous internal Boeing and FAA documents obtained by our reporters, showed how the flawed design was approved by a flawed regulatory process as the FAA increasingly delegated responsibility for safety assessments to the manufacturer, and how management at both organizations pressed for shortcuts and money-saving solutions against the recommendations of both low-level FAA officials and Boeing safety experts.
...
I first flew the Boeing 737-200 in 1977 and the Pilot Training Manual for that aircraft explained quite clearly how to use the manual trim in event of a runaway electrically operated stabilizer trim. However, following the introduction of the Boeing 737-300 and subsequent models, Boeing, for some unknown reason, withdrew those directions and instead inserted an amended version which included the words “in extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming.” The amended directions did not say precisely how this was to be accomplished and it was left to pilots to interpret what Boeing meant. Pilots with English as second language would have little idea what Boeing meant.
I first flew the Boeing 737-200 in 1977 and the Pilot Training Manual for that aircraft explained quite clearly how to use the manual trim in event of a runaway electrically operated stabilizer trim. However, following the introduction of the Boeing 737-300 and subsequent models, Boeing, for some unknown reason, withdrew those directions and instead inserted an amended version which included the words “in extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming.” The amended directions did not say precisely how this was to be accomplished and it was left to pilots to interpret what Boeing meant. Pilots with English as second language would have little idea what Boeing meant.
1) What is the exact definition of trim runaway that was trained as trim runaway? It's 100% certain that trim will always stop when the stab reaches the trim stops and that is indistinguishable from every time MCAS stopped. It cannot go further and therefore it cannot runaway if it always stops. Was the expectation to let it run until it hit the upper or lower stop and then do something?
2) What amount of force are pilots trained to use trim to offset? In the case of the Ethiopian crash the plane was 2 units out of trim before the trim motors were disabled. They felt confident with that amount, having trimmed back from nearly 4 units, and waited to disable the motors. From that point until they re-enabled the trim motors did the control forces increase and to what level?
3) Why are there cutout switches? There must have been an unavoidable potential defect in order for there to be an interrupt put in place. It seems the general feeling is that because MCAS was unknown to the pilots prior to the Lion Air crash that no pilot could do anything to avoid it because it wasn't an already known defect or set of defects that the pilot would diagnose. What defect were pilots told the switches were for?
Three questions have gone unaddressed:
1) What is the exact definition of trim runaway that was trained as trim runaway? It's 100% certain that trim will always stop when the stab reaches the trim stops and that is indistinguishable from every time MCAS stopped. It cannot go further and therefore it cannot runaway if it always stops. Was the expectation to let it run until it hit the upper or lower stop and then do something?
2) What amount of force are pilots trained to use trim to offset? In the case of the Ethiopian crash the plane was 2 units out of trim before the trim motors were disabled. They felt confident with that amount, having trimmed back from nearly 4 units, and waited to disable the motors. From that point until they re-enabled the trim motors did the control forces increase and to what level?
3) Why are there cutout switches? There must have been an unavoidable potential defect in order for there to be an interrupt put in place. It seems the general feeling is that because MCAS was unknown to the pilots prior to the Lion Air crash that no pilot could do anything to avoid it because it wasn't an already known defect or set of defects that the pilot would diagnose. What defect were pilots told the switches were for?
1) What is the exact definition of trim runaway that was trained as trim runaway? It's 100% certain that trim will always stop when the stab reaches the trim stops and that is indistinguishable from every time MCAS stopped. It cannot go further and therefore it cannot runaway if it always stops. Was the expectation to let it run until it hit the upper or lower stop and then do something?
2) What amount of force are pilots trained to use trim to offset? In the case of the Ethiopian crash the plane was 2 units out of trim before the trim motors were disabled. They felt confident with that amount, having trimmed back from nearly 4 units, and waited to disable the motors. From that point until they re-enabled the trim motors did the control forces increase and to what level?
3) Why are there cutout switches? There must have been an unavoidable potential defect in order for there to be an interrupt put in place. It seems the general feeling is that because MCAS was unknown to the pilots prior to the Lion Air crash that no pilot could do anything to avoid it because it wasn't an already known defect or set of defects that the pilot would diagnose. What defect were pilots told the switches were for?
It was something like "a continuous un-commanded trim operation" the background was a faulty locked switch or other component/electrical failure that put power to the motor. But the shorted switch was the main idea (fault) - put in an input then take thumb off and it keeps going and does not stop until the fixed stop (hopefully) but the motor could still run on a clutch eventually burning out.
100% is not certain it could shear the mechanical stops.
A runaway trim was originally believed to occur after the pilot inputted a up or down command and that command continued after the pilot stopped the input. As an example put your foot down on the car accelerator, when you lift your foot you expect the car acceleration to decrease. If speed continues to increases you have a runaway throttle.
2) What amount of force are pilots trained to use trim to offset? In the case of the Ethiopian crash the plane was 2 units out of trim before the trim motors were disabled. They felt confident with that amount, having trimmed back from nearly 4 units, and waited to disable the motors. From that point until they re-enabled the trim motors did the control forces increase and to what level?
There will not be a fixed amount of force trained for and I expect it varies a lot during changes of flight phase. I doubt very much the Ethiopian crew felt "confident" at any stage of that flight soon after lift off.
After the trim motors were deactivated the aircraft speed increased, so the forces would have increased and I expect a lot.
3) Why are there cutout switches? There must have been an unavoidable potential defect in order for there to be an interrupt put in place. It seems the general feeling is that because MCAS was unknown to the pilots prior to the Lion Air crash that no pilot could do anything to avoid it because it wasn't an already known defect or set of defects that the pilot would diagnose. What defect were pilots told the switches were for?
The cutout switches were originally there to isolate the undesired conditions including the runway trim, the switches were there long before the MAX so MCAS is not relevant to the question.
Originally the pilots were told what both of the switches isolated independently including the manual trim switch, that changed on the NG and after.
Soon after the Lion Air crash runaway trim was no longer required to be "continuous" and I doubt that to this day there is any definition of runaway trim that is taught to be correct as a standard (run time or stick force).
I would also question the simulators control forces accuracy in many conditions.
1) What is the exact definition of trim runaway that was trained as trim runaway? It's 100% certain that trim will always stop when the stab reaches the trim stops and that is indistinguishable from every time MCAS stopped. It cannot go further and therefore it cannot runaway if it always stops. Was the expectation to let it run until it hit the upper or lower stop and then do something?
It was something like "a continuous un-commanded trim operation" the background was a faulty locked switch or other component/electrical failure that put power to the motor. But the shorted switch was the main idea (fault) - put in an input then take thumb off and it keeps going and does not stop until the fixed stop (hopefully) but the motor could still run on a clutch eventually burning out.
100% is not certain it could shear the mechanical stops.
A runaway trim was originally believed to occur after the pilot inputted a up or down command and that command continued after the pilot stopped the input. As an example put your foot down on the car accelerator, when you lift your foot you expect the car acceleration to decrease. If speed continues to increases you have a runaway throttle.
2) What amount of force are pilots trained to use trim to offset? In the case of the Ethiopian crash the plane was 2 units out of trim before the trim motors were disabled. They felt confident with that amount, having trimmed back from nearly 4 units, and waited to disable the motors. From that point until they re-enabled the trim motors did the control forces increase and to what level?
There will not be a fixed amount of force trained for and I expect it varies a lot during changes of flight phase. I doubt very much the Ethiopian crew felt "confident" at any stage of that flight soon after lift off.
After the trim motors were deactivated the aircraft speed increased, so the forces would have increased and I expect a lot.
3) Why are there cutout switches? There must have been an unavoidable potential defect in order for there to be an interrupt put in place. It seems the general feeling is that because MCAS was unknown to the pilots prior to the Lion Air crash that no pilot could do anything to avoid it because it wasn't an already known defect or set of defects that the pilot would diagnose. What defect were pilots told the switches were for?
The cutout switches were originally there to isolate the undesired conditions including the runway trim, the switches were there long before the MAX so MCAS is not relevant to the question.
Originally the pilots were told what both of the switches isolated independently including the manual trim switch, that changed on the NG and after.
Soon after the Lion Air crash runaway trim was no longer required to be "continuous" and I doubt that to this day there is any definition of runaway trim that is taught to be correct as a standard (run time or stick force).
I would also question the simulators control forces accuracy in many conditions.
It was something like "a continuous un-commanded trim operation" the background was a faulty locked switch or other component/electrical failure that put power to the motor. But the shorted switch was the main idea (fault) - put in an input then take thumb off and it keeps going and does not stop until the fixed stop (hopefully) but the motor could still run on a clutch eventually burning out.
100% is not certain it could shear the mechanical stops.
A runaway trim was originally believed to occur after the pilot inputted a up or down command and that command continued after the pilot stopped the input. As an example put your foot down on the car accelerator, when you lift your foot you expect the car acceleration to decrease. If speed continues to increases you have a runaway throttle.
2) What amount of force are pilots trained to use trim to offset? In the case of the Ethiopian crash the plane was 2 units out of trim before the trim motors were disabled. They felt confident with that amount, having trimmed back from nearly 4 units, and waited to disable the motors. From that point until they re-enabled the trim motors did the control forces increase and to what level?
There will not be a fixed amount of force trained for and I expect it varies a lot during changes of flight phase. I doubt very much the Ethiopian crew felt "confident" at any stage of that flight soon after lift off.
After the trim motors were deactivated the aircraft speed increased, so the forces would have increased and I expect a lot.
3) Why are there cutout switches? There must have been an unavoidable potential defect in order for there to be an interrupt put in place. It seems the general feeling is that because MCAS was unknown to the pilots prior to the Lion Air crash that no pilot could do anything to avoid it because it wasn't an already known defect or set of defects that the pilot would diagnose. What defect were pilots told the switches were for?
The cutout switches were originally there to isolate the undesired conditions including the runway trim, the switches were there long before the MAX so MCAS is not relevant to the question.
Originally the pilots were told what both of the switches isolated independently including the manual trim switch, that changed on the NG and after.
Soon after the Lion Air crash runaway trim was no longer required to be "continuous" and I doubt that to this day there is any definition of runaway trim that is taught to be correct as a standard (run time or stick force).
I would also question the simulators control forces accuracy in many conditions.
That still leaves what "continuous" means. I didn't mean that it would shear the stop but that trim movement would cease when it got there and would not continue and until then the pilot would be unable to decide if it was continuous or not. Hence my leaning to what the pilots should cue on based on control forces.
I see some forces are mentioned in the Lion Air final report; sometimes the pilots were trimming when forces exceeded 65 pounds. Apparently the FO reached over 100 pounds of pull and still did not effectively trim. I expect the Ethiopian pilots were in the 60+ pound range when they stopped trimming, but until that report comes out I don't have any idea why they chose as they did. For that plane the trim forces increased only due to aerodynamic loads until they reenabled the trim motors, so how hard were they pulling before making that decision is still unknown to me.
Even the Lion Air report is not clear about why the FO accepted such high control forces except the FO had numerous training notations about difficulties in handling the aircraft which indicate to me a propensity to muscle the plane rather than use the trim switches or ask for guidance. The captain was quite proficient at handling, stopping MCAS with as little as 3 seconds of operation and almost completely reversing MCAS inputs, but didn't communicate the need for effective trimming under circumstances when such trimming was entirely unexpected.
Had there been an intermittent connection in the AND trim switch that produced similar effects to MCAS would the pilots have done the correct thing under the previous trim runaway training? Or would they still have crashed and the maker of the switches be examined and blamed for creating an untenable situation? The main difference would be no warnings at all; just a sudden and random AND trim because the AoA system would not be involved, so no stickshaker or stall warning.
Given that the NG switches were independent, but the guidance was to shut both off, probably to prevent crews from spending time trying to figure out which system was responsible and crashing the plane while making that decision or shutting off the wrong one and crashing the plane. The change in the MAX made it so that the crew could not leave a runaway source enabled no matter which switch they used. Since the plane was to be comparable to the NG there would have been the potential for confusion of a MAX pilot in an NG seat thinking only one switch was necessary, precluding the use on the MAX of only one switch even though only one was required. I do recognize I've heard of no case of trim runaway in an NG so I don't know if that compatibility would have mattered.
There have been multiple suggestions that, had the crews trained or been told of MCAS, the accidents would not have happened, but the evidence is that the only crew to successfully address it had no idea it existed and the crew that should have known every detail performed the worst of all three. Hence my feeling that instead of complex checklists to sort out trim problems, training should focus entirely on the symptoms regardless of cause. Any non-maneuvering forces should be trimmed out to allow maximum maneuvering inputs to be made. Calling out the pitch trim position following pilot input should be common so crews are fully aware of the typical values and can spot trim deviations more quickly.
That still leaves what "continuous" means. I didn't mean that it would shear the stop but that trim movement would cease when it got there and would not continue and until then the pilot would be unable to decide if it was continuous or not. Hence my leaning to what the pilots should cue on based on control forces.
I see some forces are mentioned in the Lion Air final report; sometimes the pilots were trimming when forces exceeded 65 pounds. Apparently the FO reached over 100 pounds of pull and still did not effectively trim. I expect the Ethiopian pilots were in the 60+ pound range when they stopped trimming, but until that report comes out I don't have any idea why they chose as they did. For that plane the trim forces increased only due to aerodynamic loads until they reenabled the trim motors, so how hard were they pulling before making that decision is still unknown to me.
Even the Lion Air report is not clear about why the FO accepted such high control forces except the FO had numerous training notations about difficulties in handling the aircraft which indicate to me a propensity to muscle the plane rather than use the trim switches or ask for guidance. The captain was quite proficient at handling, stopping MCAS with as little as 3 seconds of operation and almost completely reversing MCAS inputs, but didn't communicate the need for effective trimming under circumstances when such trimming was entirely unexpected.
Had there been an intermittent connection in the AND trim switch that produced similar effects to MCAS would the pilots have done the correct thing under the previous trim runaway training? Or would they still have crashed and the maker of the switches be examined and blamed for creating an untenable situation? The main difference would be no warnings at all; just a sudden and random AND trim because the AoA system would not be involved, so no stickshaker or stall warning.
Given that the NG switches were independent, but the guidance was to shut both off, probably to prevent crews from spending time trying to figure out which system was responsible and crashing the plane while making that decision or shutting off the wrong one and crashing the plane. The change in the MAX made it so that the crew could not leave a runaway source enabled no matter which switch they used. Since the plane was to be comparable to the NG there would have been the potential for confusion of a MAX pilot in an NG seat thinking only one switch was necessary, precluding the use on the MAX of only one switch even though only one was required. I do recognize I've heard of no case of trim runaway in an NG so I don't know if that compatibility would have mattered.
There have been multiple suggestions that, had the crews trained or been told of MCAS, the accidents would not have happened, but the evidence is that the only crew to successfully address it had no idea it existed and the crew that should have known every detail performed the worst of all three. Hence my feeling that instead of complex checklists to sort out trim problems, training should focus entirely on the symptoms regardless of cause. Any non-maneuvering forces should be trimmed out to allow maximum maneuvering inputs to be made. Calling out the pitch trim position following pilot input should be common so crews are fully aware of the typical values and can spot trim deviations more quickly.
I see some forces are mentioned in the Lion Air final report; sometimes the pilots were trimming when forces exceeded 65 pounds. Apparently the FO reached over 100 pounds of pull and still did not effectively trim. I expect the Ethiopian pilots were in the 60+ pound range when they stopped trimming, but until that report comes out I don't have any idea why they chose as they did. For that plane the trim forces increased only due to aerodynamic loads until they reenabled the trim motors, so how hard were they pulling before making that decision is still unknown to me.
Even the Lion Air report is not clear about why the FO accepted such high control forces except the FO had numerous training notations about difficulties in handling the aircraft which indicate to me a propensity to muscle the plane rather than use the trim switches or ask for guidance. The captain was quite proficient at handling, stopping MCAS with as little as 3 seconds of operation and almost completely reversing MCAS inputs, but didn't communicate the need for effective trimming under circumstances when such trimming was entirely unexpected.
Had there been an intermittent connection in the AND trim switch that produced similar effects to MCAS would the pilots have done the correct thing under the previous trim runaway training? Or would they still have crashed and the maker of the switches be examined and blamed for creating an untenable situation? The main difference would be no warnings at all; just a sudden and random AND trim because the AoA system would not be involved, so no stickshaker or stall warning.
Given that the NG switches were independent, but the guidance was to shut both off, probably to prevent crews from spending time trying to figure out which system was responsible and crashing the plane while making that decision or shutting off the wrong one and crashing the plane. The change in the MAX made it so that the crew could not leave a runaway source enabled no matter which switch they used. Since the plane was to be comparable to the NG there would have been the potential for confusion of a MAX pilot in an NG seat thinking only one switch was necessary, precluding the use on the MAX of only one switch even though only one was required. I do recognize I've heard of no case of trim runaway in an NG so I don't know if that compatibility would have mattered.
There have been multiple suggestions that, had the crews trained or been told of MCAS, the accidents would not have happened, but the evidence is that the only crew to successfully address it had no idea it existed and the crew that should have known every detail performed the worst of all three. Hence my feeling that instead of complex checklists to sort out trim problems, training should focus entirely on the symptoms regardless of cause. Any non-maneuvering forces should be trimmed out to allow maximum maneuvering inputs to be made. Calling out the pitch trim position following pilot input should be common so crews are fully aware of the typical values and can spot trim deviations more quickly.
So how will this meet grandfather rights?
One wheel on my waggon
While the FAA may be seen to reinvent the wheel, they and the industry must not overlook that this has to be applied in a context which is very different from the original issue. Different vehicle, drivers, travelling surface, increased distraction, and higher task expectation; time for a new wheel, or at least think about wheels before the next one comes off.
Aerodynamically the 737 has had many evolutionary cycles. We have yet to see if the progression of assumptions about the trim system are valid for the Max. The differences between the Max aerodynamics vs the NG might still be excessive; or the change to the smaller trim wheel in the NG, or larger tail surfaces in the classic, all may be the undetected tipping point resulting in recent accidents.
Technology advancement; increased application of highly integrated and inter-dependent systems change the crew task which might have been overlooked in the same-type rating.
Operationally; the pilots who fly the Max will have different levels of training and experience from those in previous years. The training and experience required for an older design with updated systems cannot be judged against the modern, new designed aircraft and operational tasks.
Regulators define and check the standards; the current safety level indicates that a suitable balance is being achieved - except when it isn’t. We must not judge today’s pilots against the standards required many years ago; but today’s Max pilots require those standards, so how to close the gap.
As all of these aspects above converge, increasing complexity, fudged with assumption and belief in past successes - until something fails.
Don’t expect that the old wheel to fit or be sufficient for the range of foreseeable situations in this new world.
Aerodynamically the 737 has had many evolutionary cycles. We have yet to see if the progression of assumptions about the trim system are valid for the Max. The differences between the Max aerodynamics vs the NG might still be excessive; or the change to the smaller trim wheel in the NG, or larger tail surfaces in the classic, all may be the undetected tipping point resulting in recent accidents.
Technology advancement; increased application of highly integrated and inter-dependent systems change the crew task which might have been overlooked in the same-type rating.
Operationally; the pilots who fly the Max will have different levels of training and experience from those in previous years. The training and experience required for an older design with updated systems cannot be judged against the modern, new designed aircraft and operational tasks.
Regulators define and check the standards; the current safety level indicates that a suitable balance is being achieved - except when it isn’t. We must not judge today’s pilots against the standards required many years ago; but today’s Max pilots require those standards, so how to close the gap.
As all of these aspects above converge, increasing complexity, fudged with assumption and belief in past successes - until something fails.
Don’t expect that the old wheel to fit or be sufficient for the range of foreseeable situations in this new world.
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MechEngr - the stabilizer cutoff switches are within a foot of the Captain's right hip/thigh. Possible as close as 6 inches. (12-25 cm).
I have about 4,500 hrs of listening to the trim wheel on Boeing aircraft. I have another 14,500+ hrs on Boeing jets without a trim wheel but the red guarded cutoff switches are still right next to the Captain's hip on those aircraft. In all of them utilizing the stab trim cutout switches is a known procedure for uncommanded or runaway stabilizer trim movement.
It's stunning to think a pilot wouldn't flip the red guarded cutoff switches next to their hip if the trim ran uncommanded for 9.2 secs. A fraction of pilots might let that happen. But then it starts a second time and runs ND for another 9.2 seconds and you feel the plane getting farther and farther out of trim? That's scary to even contemplate. If that happened to any of us it would be one of our top career aviation stories if it happened once. We know it's dangerous on any large jet. If we were telling the story and continued with "and then the trim started to run ND again!!!" would have every pilot listening on the edge of their seats.
Interestingly the 777 and 787 QRH say to not exceed, or perhaps maintain, the current speed. The 737/757/767 make no mention of maintaining your current speed or not exceeding a certain speed.
I have about 4,500 hrs of listening to the trim wheel on Boeing aircraft. I have another 14,500+ hrs on Boeing jets without a trim wheel but the red guarded cutoff switches are still right next to the Captain's hip on those aircraft. In all of them utilizing the stab trim cutout switches is a known procedure for uncommanded or runaway stabilizer trim movement.
It's stunning to think a pilot wouldn't flip the red guarded cutoff switches next to their hip if the trim ran uncommanded for 9.2 secs. A fraction of pilots might let that happen. But then it starts a second time and runs ND for another 9.2 seconds and you feel the plane getting farther and farther out of trim? That's scary to even contemplate. If that happened to any of us it would be one of our top career aviation stories if it happened once. We know it's dangerous on any large jet. If we were telling the story and continued with "and then the trim started to run ND again!!!" would have every pilot listening on the edge of their seats.
Interestingly the 777 and 787 QRH say to not exceed, or perhaps maintain, the current speed. The 737/757/767 make no mention of maintaining your current speed or not exceeding a certain speed.
MechEngr - the stabilizer cutoff switches are within a foot of the Captain's right hip/thigh. Possible as close as 6 inches. (12-25 cm).
I have about 4,500 hrs of listening to the trim wheel on Boeing aircraft. I have another 14,500+ hrs on Boeing jets without a trim wheel but the red guarded cutoff switches are still right next to the Captain's hip on those aircraft. In all of them utilizing the stab trim cutout switches is a known procedure for uncommanded or runaway stabilizer trim movement.
It's stunning to think a pilot wouldn't flip the red guarded cutoff switches next to their hip if the trim ran uncommanded for 9.2 secs. A fraction of pilots might let that happen. But then it starts a second time and runs ND for another 9.2 seconds and you feel the plane getting farther and farther out of trim? That's scary to even contemplate. If that happened to any of us it would be one of our top career aviation stories if it happened once. We know it's dangerous on any large jet. If we were telling the story and continued with "and then the trim started to run ND again!!!" would have every pilot listening on the edge of their seats.
Interestingly the 777 and 787 QRH say to not exceed, or perhaps maintain, the current speed. The 737/757/767 make no mention of maintaining your current speed or not exceeding a certain speed.
I have about 4,500 hrs of listening to the trim wheel on Boeing aircraft. I have another 14,500+ hrs on Boeing jets without a trim wheel but the red guarded cutoff switches are still right next to the Captain's hip on those aircraft. In all of them utilizing the stab trim cutout switches is a known procedure for uncommanded or runaway stabilizer trim movement.
It's stunning to think a pilot wouldn't flip the red guarded cutoff switches next to their hip if the trim ran uncommanded for 9.2 secs. A fraction of pilots might let that happen. But then it starts a second time and runs ND for another 9.2 seconds and you feel the plane getting farther and farther out of trim? That's scary to even contemplate. If that happened to any of us it would be one of our top career aviation stories if it happened once. We know it's dangerous on any large jet. If we were telling the story and continued with "and then the trim started to run ND again!!!" would have every pilot listening on the edge of their seats.
Interestingly the 777 and 787 QRH say to not exceed, or perhaps maintain, the current speed. The 737/757/767 make no mention of maintaining your current speed or not exceeding a certain speed.
From JT610 final report.
At 23:22:33 UTC, the flaps reached the fully retracted position and the automatic AND trim was active for about 10 seconds, during which the horizontal stabilizer pitch trim decreased from 6.1 to 3.8 units. At 23:22:41 UTC, the Captain instructed the FO to select flaps 1 and the DFDR recorded the flaps started to move. Three seconds later, the DFDR recorded the main electric trim moved the stabilizer in the aircraft nose up (ANU) direction for 5 seconds and the pitch trim gradually increased to 4.7 units.
At 23:25:27 UTC, the automatic AND trim activated by the Maneuver Characteristic Augmentation System (MCAS) for 2 seconds and was interrupted by the Captain who commanded ANU trim for 6 seconds. The pitch trim recorded 6.19 units.
At 23:25:40 UTC, MCAS activated for 6 seconds. The pitch trim recorded 4.67 units. This MCAS activation was interrupted when the Captain commanded ANU trim at 23:25:46 UTC for 7 seconds and again at 23:25:54 UTC for 1 second. The pitch trim recorded 6.27 units. At 23:26:00 UTC, MCAS activated for 7 seconds and was interrupted at 23:26:06 UTC when the Captain commanded ANU trim for 6 seconds. The pitch trim recorded 5.59 units. At 23:26:17 UTC, MCAS activated for 4 seconds until it was interrupted at 23:26:20 UTC when the Captain commanded ANU trim for 4 seconds. The pitch trim recorded 5.6 units At 23:26:29 UTC, MCAS activated for 3 seconds until it was interrupted at 23:26:32 UTC when the Captain commanded ANU trim for 3 seconds. The pitch trim recorded 5.0 units
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A related bit of history
found on the tech log thread that was locked
Shows how messed up boeing was re MCAS and software fix as in 10 days . . .
https://www.thestreet.com/investing/...e-fix-14897876
Shows how messed up boeing was re MCAS and software fix as in 10 days . . .
https://www.thestreet.com/investing/...e-fix-14897876
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