The Boeing 737 use of the Roller Coaster method of manual stab trim
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The Boeing 737 use of the Roller Coaster method of manual stab trim
Regarding the eventual return to service of the 737 800MAX. Presumably Boeing would make any necessary amendments to the FCTM. Reports indicated both crews of the ill-fated aircraft were unable to regain manual stabilizer pitch control because of powerful aerodynamic forces on the stabilizer caused by a runaway electrical stabilizer trim.
There were other factors of course, but one particular action that may have prevented these accidents was if the crews had been aware of how to "unload " this aerodynamic force on the stabilizer quickly enough before the mis-trim got out of hand. .
With regard to using the manual stabilizer trim wheel, the FCTM at the time stated "excessive air-loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air-loads to allow manual trimming."
Critically, the FCTM omitted to explain how pilots can "aerodynamically relieve air-loads" Yet an explanation of this method was contained in the pilot training manuals of the Boeing 737-200, the Boeing 727 and the Boeing 707. For example the Boeing 737 PTM explains how this is achieved by stating:
"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 column pressure and manually trim nose-up. Allow the nose to drop to the horizon while trimming. Repeat the sequence until the airplane is in trim."
That is called aerodynamically relieving air-loads and was sometimes called the Yo-Yo maneuver. Yet that advice was removed from the Boeing 737-300 FCTM and subsequent variants of the 737 series.
Interestingly I received the following comment in a letter from a former airline pilot.
He wrote: :"In late 1974 I did my B727 conversion on the TAA -100 simulator and the yo-yo manoeuvre was in the FTM and we practised it in the sim. From memory you would get about 2 to 4 winds of the trim wheel per bunt.".
When the Boeing 737-800 MAX finally returns to service it would be wise for Boeing include amplification in the FCTM of the terminology "aerodynamically relieve air-loads." The best way is to explain the use of the roller coaster method.
There were other factors of course, but one particular action that may have prevented these accidents was if the crews had been aware of how to "unload " this aerodynamic force on the stabilizer quickly enough before the mis-trim got out of hand. .
With regard to using the manual stabilizer trim wheel, the FCTM at the time stated "excessive air-loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air-loads to allow manual trimming."
Critically, the FCTM omitted to explain how pilots can "aerodynamically relieve air-loads" Yet an explanation of this method was contained in the pilot training manuals of the Boeing 737-200, the Boeing 727 and the Boeing 707. For example the Boeing 737 PTM explains how this is achieved by stating:
"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 column pressure and manually trim nose-up. Allow the nose to drop to the horizon while trimming. Repeat the sequence until the airplane is in trim."
That is called aerodynamically relieving air-loads and was sometimes called the Yo-Yo maneuver. Yet that advice was removed from the Boeing 737-300 FCTM and subsequent variants of the 737 series.
Interestingly I received the following comment in a letter from a former airline pilot.
He wrote: :"In late 1974 I did my B727 conversion on the TAA -100 simulator and the yo-yo manoeuvre was in the FTM and we practised it in the sim. From memory you would get about 2 to 4 winds of the trim wheel per bunt.".
When the Boeing 737-800 MAX finally returns to service it would be wise for Boeing include amplification in the FCTM of the terminology "aerodynamically relieve air-loads." The best way is to explain the use of the roller coaster method.
Last edited by Centaurus; 15th Mar 2020 at 10:29.
When I did my -300 conversion in 2000 I'm pretty sure the yo-yo was in the FCTM though to what degree it was described I can't remember.
However our very old-school instructor who was nothing if not punctilious demonsrated the situation and taught us to do it in the sim.
iirc in the early stages of an extreme case it required quite a short & sharp unload to free even the smallest trim wheel movement but with co-ordination these became longer and softer and more trim was recovered each time. Initially you might only get a quarter/half turn or so. I don't recall being particularly anxious over loss of altitude as we were always (both together) able to pull enough pitch to compensate for the previous bunt. That did require the PNF to link hands behind the column and pull hard so as to leave roll control unaffected. Initially it was very physical and the PNF had to be forceful with the trim wheel.
Bear in mind this is not necessary if electric trim is operable (as it was in both MAX accidents) as electric trim can override all trim loads. The problem only arises if a severe out of trim situation occurs with electric trim inop.
The moral of the story appears to be that minimum training on conversion is not a good idea, there are nice to know extras like this that can save your skin, and don't de-select electric trim when you are drastically out of trim and the electric was perfectly capable of restoring equilibrium which is so obvious it seems hardly necessary to mention it.
However our very old-school instructor who was nothing if not punctilious demonsrated the situation and taught us to do it in the sim.
iirc in the early stages of an extreme case it required quite a short & sharp unload to free even the smallest trim wheel movement but with co-ordination these became longer and softer and more trim was recovered each time. Initially you might only get a quarter/half turn or so. I don't recall being particularly anxious over loss of altitude as we were always (both together) able to pull enough pitch to compensate for the previous bunt. That did require the PNF to link hands behind the column and pull hard so as to leave roll control unaffected. Initially it was very physical and the PNF had to be forceful with the trim wheel.
Bear in mind this is not necessary if electric trim is operable (as it was in both MAX accidents) as electric trim can override all trim loads. The problem only arises if a severe out of trim situation occurs with electric trim inop.
The moral of the story appears to be that minimum training on conversion is not a good idea, there are nice to know extras like this that can save your skin, and don't de-select electric trim when you are drastically out of trim and the electric was perfectly capable of restoring equilibrium which is so obvious it seems hardly necessary to mention it.
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Though this procedure is physically valid, there's no way it would have worked in these 2 crashes. It requires a highly coordinated sequence of very unusual tasks between the two pilots, but they were at full task saturation (i.e., brain locked) that they couldn't handle the much more mundane tasks of normal flight. Like, in the Lion Air crash, the CA had the situation stabilized for many minutes, where the plane would trim itself out of trim, and the CA would (electric) trim it back, over and over and over. Very well. But there wasn't the coordination in the handoff to the FO to give him that information ("it keeps trimming down, just trim it back up"). So if they couldn't manage that, there's no way in hell they could coordinate the timing of the roller coaster unload sequence.
Plus, though it is valid, the roller coaster sequence is useless. If anybody has the control authority to pull it into a climb to begin the arc to allow the bunt, they can instead just establish the climb and hold it there for a little while until the speed comes down to where the trim wheel unjams.
Plus, though it is valid, the roller coaster sequence is useless. If anybody has the control authority to pull it into a climb to begin the arc to allow the bunt, they can instead just establish the climb and hold it there for a little while until the speed comes down to where the trim wheel unjams.
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If anybody has the control authority to pull it into a climb to begin the arc to allow the bunt, they can instead just establish the climb and hold it there for a little while until the speed comes down to where the trim wheel unjams.
If the effects of trim runaway (stick force per deviation) in the Max are greater than previous 737 versions (aerodynamic differences), then the regulators might have to consider alternatives to a crew procedure
Investigations into the recent accidents suggest that there are conditions where the pitch control with large trim displacement in the Max is ineffective, thus a crew would be unable to fly a procedure even after reducing thrust / speed with a nose down attitude.
Rumours re MCAS modifications suggest that there will be a MCAS fail light so that the crew can isolate the trim system within the short time period whilst there is reasonable opportunity to have pitch control.
Also rumours of some aerodynamic 'adjustments'.
For trim runaway there could be a new 'trim fail' light (difficult to engineer) or a very strong argument why trim runaway differs from MCAS fail (intermittent vs continuous - Pprune debates).
i.e. Having an alert to trigger quick pilot action, or reliance on crew identifying a critical situation requiring urgent action; who's choice, safety basis, man or machine, human factors, history, …
Investigations into the recent accidents suggest that there are conditions where the pitch control with large trim displacement in the Max is ineffective, thus a crew would be unable to fly a procedure even after reducing thrust / speed with a nose down attitude.
Rumours re MCAS modifications suggest that there will be a MCAS fail light so that the crew can isolate the trim system within the short time period whilst there is reasonable opportunity to have pitch control.
Also rumours of some aerodynamic 'adjustments'.
For trim runaway there could be a new 'trim fail' light (difficult to engineer) or a very strong argument why trim runaway differs from MCAS fail (intermittent vs continuous - Pprune debates).
i.e. Having an alert to trigger quick pilot action, or reliance on crew identifying a critical situation requiring urgent action; who's choice, safety basis, man or machine, human factors, history, …
Regarding the eventual return to service of the 737 800MAX. Presumably Boeing would make any necessary amendments to the FCTM. Reports indicated both crews of the ill-fated aircraft were unable to regain manual stabilizer pitch control because of powerful aerodynamic forces on the stabilizer caused by a runaway electrical stabilizer trim.
There were other factors of course, but one particular action that may have prevented these accidents was if the crews had been aware of how to "unload " this aerodynamic force on the stabilizer quickly enough before the mis-trim got out of hand. .
With regard to using the manual stabilizer trim wheel, the FCTM at the time stated "excessive air-loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air-loads to allow manual trimming."
Critically, the FCTM omitted to explain how pilots can "aerodynamically relieve air-loads" Yet an explanation of this method was contained in the pilot training manuals of the Boeing 737-200, the Boeing 727 and the Boeing 707. For example the Boeing 737 PTM explains how this is achieved by stating:
"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 column pressure and manually trim nose-up. Allow the nose to drop to the horizon while trimming. Repeat the sequence until the airplane is in trim."
That is called aerodynamically relieving air-loads and was sometimes called the Yo-Yo maneuver. Yet that advice was removed from the Boeing 737-300 FCTM and subsequent variants of the 737 series.
Interestingly I received the following comment in a letter from a former airline pilot.
He wrote: :"In late 1974 I did my B727 conversion on the TAA -100 simulator and the yo-yo manoeuvre was in the FTM and we practised it in the sim. From memory you would get about 2 to 4 winds of the trim wheel per bunt.".
When the Boeing 737-800 MAX finally returns to service it would be wise for Boeing include amplification in the FCTM of the terminology "aerodynamically relieve air-loads." The best way is to explain the use of the roller coaster method.
There were other factors of course, but one particular action that may have prevented these accidents was if the crews had been aware of how to "unload " this aerodynamic force on the stabilizer quickly enough before the mis-trim got out of hand. .
With regard to using the manual stabilizer trim wheel, the FCTM at the time stated "excessive air-loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air-loads to allow manual trimming."
Critically, the FCTM omitted to explain how pilots can "aerodynamically relieve air-loads" Yet an explanation of this method was contained in the pilot training manuals of the Boeing 737-200, the Boeing 727 and the Boeing 707. For example the Boeing 737 PTM explains how this is achieved by stating:
"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 column pressure and manually trim nose-up. Allow the nose to drop to the horizon while trimming. Repeat the sequence until the airplane is in trim."
That is called aerodynamically relieving air-loads and was sometimes called the Yo-Yo maneuver. Yet that advice was removed from the Boeing 737-300 FCTM and subsequent variants of the 737 series.
Interestingly I received the following comment in a letter from a former airline pilot.
He wrote: :"In late 1974 I did my B727 conversion on the TAA -100 simulator and the yo-yo manoeuvre was in the FTM and we practised it in the sim. From memory you would get about 2 to 4 winds of the trim wheel per bunt.".
When the Boeing 737-800 MAX finally returns to service it would be wise for Boeing include amplification in the FCTM of the terminology "aerodynamically relieve air-loads." The best way is to explain the use of the roller coaster method.
I know it sounds like heresy but shouldn’t an aircraft trim system be able to function reliably throughout the flight envelope? If it didn’t really work on the -200, there’s not much hope for the MAX with larger control surfaces and smaller wheels...
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B737 was made out of 707 fuselage to save production cost. Ever since the bigger fan fuel saving engines came on the scene it became a handicap. It should have been ended but for commercial reasons they developed work arounds and kept selling. They wanted to end at NG but again for commercial reasons hammered this old design into a MAX with MCAS which was without any redundancy and without any information to operators leave alone any training. Now that 364 people have lost their lives all the skeletons are tumbling out of cupboards. Position of centre of pressure on the stabilizer is yet another one. In the mean time they have sold thousands of these. Irony is perhaps the commercially most successful aircraft is perhaps the most unsafe Boeing ever made.
A technique such as this, which dates back to the first generation of jet transports, shouldn't be required in the 21 century to counteract undesirable handling characteristics. The fact that it is being talked about shows how out of date this aircraft is.
FullWings, # 8 
Some certification aspects were discussed on Pprune during the 'early days'. However, reconsidering these with what is now known could change thoughts and understanding; not necessarily for the better.
EASA requested certification clarification (*);
'Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, … The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only. '
The Boeing / FAA response to use the trim wheel as an alternative, was accepted:-
'The need to use the trim wheel is considered unusual, as it is only required for manual flight in those corners of the envelope. The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides a compensating factor for the inability to use the thumb switches throughout the entire flight envelope. Furthermore, the additional crew procedures and training material will clearly explain to pilots the situations where use of the trim wheel may be needed due to lack of trim authority with the wheel mounted switches. The trim systems on the 737Max provide an appropriate level of safety relative to longitudinal trim capability. '
This acceptance appears to be based on the engineering simulation at that time, which subsequently has been shown not to cover all aspects ( Ethiopian accident report ).
If the simulation deficiencies only applied to the specific conditions of an MCAS induced trim deviation, additional stick force (trim wheel) due to 'feel shift', then the MCAS mod should negate that extreme.
However, if the revised trim model now shows that the previous acceptable position is questionable - that trim cannot be achieved, or that the additional safety from limiting electric trim input no longer applies (assumes elect trim runaway is possible beyond thumb switch limiting), then the certification aspect must be addressed by other means.
* https://www.easa.europa.eu/sites/def...20ISS%2010.pdf (Page 15)
Some certification aspects were discussed on Pprune during the 'early days'. However, reconsidering these with what is now known could change thoughts and understanding; not necessarily for the better.
EASA requested certification clarification (*);
'Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, … The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only. '
The Boeing / FAA response to use the trim wheel as an alternative, was accepted:-
'The need to use the trim wheel is considered unusual, as it is only required for manual flight in those corners of the envelope. The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides a compensating factor for the inability to use the thumb switches throughout the entire flight envelope. Furthermore, the additional crew procedures and training material will clearly explain to pilots the situations where use of the trim wheel may be needed due to lack of trim authority with the wheel mounted switches. The trim systems on the 737Max provide an appropriate level of safety relative to longitudinal trim capability. '
This acceptance appears to be based on the engineering simulation at that time, which subsequently has been shown not to cover all aspects ( Ethiopian accident report ).
If the simulation deficiencies only applied to the specific conditions of an MCAS induced trim deviation, additional stick force (trim wheel) due to 'feel shift', then the MCAS mod should negate that extreme.
However, if the revised trim model now shows that the previous acceptable position is questionable - that trim cannot be achieved, or that the additional safety from limiting electric trim input no longer applies (assumes elect trim runaway is possible beyond thumb switch limiting), then the certification aspect must be addressed by other means.
* https://www.easa.europa.eu/sites/def...20ISS%2010.pdf (Page 15)
Last edited by safetypee; 17th Mar 2020 at 09:52. Reason: sp
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Plus, though it is valid, the roller coaster sequence is useless
Maybe you should contact Boeing suggesting an alternative method of regaining some semblance of manual stab trim pitch control following a electrical runaway nose down change of trim?
Last edited by Centaurus; 17th Mar 2020 at 11:59.
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Apologies if I have the wrong interpretation of your comment. But are you implying the original Boeing test pilots who tested the roller coaster method on the 707,727and 737, were talking through their arse?
Maybe you should contact Boeing suggesting an alternative method of regaining some semblance of manual stab trim pitch control following a electrical runaway nose down change of trim?
Maybe you should contact Boeing suggesting an alternative method of regaining some semblance of manual stab trim pitch control following a electrical runaway nose down change of trim?
To be fair to the original poster of that comment, if you are already pointing earthwards and you have no functioning pitch trim and no elevator authority, then the existence of the roller-coaster technique is indeed academic.
I agree with DaveReidUK. The roller coaster manoeuvre would have been next to impossible - it is only to be practised in the sim or at a reasonable altitude in the aircraft and always with great care. It had to be done for real years ago in a 707 by a BOAC crew at very low altitude after a stabiliser runaway soon after take-off in Canada. It was sucessful, but a very near run thing!
weemonkey, 'single warning system'
Not necessarily so. Assuming that the purpose is to alert the crew that MCAS is inoperative, because the new dual-monitored system has shut that system down (after a small trim deviation), the alert does not require any higher integrity.
Conversely, a hypothetical 'trim runaway' alert would be more difficult because there does not appear to be any means of monitoring trim - reliance on the crew.
Is trim movement normal, manual, autopilot, or is there a failure; how could this be judged by the system so that the crew could inhibit trim before a roller coaster (feasible or not), would be required.
One outstanding question is why does the 737 require such a large trim range - configuration, cg, speed ?
Not necessarily so. Assuming that the purpose is to alert the crew that MCAS is inoperative, because the new dual-monitored system has shut that system down (after a small trim deviation), the alert does not require any higher integrity.
Conversely, a hypothetical 'trim runaway' alert would be more difficult because there does not appear to be any means of monitoring trim - reliance on the crew.
Is trim movement normal, manual, autopilot, or is there a failure; how could this be judged by the system so that the crew could inhibit trim before a roller coaster (feasible or not), would be required.
One outstanding question is why does the 737 require such a large trim range - configuration, cg, speed ?
to be fair to the original poster of that comment, if you are already pointing earthwards and you have no functioning pitch trim and no elevator authority, then the existence of the roller-coaster technique is indeed academic.
If they had promptly turned off the electrics to the stabiliser as the first action, the mis-trim would not have so bad as it eventually came. Once the nose was allowed to drop well below the horizon before pilot intervention, the airspeed rapidly increased making it difficult for even the combined efforts of both pilots (assuming they were coordinated in the first place) to fall back on the roller coaster method to regain pitch authority.
It was probable that neither of the two crews were aware of the roller coaster method of recovery to trimmed flight. I am aware of at least one operator who practices the roller coaster in the simulator. The simulator instructor places the simulator into a dive to simulate a runaway stab trim. He then instructs the crew to heave back on the control column to the horizon only and then relax the elevator back pressure and attempt to operate the manual trim backwards during the momentary unloading process. By then the aircraft has increased speed so quickly that it becomes almost impossible to pull the nose as high as possible above the horizon to commence the roller coaster.
In other words the instructor is at fault for setting up the scenario badly in the first instance and giving the crew no hope of regaining control by manual stab trim operation.
The crew wonder WTF and go away to have their coffee without having learned a thing. Of course there are limits to the successful operation of the manual stabilizer recovery to trimmed flight. But if the simulator instructors don't have a clue then no wonder the flight crew can be caught out
Only if you define elevator authority as being able to simply move the control surface.
But the usual definition concerns the degree of control that the elevators can exert over the pitch of an aircraft. When the elevators are competing with full AND stab trim (that you can't reverse) combined with a significantly higher than normal IAS, the degree of control that the elevator can provide is pretty well non-existent, as events proved.
If it was otherwise, we wouldn't be having this discussion.
But the usual definition concerns the degree of control that the elevators can exert over the pitch of an aircraft. When the elevators are competing with full AND stab trim (that you can't reverse) combined with a significantly higher than normal IAS, the degree of control that the elevator can provide is pretty well non-existent, as events proved.
If it was otherwise, we wouldn't be having this discussion.
Judd, elevator authority - movement, as per DR #15.
Words such as 'evident', 'if', 'probable' imply assumption - often leading to blaming the human; which as with previous discussions is unhelpful for understanding the point of discussion.
What appears to be unknown (to us) is the actual aircraft characteristics at extreme trim settings, in combination with control deflection, speed, thrust setting, nose attitude, and also how these differ amongst 737 variants.
What are the effects of increased tail surface, smaller trim wheel, higher thrust levels, and aerodynamic changes affecting the ability to move the trim manually at extreme settings; none AFAIR have been disclosed by Boeing.
Details from the accidents suggest that there were similar unknowns in the design/operation and certification of the Max, many were mitigated by assumption, ill judged extrapolation from previous variants, and in service experience with previous variants.
Whereas the manufacturer and regulators have opportunity to learn and change, as yet we do not have such understanding re 737 Max, thus there is little point in attempting to relate what appears to be an impractical procedure, (but blame the pilot anyway), with what will be required in a modified aircraft.
Words such as 'evident', 'if', 'probable' imply assumption - often leading to blaming the human; which as with previous discussions is unhelpful for understanding the point of discussion.
What appears to be unknown (to us) is the actual aircraft characteristics at extreme trim settings, in combination with control deflection, speed, thrust setting, nose attitude, and also how these differ amongst 737 variants.
What are the effects of increased tail surface, smaller trim wheel, higher thrust levels, and aerodynamic changes affecting the ability to move the trim manually at extreme settings; none AFAIR have been disclosed by Boeing.
Details from the accidents suggest that there were similar unknowns in the design/operation and certification of the Max, many were mitigated by assumption, ill judged extrapolation from previous variants, and in service experience with previous variants.
Whereas the manufacturer and regulators have opportunity to learn and change, as yet we do not have such understanding re 737 Max, thus there is little point in attempting to relate what appears to be an impractical procedure, (but blame the pilot anyway), with what will be required in a modified aircraft.