737 Trim
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737 Trim
Avoiding the closed threads, I'd like to enquire about the 737NG HS trim system. Evidence tends to suggest that in a specific and perhaps very niche corner of the flight envelope, due to powerful aerodynamic forces, the manual HS trim on at least one version of the 737 doesn't function as intended and, in all probability based on known facts at this time, the electric trim system can also becone overwhelmed in extreme circumstances. My question is: regardless of how unlikely it may be, how certain is it that other variants of the 737 can not end up in a flight regime, through some chain of undesirable events, whereby the trim system essentially becomes inoperable?
If it is certain this can't occur, how do we know with any certainty?
If it is certain this can't occur, how do we know with any certainty?
deadheader,
Many aspects of your question were considered in prior, mostly closed threads, but never pursued to a meaningful conclusion.
Reasons for this are difficult to establish; thus specific answers remain unknown because they would depend on what tests Boeing conducted over the many years of 737 development. The results of these tests should have been available to the FAA as submissions or supporting data in certification.
Starting with the original 737, it could be expected that a full range of evaluations were conducted and used to justify safety, procedures, and training. But it may be difficult to establish how much of this data has been transferred with grandfather rights to newer variants, not always repeating the tests.
Spot checks within the normal operating envelope would support a new series certification, so too data from mandatory extreme structural and handling flights VD, MDF.
However, how far the original assumptions have been stretched for abnormal operation might be less clear, e.g. recovery from trim runaway. See the related thread Evolution of 737 Runaway Trim Procedure
Changes in the emergency drill suggest adaptions to modifications to the trim system / electrical system; but not necessarily to any aerodynamic changes with aircraft growth.
Thus a key question is to what extent was the runaway trim procedure validated in new variants. Critical assumptions involve pilots’ recognition of the failure and reaction time in isolating the electrical trim, but amended drills appear to lengthen this period. If so, then the likelihood that the extent of the tail plane deviation will increase with failure, with matching difficulty in maintaining control and establishing a safe trimmed condition using manual trim wheel / elevator. Note changes involving grasping the trim wheel, together with a reduced size of the trim wheel.
EASA posed certification questions for the NG, which were satisfactorily addressed by manual trim in situations where electrical trim was unavailable - either by design (minimising effect of runaway), by motor characteristics, or crew effort; still unknown ?
The effect of the reduced size of trim wheel in recovery was not published, thus assumed to be negligible; however, with accumulated assumption, extent of trim runaway, reaction time, this could now be questioned - see FAA review. Anyway it might be difficult to validate or justify this situation based on flight test - particularly if the test was unsuccessful !
It is probable that the response to EASA only considered normal trim operation (that was the question), and not that of extreme tail plane deviation nor further adverse effect of elevator input for recovery.
There are no simple answers; if any at all.
We cannot be certain about the effect of a failure; there is no ‘certainty’ in certification. Judgements are made about systems’ design and evaluation, technical, human performance, reliability, etc. This is a forecast based on what is known (assumed) about the systems probability of failure and the likely outcome, the latter being most difficult to judge particularly if relying on human input. Whilst the failure rate might be the same, the outcome perhaps less certain based on recent events.
How far back this issue applies - previous variants. This, as above is unknown (unknowable), but growing concerns from pilots and operators, and more recently regulators might reflect a serious safety issue.
Many aspects of your question were considered in prior, mostly closed threads, but never pursued to a meaningful conclusion.
Reasons for this are difficult to establish; thus specific answers remain unknown because they would depend on what tests Boeing conducted over the many years of 737 development. The results of these tests should have been available to the FAA as submissions or supporting data in certification.
Starting with the original 737, it could be expected that a full range of evaluations were conducted and used to justify safety, procedures, and training. But it may be difficult to establish how much of this data has been transferred with grandfather rights to newer variants, not always repeating the tests.
Spot checks within the normal operating envelope would support a new series certification, so too data from mandatory extreme structural and handling flights VD, MDF.
However, how far the original assumptions have been stretched for abnormal operation might be less clear, e.g. recovery from trim runaway. See the related thread Evolution of 737 Runaway Trim Procedure
Changes in the emergency drill suggest adaptions to modifications to the trim system / electrical system; but not necessarily to any aerodynamic changes with aircraft growth.
Thus a key question is to what extent was the runaway trim procedure validated in new variants. Critical assumptions involve pilots’ recognition of the failure and reaction time in isolating the electrical trim, but amended drills appear to lengthen this period. If so, then the likelihood that the extent of the tail plane deviation will increase with failure, with matching difficulty in maintaining control and establishing a safe trimmed condition using manual trim wheel / elevator. Note changes involving grasping the trim wheel, together with a reduced size of the trim wheel.
EASA posed certification questions for the NG, which were satisfactorily addressed by manual trim in situations where electrical trim was unavailable - either by design (minimising effect of runaway), by motor characteristics, or crew effort; still unknown ?
The effect of the reduced size of trim wheel in recovery was not published, thus assumed to be negligible; however, with accumulated assumption, extent of trim runaway, reaction time, this could now be questioned - see FAA review. Anyway it might be difficult to validate or justify this situation based on flight test - particularly if the test was unsuccessful !
It is probable that the response to EASA only considered normal trim operation (that was the question), and not that of extreme tail plane deviation nor further adverse effect of elevator input for recovery.
There are no simple answers; if any at all.
We cannot be certain about the effect of a failure; there is no ‘certainty’ in certification. Judgements are made about systems’ design and evaluation, technical, human performance, reliability, etc. This is a forecast based on what is known (assumed) about the systems probability of failure and the likely outcome, the latter being most difficult to judge particularly if relying on human input. Whilst the failure rate might be the same, the outcome perhaps less certain based on recent events.
How far back this issue applies - previous variants. This, as above is unknown (unknowable), but growing concerns from pilots and operators, and more recently regulators might reflect a serious safety issue.
Avoiding the closed threads, I'd like to enquire about the 737NG HS trim system. Evidence tends to suggest that in a specific and perhaps very niche corner of the flight envelope, due to powerful aerodynamic forces, the manual HS trim on at least one version of the 737 doesn't function as intended and, in all probability based on known facts at this time, the electric trim system can also becone overwhelmed in extreme circumstances. My question is: regardless of how unlikely it may be, how certain is it that other variants of the 737 can not end up in a flight regime, through some chain of undesirable events, whereby the trim system essentially becomes inoperable?
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Avoiding the closed threads, I'd like to enquire about the 737NG HS trim system. Evidence tends to suggest that in a specific and perhaps very niche corner of the flight envelope, due to powerful aerodynamic forces, the manual HS trim on at least one version of the 737 doesn't function as intended and, in all probability based on known facts at this time, the electric trim system can also becone overwhelmed in extreme circumstances. My question is: regardless of how unlikely it may be, how certain is it that other variants of the 737 can not end up in a flight regime, through some chain of undesirable events, whereby the trim system essentially becomes inoperable?
If it is certain this can't occur, how do we know with any certainty?
If it is certain this can't occur, how do we know with any certainty?
Since the B-707 days, Boeing has recognized that in a situation combining high airspeeds with a severely out of trim stabilizer, the ability of the either the manual trim wheels or electric drive to move the trim jack screw could be overwhelmed. I quote from the "FAA APPROVED AIRPLANE (B-707) FLIGHT MANUAL, STABILIZER TRIM EMERGENCY OPERATION: "If the stabilizer electric trim becomes inoperative, the manual trim can be used. Due to the flight loads imposed in an out of trim condition, it may be necessary to release the control column to a low force position and retrim while in an "unloaded" condition. If the airplane is much out of trim, this action will have to be repeated until correct trim is obtained." It should be noted that the foregoing is quoted from a November 08, 1960 revision to the manual.
Hopefully following the links in the Posts by PEI 3721 will answer the balance of your questions.
Cheers,
Grog
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More important than MCAS...
Thank you for starting a thread on this important topic.
Lost in the back and forth of the previous epic threads was the fact that at some point in (probably) all late-model 737s the ability of the aircrew to move the horizontal stabilizer using either manual (wheel) trim or manual (electronic) trim becomes impossible. Anecdotal contributions indicate that this design flaw traces its way all the way back to the 707 at least and may have been present in most or all 737s ever built.
The bottom line is this: The 737 series has been certified with a (possibly) unknown portion of the normal flight operations envelope within which you can inadvertently render your primary pitch control surface completely inoperable!! I have spent significant time reviewing the FARs as they relate to certification and I have yet to find the one that covers Control surface stops functioning There are all kinds of tests for failure and overstress, but it never seems to have occurred to anyone that they would need to have a certification for Control surface operates and is operable throughout its range in all quadrants of the performance envelope.
For you 737 drivers out there consider these thoughts:
1. What is the airspeed/altitude/pitch combination that you should not cross- after which you will not have control authority over the horizontal stabilizer??
2. If you TOGA on short-final and your PF holds in a bunch of nose-down trim as you accelerate out of 130Kts, at what point does the airplane become unrecoverable after you come over the top and start back down due to the severe out of trim situation?? It seems that 9 seconds of trim may well be enough to start this ball rolling downhill, so at what point does your airplane become unrecoverable due to a slow/non-moving horizontal stab trim due to aerodynamic overstresses??
I have never heard of an airplane being certified wherein a single control surface (the most important one really) can completely cease operation and lock-up within the normal flight envelope. And if such a situation was allowed to be certified youd think there would be placards, warnings, airspeed bugs etc. to prevent one from unintentionally straying into uncharted territory.
Its actually enough to make one look again at Rostov-on-Don and other incidents from the past, where airplanes went from happily flying to a nose-down dive and crash within 45 seconds or so after a TOGA. For some reason the 737 seems oddly prone to this type of unfortunate end.
MCAS is worth parsing to the nth degree of course, but this issue, the one that involves the total inoperability of the primary pitch control for the airplane, this is the one that has had me reading and researching everything I can find over the past month.
Regards,
dce
Lost in the back and forth of the previous epic threads was the fact that at some point in (probably) all late-model 737s the ability of the aircrew to move the horizontal stabilizer using either manual (wheel) trim or manual (electronic) trim becomes impossible. Anecdotal contributions indicate that this design flaw traces its way all the way back to the 707 at least and may have been present in most or all 737s ever built.
The bottom line is this: The 737 series has been certified with a (possibly) unknown portion of the normal flight operations envelope within which you can inadvertently render your primary pitch control surface completely inoperable!! I have spent significant time reviewing the FARs as they relate to certification and I have yet to find the one that covers Control surface stops functioning There are all kinds of tests for failure and overstress, but it never seems to have occurred to anyone that they would need to have a certification for Control surface operates and is operable throughout its range in all quadrants of the performance envelope.
For you 737 drivers out there consider these thoughts:
1. What is the airspeed/altitude/pitch combination that you should not cross- after which you will not have control authority over the horizontal stabilizer??
2. If you TOGA on short-final and your PF holds in a bunch of nose-down trim as you accelerate out of 130Kts, at what point does the airplane become unrecoverable after you come over the top and start back down due to the severe out of trim situation?? It seems that 9 seconds of trim may well be enough to start this ball rolling downhill, so at what point does your airplane become unrecoverable due to a slow/non-moving horizontal stab trim due to aerodynamic overstresses??
I have never heard of an airplane being certified wherein a single control surface (the most important one really) can completely cease operation and lock-up within the normal flight envelope. And if such a situation was allowed to be certified youd think there would be placards, warnings, airspeed bugs etc. to prevent one from unintentionally straying into uncharted territory.
Its actually enough to make one look again at Rostov-on-Don and other incidents from the past, where airplanes went from happily flying to a nose-down dive and crash within 45 seconds or so after a TOGA. For some reason the 737 seems oddly prone to this type of unfortunate end.
MCAS is worth parsing to the nth degree of course, but this issue, the one that involves the total inoperability of the primary pitch control for the airplane, this is the one that has had me reading and researching everything I can find over the past month.
Regards,
dce
1. What is the airspeed/altitude/pitch combination that you should not cross- after which you will not have control authority over the horizontal stabilizer??
From that, someone (not me) ought to be able to calculate the net pounds or kilos of force needed to move the stabilizer against the force of a given ram-airspeed. As modified by the pitch angle of the stab (trigonometry: sines, cosines, tangents etc) to the relative wind.
IF that value can reach as high as 1 lb/inˆ2 or 704.5 kg/mˆ2, obviously we're looking at fighting up to 46500-51000 lbs or 21136-23181 kg of force. I doubt the reality is anywhere near that high, since the THS trim range can generally only be a few degrees out of alignment with the slipstream/airspeed - that's where the trigonometry fits in. Along with the other factors mentioned - altitude/air density, speed, pitch of the overall aircraft.
With which we can compare the available trimming force available (motor power or pilot muscle).
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Not a pilot but I was talking to senior NG driver recently and asked this question. Has he ever had a stab trim that could not be moved manually due to air load?
The answer was no.
The answer was no.
Dave, #3
The diagram of trim range is shown in the link below: trim authority and mechanism, AP, electric, and overall manual. Note flap criteria. https://www.satcom.guru/2019/04/what...-on-et302.html
And the combined effects of elevator with trim offset; https://www.satcom.guru/2019/04/stab...and-range.html
EASA document: https://www.easa.europa.eu/sites/def...20ISS%2010.pdf page 15.
This specifically applies to the Max; of note The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only.
Elect trim is limited for safety reasons; manual trim is available to meet certification requirements. The statement refers to dive conditions, but as in #2 did EASA mean normal operations or to include trim runaway.
And
It specifically noted that at speeds greater than 230 knots (425 km/h) with flaps retracted, pilots might have to use the wheel in the cockpits centre console rather than an electric thumb switch on the control yoke.
EASA and the US Federal Aviation Administration (FAA) ultimately determined that set-up was safe enough for the plane to be certified, with the European agency citing training plans and the relative rarity of conditions requiring the trim wheel.
An American Airlines, the 1400 pages long flight manual for 737 MAX pilots dated October 2017 said the thumb switches had less ability to move the nose than the manual wheel. and did not specify the flight conditions in which the wheel might be needed.
FAA and EASA knew that 737MAX trim control could be confusing?two years before the lion air MAX crash ? News In Flight
The diagram of trim range is shown in the link below: trim authority and mechanism, AP, electric, and overall manual. Note flap criteria. https://www.satcom.guru/2019/04/what...-on-et302.html
And the combined effects of elevator with trim offset; https://www.satcom.guru/2019/04/stab...and-range.html
EASA document: https://www.easa.europa.eu/sites/def...20ISS%2010.pdf page 15.
This specifically applies to the Max; of note The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only.
Elect trim is limited for safety reasons; manual trim is available to meet certification requirements. The statement refers to dive conditions, but as in #2 did EASA mean normal operations or to include trim runaway.
And
It specifically noted that at speeds greater than 230 knots (425 km/h) with flaps retracted, pilots might have to use the wheel in the cockpits centre console rather than an electric thumb switch on the control yoke.
EASA and the US Federal Aviation Administration (FAA) ultimately determined that set-up was safe enough for the plane to be certified, with the European agency citing training plans and the relative rarity of conditions requiring the trim wheel.
An American Airlines, the 1400 pages long flight manual for 737 MAX pilots dated October 2017 said the thumb switches had less ability to move the nose than the manual wheel. and did not specify the flight conditions in which the wheel might be needed.
FAA and EASA knew that 737MAX trim control could be confusing?two years before the lion air MAX crash ? News In Flight
As far as I recall, the references were to Boeing inhibiting electric trim movement of the THS in the direction that would put the aircraft further out-of-trim, at some points in the flight envelope. If it's a "fact" that electric trim can't move the stab back towards a trimmed state, either, then can we have some non-anecdotal evidence of that, please ?
Because of the relevance of trim history in this thread, the opening post in the tech log link is restated: - that thread will be closed if able.
FAA officials have told airline safety officials and pilots unions in April 2019 that they are reviewing the Runaway Stabilizer procedure in the wake of the 737 MAX MCAS accidents.
737 Runaway Stabilizer Procedure (Chris Brady)
Note changes; aircraft evolution, system complexity, aerodynamic effectiveness, switching - order / evaluation time, symptom definition. Human performance, training.
1967-1987
1987-2000
2000-2013
2013- current
and next
Dennis Tajer of the Allied Pilots Association, which represents 15,000 pilots of American Airlines, has said of the FAA review: "I want them to be invasive. I want them to be aggressive. I want them to be obstinate about knowing all of the information, I want them to be more than trust but verify." He went on to say that even if the basic steps do not change, the spartan checklist language could be augmented with instructions such as those contained in Boeing supplemental materials: "It may take two pilots to manually trim the aircraft." "It may require elevator load alleviation in order to manually trim the aircraft." I have to say that I agree that more information in the QRH would be helpful.
related - Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim
FAA officials have told airline safety officials and pilots unions in April 2019 that they are reviewing the Runaway Stabilizer procedure in the wake of the 737 MAX MCAS accidents.
737 Runaway Stabilizer Procedure (Chris Brady)
Note changes; aircraft evolution, system complexity, aerodynamic effectiveness, switching - order / evaluation time, symptom definition. Human performance, training.
1967-1987
1987-2000
2000-2013
2013- current
and next
Dennis Tajer of the Allied Pilots Association, which represents 15,000 pilots of American Airlines, has said of the FAA review: "I want them to be invasive. I want them to be aggressive. I want them to be obstinate about knowing all of the information, I want them to be more than trust but verify." He went on to say that even if the basic steps do not change, the spartan checklist language could be augmented with instructions such as those contained in Boeing supplemental materials: "It may take two pilots to manually trim the aircraft." "It may require elevator load alleviation in order to manually trim the aircraft." I have to say that I agree that more information in the QRH would be helpful.
related - Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim
As far as I recall, the references were to Boeing inhibiting electric trim movement of the THS in the direction that would put the aircraft further out-of-trim, at some points in the flight envelope. If it's a "fact" that electric trim can't move the stab back towards a trimmed state, either, then can we have some non-anecdotal evidence of that, please ?
*= pilot selected electric trim. MCAS apparently has trim authority all the way to zero units.
Ironically, when I did my first 737-400 transition course 30 years ago I was told the electric trim was limited to 2.9 units nose down to prevent a runaway from causing an unrecoverable loss of control. That was at Boeing, btw.
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At least two of our Level D B737-800 FSTDs exhibit the characteristic being discussed with regard to electric trim being ‘overwhelmed’ by aerodynamic loads… the scenario being AND ‘loaded’ trim situations, significant aft elevator deflection/input to arrest ROD/pitch down and higher KIAS (albeit within the overall AFM envelope, however, over flap limit speed in the given scenario). ANU electric trim stops in this scenario, unless the elevator input is neutralised. Centralised control wheel/column allows electric trim to re-start, progressively, similar to an aerodynamic unloading of the forces acting on the motor/screw jack. It is not like a ‘switch’.
This effect was discovered during mandated UPRT, a ‘Rostov’ type of scenario (from the interim report data).
I would add during these simulated scenarios (which do involve mis-handling to get there), without ‘unloading/neutralising’ the elevator and allowing AUN stab trim movement as KIAS increases the pitch attitude further lowers despite full aft elevator, the outcome being wholly predictable.
The FFS concerned are SPF1 and Non SFP (W) variants, both built by the same very creditable and long term FSTD manufacturer.
Questions have been asked as to validity of the simulation, however there does appear clear historical evidence (FCTM advice etc) that previous Boeing models had similar effects. Some have suggested that what’s being observed is some form of ‘historical’ simulator data and may well not be valid.
This effect was discovered during mandated UPRT, a ‘Rostov’ type of scenario (from the interim report data).
I would add during these simulated scenarios (which do involve mis-handling to get there), without ‘unloading/neutralising’ the elevator and allowing AUN stab trim movement as KIAS increases the pitch attitude further lowers despite full aft elevator, the outcome being wholly predictable.
The FFS concerned are SPF1 and Non SFP (W) variants, both built by the same very creditable and long term FSTD manufacturer.
Questions have been asked as to validity of the simulation, however there does appear clear historical evidence (FCTM advice etc) that previous Boeing models had similar effects. Some have suggested that what’s being observed is some form of ‘historical’ simulator data and may well not be valid.
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Riddle me this
So if MCAS has put your 737 out trim limits which the buttons on your steering wheel are allowed to do.
Will up still work when MCAS just put you nose full down and will up still even work , because if i read the schematics right it means MCAS can go beyond the limit.
Will up still work when MCAS just put you nose full down and will up still even work , because if i read the schematics right it means MCAS can go beyond the limit.
