MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
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Watching it made me wonder whether there may be a lubrication solution to the problem.
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Clamp a hex nut tightly in a vice.
Thread a six inch long bolt through the nut and keep turning. Now apply a side load to the bolt and repeat the exercise. The more side pressure you apply the more torque is needed to turn the bolt. This is because the side load is relieving pressure on one side of the threads at the top and opposite bottom side but is increasing it on the other two sides. If you keep applying the side load eventually you won’t be able to turn the bolt at all. The bolt is the stabiliser screw jack. The side load is the aerodynamic forces transferred from surface of the stabiliser to the jack screw. The point where it no longer turns is the point where the clutch disengages the electric trim motor or the trim wheel no longer turns without using the elevator to momentarily unload the aerodynamic pressure on the screw jack.
As I said in an earlier post, a redundant electric servo to assist/replace the manual trim wheel will not only need to be able to apply more torque than the existing electric trim motor but the clutch and screw jack themselves may need to be beefed up to cope with the increased stresses on those parts. Adding an electric motor and motor control to each of the 7,500 NGs is not something that I believe Boeing will relish. Redesigning and retrofitting an HS screw jack is something that they will relish even less.
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Basic AERO LOAD issue. Since NG uses ( except for mcas ) the same hardware and trim system and has fore a few decades, the solution has zip to do with lube.
Meanwhile, here is the latest Boeing contribution to the discussion...
... complete with the standard piano track that is overused in movies to indicate that someone has The Real Feels.
... complete with the standard piano track that is overused in movies to indicate that someone has The Real Feels.
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At the point where the clutch connecting the screw jack to the electric trim motor starts to slip or the manual trim wheels can no longer be turned, then the friction on the screw jack threads exceeds the torque applied.
I'm struggling to see how the ballnut/jackscrew interface can be subject to any significant load other than an axial one.
As for lubrication, even an (imaginary) entirely friction-free mechanism would still be subject to pretty well the same aerodynamic loads.
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If you keep applying the side load eventually you won’t be able to turn the bolt at all. The bolt is the stabilizer screw jack. The side load is the aerodynamic forces transferred from surface of the stabilizer to the jack screw.
see--- satcom.guru/2019/08/connecting-dots-from-command-to-action
Wednesday, August 21, 2019
Connecting the Dots: From Command to Action - scroll down to excellent illustrations of system.
As to helping the gearing/use of the trim wheel a very basic principle applies. To increase torgue one must gear down the system which then requires MORE revolutions to get the same 'displacement ' For example if 20 revolutions move the system one degree with x torgue applied, then 40 revolutions at 1/2 the torgue would also move the system one degree.
Thus we must consider some sort of ' high speed ' motor to assist- appropriately geared.
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And it is not at all misleading. My example was of a screw system where a predominantly axial load gradually shifts to a lateral load as the force applied deviates from the axial. This is exactly what happens in the real jack screw mechanism regardless of how it is mounted.
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If you keep applying the side load eventually you won’t be able to turn the bolt at all. The bolt is the stabilizer screw jack. The side load is the aerodynamic forces transferred from surface of the stabilizer to the jack screw
see--- satcom.guru/2019/08/connecting-dots-from-command-to-action
Wednesday, August 21, 2019
Connecting the Dots: From Command to Action - scroll down to excellent illustrations of system.
And carefully note the use of the word gimbal and where used.
Although there is no doubt some side loads, IMHO and that of others- that design shows the side loads are minimal and most likley a small percent of the axial loads which when applied to a ball screw " nut " results in little friction.
Thank you
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[QUOTE=Grebe;10582708][QUOTE]
If you keep applying the side load eventually you won’t be able to turn the bolt at all. The bolt is the stabilizer screw jack. The side load is the aerodynamic forces transferred from surface of the stabilizer to the jack screw
Please read the following
see--- satcom.guru/2019/08/connecting-dots-from-command-to-action
Wednesday, August 21, 2019
Connecting the Dots: From Command to Action - scroll down to excellent illustrations of system.
And carefully note the use of the word gimbal and where used.
Although there is no doubt some side loads, IMHO and that of others- that design shows the side loads are minimal and most likley a small percent of the axial loads which when applied to a ball screw " nut " results in little friction.
Thank you
regarding this thread about stab loads becoming excessive, you can put away your theories to save energy. The recommended method is not to get seriously out of trim in the first place by carrying out the correct procedures. Taking ET, the excessive speed ‘was due to permitting the airspeed to build following an IAS fault. The priority with that is to fly the plane in steady state pitch and power conditions. That would have kept the speed around 220 kts where stab loads are very light. The ensuing double fault of MCAS would have been easily accomplished using STAB OFF switches and manual trim. Even if it took a while to recognize the MCAS runaway, the loads would be light and the elec trim would have trimmed back to neutral prior to switches OFF. Very high airspeed with massive out of trim has, since the 707 been a death trap to be avoided. But, have the current generation of pilots been taught this or has it been forgotten. Time to get back to basics.
Cheers
If I sound a bit terse, it’s because I see so many times people trying to explain complex engineering issues when the problem is fully understood and procedures already in place.
If you keep applying the side load eventually you won’t be able to turn the bolt at all. The bolt is the stabilizer screw jack. The side load is the aerodynamic forces transferred from surface of the stabilizer to the jack screw
Please read the following
see--- satcom.guru/2019/08/connecting-dots-from-command-to-action
Wednesday, August 21, 2019
Connecting the Dots: From Command to Action - scroll down to excellent illustrations of system.
And carefully note the use of the word gimbal and where used.
Although there is no doubt some side loads, IMHO and that of others- that design shows the side loads are minimal and most likley a small percent of the axial loads which when applied to a ball screw " nut " results in little friction.
Thank you
Cheers
If I sound a bit terse, it’s because I see so many times people trying to explain complex engineering issues when the problem is fully understood and procedures already in place.
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So what procedure is in place to aid a pilot manually trimming a seriously out of trim stabiliser?
EASA among others seem to think that it is still a problem. Saying that a pilot shouldn’t allow the aircraft to get out of trim in the first place is hardly helpful to a pilot who finds that the manual trim wheel can barely be turned at high speed and increasingly lower altitude.
EASA among others seem to think that it is still a problem. Saying that a pilot shouldn’t allow the aircraft to get out of trim in the first place is hardly helpful to a pilot who finds that the manual trim wheel can barely be turned at high speed and increasingly lower altitude.
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lets not forget ET was going out of a hot and high airport at MTOW. Conditions not experienced much in the USA, so any talk of cut thrust, nose down and manage it are very US-centric. The "high" airports in the US don't tend to be hot!
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So what procedure is in place to aid a pilot manually trimming a seriously out of trim stabiliser?
EASA among others seem to think that it is still a problem. Saying that a pilot shouldn’t allow the aircraft to get out of trim in the first place is hardly helpful to a pilot who finds that the manual trim wheel can barely be turned at high speed and increasingly lower altitude.
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So what procedure is in place to aid a pilot manually trimming a seriously out of trim stabiliser?
EASA among others seem to think that it is still a problem. Saying that a pilot shouldn’t allow the aircraft to get out of trim in the first place is hardly helpful to a pilot who finds that the manual trim wheel can barely be turned at high speed and increasingly lower altitude.
I've been on Boeing's most of my 30+ years in aviation, and I am very familiar with the trim system. Quite a few years back, I was actually taught the "roller coaster" technique. Somewhere along the way, the stab system in general, and runaway trim in particular, became less and less of a feature in our training syllabus. Despite the fact that runaway trim is a "memory" procedure, a pilot might see it once on initial checkout but it was not a regular feature of our normal recurrent training cycle (Stop and think about that for a moment.) In the early days of the MAX accident investigations, I was absolutely flabbergasted by the number of 737 pilots at my own airline who were unaware of many aspects of this trim system. To the extent that this was also the case at Lion Air and Ethiopian, then the pilots' reactions are more understandable.
Last edited by Tomaski; 30th Sep 2019 at 13:44.
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Only an inch I know, but this reduces the turning moment by over 10%. This seems to have reduced the gearing to an extent where something which was ‘difficult but not impossible’ has become ‘impossible’ in some cases.
Earlier in the thread, a poster described using the ‘rollercoaster method’ in a former life and said that he lost 5,500 feet of altitude from start to finish. As a method to reduce aerodynamic load on the HS this is therefore only satisfactory above altitudes of 6000 feet plus.
And it is not just EASA expressing concern, the FAA is also reassessing the amount of force needed to move the trim wheel in extreme circumstances.
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Off topic: The Mentour video is an attempt to emulate a runaway trim, since the B737 NG simulator is not programmed to do that! The crew inputs do not match a real world situation, but the idea is to show the out of balance forces on the stabiliser trim wheels. That is all...
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Which has been the same size since the 737NG was first produced 23 years ago.
I'm not saying it's not a good idea to revisit this design, but it is not a new design. The fact that it is difficult to use the manual trim wheel in extreme out of trim situations has always been there, but somewhere along the way this knowledge fell out of general circulation and with it an appreciation for the seriousness of runaway trim and the need for flight crews to be regularly exposed to this malfunction and the proper steps to counter it.
To use an analogy, there was a time when the hazards of low-level windshear and microburst phenomenon were not well-appreciated. Now we practice windshear recoveries all the time because prompt recognition and response can be critical to a successful outcome. Why do we not do the same with Runaway Stab Trim? It is a "memory item" after all.
I'm not saying it's not a good idea to revisit this design, but it is not a new design. The fact that it is difficult to use the manual trim wheel in extreme out of trim situations has always been there, but somewhere along the way this knowledge fell out of general circulation and with it an appreciation for the seriousness of runaway trim and the need for flight crews to be regularly exposed to this malfunction and the proper steps to counter it.
To use an analogy, there was a time when the hazards of low-level windshear and microburst phenomenon were not well-appreciated. Now we practice windshear recoveries all the time because prompt recognition and response can be critical to a successful outcome. Why do we not do the same with Runaway Stab Trim? It is a "memory item" after all.
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Which has been the same size since the 737NG was first produced 23 years ago.
I'm not saying it's not a good idea to revisit this design, but it is not a new design. The fact that it is difficult to use the manual trim wheel in extreme out of trim situations has always been there, but somewhere along the way this knowledge fell out of general circulation and with it an appreciation for the seriousness of runaway trim and the need for flight crews to be regularly exposed to this malfunction and the proper steps to counter it.
I'm not saying it's not a good idea to revisit this design, but it is not a new design. The fact that it is difficult to use the manual trim wheel in extreme out of trim situations has always been there, but somewhere along the way this knowledge fell out of general circulation and with it an appreciation for the seriousness of runaway trim and the need for flight crews to be regularly exposed to this malfunction and the proper steps to counter it.
) due to some additional stuff on center console, they reduced the dia of trim wheel. And that in a mockup they used a well conditioned female to try and turn the wheel against some load. specifics of load were not given- but that she was unable. Someone else here may remember the post at least 6 months ago in the ' first' ultra long thread now found in tech log and locked, but still available ???
Last edited by Grebe; 30th Sep 2019 at 14:37. Reason: Corrected wheel change from MAX to NG mea culpa
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Yes, the diameter of the manual trim wheels changed and, yes, it was a long time ago. It's possible that there is some side loading of the jackscrew, but probably not enough to be a major factor, considering the angle it forms with the stab lever arm and the fact that the ball nut is in a gimbal ring. Yes, pilots are no longer taught the porpoise maneuver, but it has no chance of saving the day unless there's sufficient altitude AGL.
Almost certainly, the real problem with manual trimming in the sorts of situations we are considering is just that the H-stab is huge and, at some combinations of attitude, (out-of-) trim positions and airspeed, the aero loads make it either unlikely or impossible to crank those wheels enough to recover.
Almost certainly, the real problem with manual trimming in the sorts of situations we are considering is just that the H-stab is huge and, at some combinations of attitude, (out-of-) trim positions and airspeed, the aero loads make it either unlikely or impossible to crank those wheels enough to recover.