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Originally Posted by yoko1
(Post 10507066)
3. Trimming forces. Speaking as someone who has actually used the manual trim wheel on the Boeing, I can assure you that the trim resistance increases in both directions. It is not like rolling a boulder uphill where it is very difficult in one direction and easy in the other. The increased resistance is due to binding forces at the jackscrew thread/nut interface. Not sure if it is strictly frictional force or some type of deformation or misalignment. Keep in mind that while the aerodynamic forces are acting roughly parallel to the jackscrew body, those forces are mostly perpendicular at the thread/nut interface. There will be some directional bias, but that is a relatively small component. BTW, this is one of the reasons why a jackscrew mechanism is used in the first place - to minimize those directional forces and make it easier for the motor. However, one point needs to be emphasized again: You completely avoid these issues by not letting the stab get out of trim in the first place. .... And yes, by not being attentive to such matters, it is absolutely possible to place the aircraft in a state where things don't work as they should. Avoiding that nasty little killer should be avoided at all costs. What silly pilots, for "not being attentive to such matters". Shame on you Yoyo, shame on you, Boeing. |
Originally Posted by DaveReidUK
(Post 10506118)
ICAO Annex 6 lists mandatory FDR parameters, and is well worth a read. One of those parameters is "Cockpit pitch trim control input position" (my emphasis, not ICAO's).
That's why FDAUs came along in the 1970s. |
Originally Posted by FrequentSLF
(Post 10507501)
however the information published does not validate a theory of the trace representing the thumb switch at the source
But the fact remains that if it turns out that the FDR isn't showing the thumb switch position, then it contravenes the Annex 6 requirements. Boeing have been building aeroplanes for long enough that one would hope they know what an FDR is supposed to record. |
Servo motors... The reason that I askd if the stab motor was a servo is that servos behave differently to the normal motors we are all used to. They were developed for cnc machines...they accurately will move about 1/4000th of a revolution per step signal, and then lock their rotor. So they need a command to move x steps in Y direction at whatever speed. That can be a simple analogue instruction; run clockwise at speed number 3 until the limit stops are reached. But it would make more sense for the FCC to signal a step + direction command. However many steps to get 2.5 units of nose down trim. The critical feature is again the locked rotor when not turning. If the clutch doesn’t work, or doesn’t work well, a human will not be able to move the rotor from its last commanded position. I have had four big servo motors in my garage machine shop so am quite familiar with their characteristics. I am also a 737 pilot, and have had a failure of the stab trim brakes in flight...a soft runaway requiring cranking the wheel. It wasn’t a big deal, but then again we were smack in the middle of the envelope, and the -400 had a both a shorter moment arm and a 10% bigger trim wheel to crank. Regarding motor overheating: I really doubt it. I have run servos in 45° heat with a big load running constantly for a couple of hours relying only on their passive air cooling. Never had a thermal shutdown. |
Originally Posted by MemberBerry
(Post 10507382)
https://aviaforum.ru/threads/katastr...2/post-2392734 (contains a more detailed schematic for the NG version). |
Originally Posted by Australopithecus
(Post 10507574)
Servo motors... The reason that I askd if the stab motor was a servo is that servos behave differently to the normal motors we are all used to. They were developed for cnc machines...they accurately will move about 1/4000th of a revolution per step signal, and then lock their rotor. So they need a command to move x steps in Y direction at whatever speed. That can be a simple analogue instruction; run clockwise at speed number 3 until the limit stops are reached. But it would make more sense for the FCC to signal a step + direction command. However many steps to get 2.5 units of nose down trim. The critical feature is again the locked rotor when not turning. If the clutch doesn’t work, or doesn’t work well, a human will not be able to move the rotor from its last commanded position. I have had four big servo motors in my garage machine shop so am quite familiar with their characteristics. I am also a 737 pilot, and have had a failure of the stab trim brakes in flight...a soft runaway requiring cranking the wheel. It wasn’t a big deal, but then again we were smack in the middle of the envelope, and the -400 had a both a shorter moment arm and a 10% bigger trim wheel to crank. Regarding motor overheating: I really doubt it. I have run servos in 45° heat with a big load running constantly for a couple of hours relying only on their passive air cooling. Never had a thermal shutdown. Dont forget though, that the workshop motors are designed for continuous running with appropriate cooling. Stab motors are designed for intermittent use with cooling appropriate to that task. As Yoko says, earlier 737 types (which had a different motor(s)) could suffer from stab motor overheat, so it is not unknown. Air speed is also possibly relevant depending on cooling air source - and OAT. |
Originally Posted by Australopithecus
(Post 10507574)
Servo motors... The reason that I askd if the stab motor was a servo is that servos behave differently to the normal motors we are all used to. They were developed for cnc machines...they accurately will move about 1/4000th of a revolution per step signal, and then lock their rotor. So they need a command to move x steps in Y direction at whatever speed. That can be a simple analogue instruction; run clockwise at speed number 3 until the limit stops are reached. But it would make more sense for the FCC to signal a step + direction command. However many steps to get 2.5 units of nose down trim. The critical feature is again the locked rotor when not turning. If the clutch doesn’t work, or doesn’t work well, a human will not be able to move the rotor from its last commanded position. I have had four big servo motors in my garage machine shop so am quite familiar with their characteristics. I am also a 737 pilot, and have had a failure of the stab trim brakes in flight...a soft runaway requiring cranking the wheel. It wasn’t a big deal, but then again we were smack in the middle of the envelope, and the -400 had a both a shorter moment arm and a 10% bigger trim wheel to crank. Regarding motor overheating: I really doubt it. I have run servos in 45° heat with a big load running constantly for a couple of hours relying only on their passive air cooling. Never had a thermal shutdown. Eaton’s new Model 6355C Stabilizer Trim Motor features: • Brushless three phase motor design • Low loss power bridge with IGBT switches • Processor based motor commutation and velocity control • Dual current limit (torque) control circuits • Power up built-in test • Continuous fault monitoring • Fault storage (non-volatile memory) • RS-232 test/maintenance interface • Investment cast housing • Two stage spur gear train • Modular, bottom up assembly — two electronic sub-assemblies, motor, housing with gear train |
Originally Posted by BDAttitude
(Post 10507592)
From Eaton:
Eaton’s new Model 6355C Stabilizer Trim Motor features: • Brushless three phase motor design • Low loss power bridge with IGBT switches • Processor based motor commutation and velocity control • Dual current limit (torque) control circuits • Power up built-in test • Continuous fault monitoring • Fault storage (non-volatile memory) • RS-232 test/maintenance interface • Investment cast housing • Two stage spur gear train • Modular, bottom up assembly — two electronic sub-assemblies, motor, housing with gear train overexertion during high loads. It may of course slow the motor down, or even stop it completly... |
A brushless three phase motor could be a "normal" induction motor. Or it could be a permanent magnet motor. And it can have some position feedback added to it. And even though it's not specifically mentioned it can very well have termistors or temperature sensors embedded in the windings for overheat protection.
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Originally Posted by DaveReidUK
(Post 10507494)
Can you point to the entry on the CVR extract (i.e. the timestamp) where you believe that happened ?
http://www.ecaa.gov.et/documents/204...+,(ET-AVJ).pdf
Originally Posted by Aircraft Accident Investigation Preliminary Report - ET-AVJ
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer confirmed stab trim cut-out.
At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance of AND automatic trim command occurred without any corresponding motion of the stabilizer, which is consistent with the stabilizer trim cutout switches were in the ‘’cutout’’ position. ... At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working. Luc |
Originally Posted by SteinarN
(Post 10507623)
A brushless three phase motor could be a "normal" induction motor. Or it could be a permanent magnet motor. And it can have some position feedback added to it. And even though it's not specifically mentioned it can very well have termistors or temperature sensors embedded in the windings for overheat protection.
• Low loss power bridge with IGBT switches • Processor based motor commutation and velocity control |
Originally Posted by BDAttitude
(Post 10507633)
It says:
And I guess that says it all. Not having thermal sensing on IGBT bridges would be very unusual. |
Dear All
Been a busy month June 2019. Will never forget it, hell! But enough about my problems. As some of You have observed I am a simple chap. And here is a simple observation: MCAS is the most retarded system I have ever encountered! It now is clear to me that Boeing has lost the plot , curtsy of FAA. I am afraid the treason by Boeing can not be forgiven, as it is systemic! The MCAS should be scraped and a aerodynamic improvement applied. Or its Coke Cane Time! Flight safety on a basic aerodynamic level is not a political thing. Looking forward to fly the MAX 2.0. Regards Cpt B |
Originally Posted by FrequentSLF
(Post 10507501)
The FDR trace show only one cockpit pitch trim control input position, it does now show BOTH thumn switches, that might lead to assume is an elaborated signal that is taken at the servo motor controller. Of course the traces might have been elebaorated before been published, however the information published does not validate a theory of the trace representing the thumb switch at the source
https://i.imgur.com/NOQGETw.png |
Originally Posted by yoko1
(Post 10507066)
3. Trimming forces. Speaking as someone who has actually used the manual trim wheel on the Boeing, I can assure you that the trim resistance increases in both directions. It is not like rolling a boulder uphill where it is very difficult in one direction and easy in the other. The increased resistance is due to binding forces at the jackscrew thread/nut interface. Not sure if it is strictly frictional force or some type of deformation or misalignment. Keep in mind that while the aerodynamic forces are acting roughly parallel to the jackscrew body, those forces are mostly perpendicular at the thread/nut interface. There will be some directional bias, but that is a relatively small component. BTW, this is one of the reasons why a jackscrew mechanism is used in the first place - to minimize those directional forces and make it easier for the motor. However, one point needs to be emphasized again: You completely avoid these issues by not letting the stab get out of trim in the first place. There is no, none, not one iota of evidence that the Main Electric Trim did not move the stab when it was used. And yes, by not being attentive to such matters, it is absolutely possible to place the aircraft in a state where things don't work as they should. 4. There is a lot of reference of what are perceived as anomalies in the trim system in the final moments of both accidents. Personally, that's not what I see, however I'm not going to claim I have a better microscope than anyone else here. That being said, I must again point out that in those final moments, the aircraft had exceeded the certified flight envelope. Engineering safety factors aside, there is not any expressed or implied guarantee that any system will work as it is supposed to once you exceed the limits of that system. The lesson here is that you keep that aircraft within its design envelope - just like the crew of the penultimate Lion Air 610 flight did. Frankly the lesson here is that Boeing should never, ever, ever again ship an aircraft with functionality that can programmatically take the aircraft out of its design envelope due to an easily foreseeable fault. Boeing made that mistake all by itself and should cease trying to muddy the waters by pointing the finger at overwhelmed crew trying to deal with multiple alarms caused by that fault. |
Originally Posted by Luc Lion
(Post 10507627)
Here it is:
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer confirmed stab trim cut-out.
At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance of AND automatic trim command occurred without any corresponding motion of the stabilizer, which is consistent with the stabilizer trim cutout switches were in the ‘’cutout’’ position. ... At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working. Luc It could equally have been him just confirming that (as you rightly say) just over a minute previously both pilots had agreed to disable the stab trim (05:40:35), given that it was followed immediately by asking "if he could try it manually". In other words, the electric trim switches obviously weren't going to work, and there wouldn't have been any point in trying them. |
Originally Posted by DaveReidUK
(Post 10507767)
You are assuming that "The First-Officer has replied that the trim was not working" means that he attempted unsuccessfully to use the electric trim switches.
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Originally Posted by MemberBerry
(Post 10507722)
Well, the trim schematic shows the thumb switch outputs for "up" and "down" connected together, and there are direct connections that go to the DFDAU (Digital Flight Data Acquisition Unit). This strongly suggests that the FDR traces are based on the actual position of the switches. I took a section of the schematic and colored in green and turquoise the paths of the signal from thumb switches to the DFDAU:
https://i.imgur.com/NOQGETw.png Thanks Alchad |
Originally Posted by Speed of Sound
(Post 10507809)
Given that the reply came about 8 seconds after asking if he could try, it is highly likely (though not 100% certain) the the FO was attempting to operate the trim switch during those 8 seconds.
According to the transcript "The First-Officer has replied that the trim was not working and asked if he could try it manually", followed (8 seconds later) by "the First-Officer replied that it is not working". That would suggest that what he was doing during those 8 seconds (having already acknowledged that electric trim was inop) was to try to wind the trim wheel (unsuccessfully, for the reasons already discussed in the thread), followed by his statement that it wouldn't move. |
Lot’s of very interesting and detailed discussion here. I’ve learned quite a bit about electric motors and their control circuits, so thanks. I should point out, however, that all this is far, far above the level of detail that would ever be included in the FCOM. Now back to the elephant in the room. All of us are basically amateurs when it comes to accident investigations. Many of us like to know how things work, or should work, and enjoy puzzling things out from the limited information that is available. Perhaps some of us just like a good mystery. Perhaps some don’t have the patience to wait for the investigative process to run its course or are uncomfortable with what is perceived as an information void. I absolutely get the psychology at work. While I consider myself fairly knowledgeable about the 737 and it’s operations, I will acknowledge that the information and resources I (or anyone else here) possess is absolutely dwarfed by those available to the collective body of professional accident investigators, multiple certificate authorities representing dozens of nations, subject matter experts, and other interested parties such as the aerospace-centric media (i.e. Aviation Week and the like) and the massive army of tort lawyers lining up to sue Boeing. Is anyone here seriously suggesting that it has not occurred to any of these parties that they should investigate the Main Electric Trim system to see if there were any issues? Please speak up, because I would really like to hear your thinking here. In the past couple of months, we have heard a litany of items that various certificate authorities want to see addressed before the MAX is cleared to fly. Conspicuously absent is any mention of the Main Electric Trim system. No suggestion of failures, stalls, overheats, or anything else. No one is asking for any redesign or replacement of switches, relays, wires, or motors. If this was still early in the investigation, sure, absolutely valid questions have been raised. And all those questions have been found wanting of actual hard, verifiable evidence to back them up. So really, folks, why the continued obsession with this system? No party who actually has all the data, resources, and experts is saying anything about it. Do you really think you are more knowledgeable than the collective body of all these very smart people? There is far too much grasping for straws that simply aren’t there. Time to move on. |
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