flybucky
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:49 am

rheinwaldner wrote:
Check the FDR plot.
Hint: there is no electric trim input visible so we absolutely know that he did not continue using the yoke switch.

That's what I initially thought too. But after reading some additional analysis, I'm leaning towards that when Stab Trim is Cutout, Electric Trim is not recorded in the FDR.

Now, there is one unexplained data where an Auto Trim (presumably MCAS) pitch down command did not result in actual Pitch Trim change. But I think that may be some special corner case.
 
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speedbored
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 7:14 am

flybucky wrote:
I'm leaning towards that when Stab Trim is Cutout, Electric Trim is not recorded in the FDR.

Looking at the schematic, it's clear that power to the pilot's trim switches is disconnected when either of the cutout switches are set to cutout. Without power, I fail to see how either of the trim switches could possibly send a signal to be recorded in the FDR.
 
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speedbored
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 7:24 am

mxaxai wrote:
I wonder, though, why MCAS gave one automatic AND input after the trim cut out switches had been flipped, then remained dormant, and only reactivated after (presumably) electric trim had been reactived. Could it be that inputs to the electric trim - either manually on the yoke or from MCAS - were not recorded while electric trim was off (after some initial transition time)?

I wonder whether the explanation might just be that the STS/MCAS signals to the FDR are derived from somewhere after the cutout switches, and that the pilot paused momentarily between operating each switch. The STS/MCAS signal from the FCC is only cut off by one of the switches, not both switches like the stab trim control power, and thumb switch power are.
 
XRAYretired
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:00 am

speedbored wrote:
mxaxai wrote:
I wonder, though, why MCAS gave one automatic AND input after the trim cut out switches had been flipped, then remained dormant, and only reactivated after (presumably) electric trim had been reactived. Could it be that inputs to the electric trim - either manually on the yoke or from MCAS - were not recorded while electric trim was off (after some initial transition time)?

I wonder whether the explanation might just be that the STS/MCAS signals to the FDR are derived from somewhere after the cutout switches, and that the pilot paused momentarily between operating each switch. The STS/MCAS signal from the FCC is only cut off by one of the switches, not both switches like the stab trim control power, and thumb switch power are.

I'm of the opinion that the MCAS related FDR trace (trim) is the MCAS command status from the FCC. I.e its not power its signal. This explains what we see in the FDR. I suspect this may also apply for STS and Thumb switch commands.

Ray
 
jollo
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:11 am

mzlin wrote:
T
MCAS was passed off as a STS modification to the FAA and hidden from the manual, which appears to have been done in order to keep common type certification with the 737NG (at least that would seem to be the only motivation strong enough to piggyback MCAS on the non-redundant STS structure and hide a safety-impacting system from the manual)


I don't agree: common type rating doesn't seem to be sufficient grounds for the kind of obfuscation Boeing enacted (MCAS expunged form the FCOM, misrepresented in FAA documents, etc.). "Grandfathering" MAX's certification would still have been possible if MCAS had had a cutoff switch (or better yet, with an automatic MCAS disconnect logic on AoA disagree flag, as ITAF's KC-767A already have, and as probably 737 MAX will have after the software patch). A MAX with an inoperative MCAS due to an AoA disagree condition would still have been safe to fly as long as the pilot knew to keep out of the specific corners of the flight envelope MCAS was needed for (e.g. high AoA, high speed coordinated turns). However this would have required new memory items, hence classified as level D (note: common type rating would still have been still possible, but additional training in a level 6 sim would have been mandatory).

IMO it was the "no sim training required" commercial imperative that set off the chain of appallingly bad (bordering on criminal?) decisions that led to MCAS being released and certified the way it was.

The clue: NG has 2 STAB TRIM CUTOFF switches, one for disabling autopilot stab trim commands and one for cutting off the electric stab trim actuator. On the MAX, they were replaced by 2 redundant (PRImary and Back/Up) switches, either of which will directly disconnect the electric stab trim actuator. Why replacing 2 switches with distinct functions (A and B) with 2 redundant switches with exactly the same function (only B)? I don't think this change was introduced for common type rating: if anything, leaving the cutoff switches logic alone would have actually helped.

I believe the AUTOPILOT cutout switch had to go because, by logic, it should have disabled all automatic control augmentations as well, MACS included.

Of course this is only speculation at this point, but would gain much credibility with an additional bit of information, asked several times in this and in the grounding thread but never AFAIK answered: in the 737 NG, does switching AUTOPILOT STAB TRIM CUTOFF to CUTOFF disable just the A/P, of does it disable STS also? If it does disable STS too (as would be logical), then piggy-backing MCAS on STS would mean AUTOPILOT STAB TRIM CUTOFF would have disabled MCAS too. And as an "MCAS inop" mode would have called for additional sim training, that would be the trigger for the chain of bad decisions.

So I will again put forward the question for members with actual NG experience: in the 737 NG switching only AUTOPILOT STAB TRIM CUTOFF to CUTOFF disables STS or not?
 
therealswede
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 9:49 am

PW100 wrote:
...

The point being constantly missed is the following: it appears that the electric trimming was not allowing, or unable to trim further nose up, for reason(s) we are not able to comprehend at this point (*).
And we see this behavior repeatedly in the FDR traces: from the moment the flaps are up, every electric nose up trimming stops way before the aft column force is balanced out. Why would a pilot stop nose up trimming, if he is pulling with most/if not all of his strength on the column? It just does not make any sense whatsoever. There must be some mechanism behind that, and I refuse to accept - without solid evidence - that that mechanism is crew/training related.

...


This is the key point in this accident!
 
jollo
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 10:13 am

therealswede wrote:
PW100 wrote:
...

The point being constantly missed is the following: it appears that the electric trimming was not allowing, or unable to trim further nose up, for reason(s) we are not able to comprehend at this point (*).
And we see this behavior repeatedly in the FDR traces: from the moment the flaps are up, every electric nose up trimming stops way before the aft column force is balanced out. Why would a pilot stop nose up trimming, if he is pulling with most/if not all of his strength on the column? It just does not make any sense whatsoever. There must be some mechanism behind that, and I refuse to accept - without solid evidence - that that mechanism is crew/training related.

...


This is the key point in this accident!


Maybe (but this is only an hypothesis) the stab was so much out of trim that thumb switch ANU commands seemed to have no prompt effect, i.e. did not alleviate appreciably column forces in the time frame pilots normally expect feedback from trim inputs (at 0.2°/sec --> more than 20 seconds of continuous ANU input to re-trim a full nose-down stab). So maybe this delay reinforced their perception that elec stab trim was "not working", in a scenario where nothing (including NNC) seemed to work as expected?
 
flybucky
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 10:13 am

speedbored wrote:
I wonder whether the explanation might just be that the STS/MCAS signals to the FDR are derived from somewhere after the cutout switches, and that the pilot paused momentarily between operating each switch. The STS/MCAS signal from the FCC is only cut off by one of the switches, not both switches like the stab trim control power, and thumb switch power are.

This thought crossed my mind as well. Prior to the MAX, the Stab Trim Cutouts were labelled "MAIN ELECT" and "AUTO PILOT". Apparently, those cutout switches used to serve different purposes in the NG. If you only cutout Stab Trim Auto Pilot, you could still use the Electric Trim. So it's not a stretch to theorize that if you only cutout MAIN ELECT, you would be disabling all electrical trim (Electric Trim and Auto Trim), however, the Auto Trim commands could still be powered and recorded (even though they had no effect).

Even though the Cutouts were relabelled to "PRI" and "B/U" on the MAX, their history may be related. So if the pilots first cutout "PRI (MAIN ELECT)", that might still allow the "B/U (AUTO PILOT)" to be powered and recorded (although with no effect).
 
xmp125a
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 10:58 am

flybucky wrote:
mwmav8r01 wrote:
I must be Arnold Schwarzenegger. Its not hard to more the trim wheel. Takes a while longer than electric.

Here's a video of pilots demonstrating manual wheel trim in a parked 737-800 NG. They set the trim to 0 units (full nose down) and attempted the manual wheel to trim back to normal 5 units.

That was in a parked 737 with no aerodynamic forces on the stabilizer. The FO struggled and was only able to trim 1 unit in 1 minute, and was exhausted afterwards. Using the left hand may have been a factor. There have been several experts that have said that the aerodynamic forces on an out-of-trim stabilizer when the elevator is up (which it was, because the FDR Control Column Position was 5-10º nose up for most of the flight) at high speed would likely make it impossible for the manual wheel to be able to trim up.

Even the Captain in the demo, who had no problems turning the manual trim wheel, took 30 seconds to trim back to normal 5 units. According to the Preliminary Report graph, ET302 crashed ~20 seconds after the final MCAS command.


Ok, I am frankly appalled. Does this effectively mean that a standard part of the 737 controls, the trim wheel, may be inoperable in large parts of the flight envelope by less muscular pilots? For christ sake, we are in 2019! Should I run away if both pilot and copilot are females and the plane is 737MAX or 737NG? I mean, people would say, well, that trim wheel is rarely used, which may be true, but the airplane is certified with the wheel... for all practical purposes the wheel that cannot be turned/cranked easily by the least physically able pilot that is certified to fly 737, does not exist and it should be perhaps removed and plane recertified without it?

Or did I miss something?
 
mjoelnir
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 11:38 am

xmp125a wrote:
flybucky wrote:
mwmav8r01 wrote:
I must be Arnold Schwarzenegger. Its not hard to more the trim wheel. Takes a while longer than electric.

Here's a video of pilots demonstrating manual wheel trim in a parked 737-800 NG. They set the trim to 0 units (full nose down) and attempted the manual wheel to trim back to normal 5 units.

That was in a parked 737 with no aerodynamic forces on the stabilizer. The FO struggled and was only able to trim 1 unit in 1 minute, and was exhausted afterwards. Using the left hand may have been a factor. There have been several experts that have said that the aerodynamic forces on an out-of-trim stabilizer when the elevator is up (which it was, because the FDR Control Column Position was 5-10º nose up for most of the flight) at high speed would likely make it impossible for the manual wheel to be able to trim up.

Even the Captain in the demo, who had no problems turning the manual trim wheel, took 30 seconds to trim back to normal 5 units. According to the Preliminary Report graph, ET302 crashed ~20 seconds after the final MCAS command.


Ok, I am frankly appalled. Does this effectively mean that a standard part of the 737 controls, the trim wheel, may be inoperable in large parts of the flight envelope by less muscular pilots? For christ sake, we are in 2019! Should I run away if both pilot and copilot are females and the plane is 737MAX or 737NG? I mean, people would say, well, that trim wheel is rarely used, which may be true, but the airplane is certified with the wheel... for all practical purposes the wheel that cannot be turned/cranked easily by the least physically able pilot that is certified to fly 737, does not exist and it should be perhaps removed and plane recertified without it?

Or did I miss something?


That is why I continue to talk about reviewing the complete 737 trim system. The including of "now trim manually with the trim wheel" should be removed from being an acceptable fall back when things go down the drain with the trimming system.

Perhaps there should also be a review of the amount of back pressure the column is able to produce, as part of the forces are produced by the system, being not really an indication of the forces on the elevator.

All that stuff seems to depend on one sensor only and one could start to get philosophically about, how this stuff ever made it through certification. The answer I assume is bit by bit.
 
dragon6172
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 11:49 am

mjoelnir wrote:
Perhaps there should also be a review of the amount of back pressure the column is able to produce, as part of the forces are produced by the system, being not really an indication of the forces on the elevator.

That force comes from certification requirements though correct? 1lb for every 6 knots out of trim?
Phrogs Phorever
 
mjoelnir
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 12:43 pm

dragon6172 wrote:
mjoelnir wrote:
Perhaps there should also be a review of the amount of back pressure the column is able to produce, as part of the forces are produced by the system, being not really an indication of the forces on the elevator.

That force comes from certification requirements though correct? 1lb for every 6 knots out of trim?


How can you be knots out of trim? And do the forces also depend on an AoA sensor and again only on one?

And what use is it, if the force overwhelms the pilot?
 
luksimir
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 1:05 pm

patplan wrote:
flybucky wrote:
PW100 wrote:
Here he explains why it was removed, but confirms his findings:
https://www.youtube.com/watch?v=q17vykscK0w

Thanks for that link regarding Mentour Pilot's decision to pull his video. But I did not have time to watch the 50 minute (!) video. Does someone have a summary? I only watched the first few minutes, where he said he pulled the video purely on his own decision, not due to any pressure from his company, Boeing, etc.


I watched the original sim session that was being pulled and also was smart enough to save the video as well...

The original sim session transcript [C=CAPT; F=FO]:
C: We have an IAS disagree.
C: So, IAS disagree memory items.
F: Autopilot if engaged, disengage.
C: Disengaged!
F: Autothrottle if engaged, disengage.
C: Disengaged!
F: Flight directors - Both up
F: With flaps up established a flight path 4 degrees and 75% N1.
C: So, 75% N1.
F: We have 77, 76,...
C: A little bit less...
F: And, there you go.
C: 4 degrees.
F: 4 degrees.

C: So I am trying to establish this now.
F: Check!
F: We are descending...?
F: We probably... Are you feeling troubled with...
F :Any trouble with the flight control?
C: Yeah, I'm trying to trim it but...
C: It continues to trim against me when I'm trimming
C: So state the malfunction, please.
F: Yeah, this doesn't look right. Looks like uh...
F: How do you feel the stabilizer, the trim system?
F: Can you control it?
C: I'm trimming it. It is responding but...
F: It's a runaway stabilizer, if you agree?
C: For every time that I trim backward, it keeps trimming forward.
F: It's trimming forward. Yeah, it's runaway stabilizer.
C: So, runaway stabilizer memory items...
C: And i'm trying to keep this thing at 4 degrees.
F: Control column, hold firmly.
C: I am... [CAPT is holding the yoke firmly with both hands]
F: Autopilot - if engaged, disengage.
C: It's disengaged.
F: Autothrottle - if engaged, disengage.
C: It's..., if you can disengage it for me, make sure that it's disengaged.
F: It's disengaged.
F: And, do you feel that the failure stop?
F: Negative?
C: No, it's still moving.
F: Stab trim cutoff switches to cutoff.
F: OK. It stops. It looks like it stops.
C: You can see now I'm using almost full back pressure here.
F: Exactly.
C: How many degrees nose down?
F: We have 4 units nose down now
C: 4 units nose down?
F: Yup.
C: OK, I'm struggling.
C: I'm actually using almost my full force to keep the aircraft level here.
F: Do you want me to help you?
C: What I would like to do.
C: Just for the sake of exercise, can you trim this forward? [to simulate MCAS trim AND]
C: See if we can reach even zero nose down.
C: And see if I can even hold it.

[FO is trying to crank the trim wheel to reach zero nose down, simulating MCAS AND]

C: So, now we are doing this just as an exercise!
C: Do not try this at home.
C: This...
C: We are at 300 knots now.
F: I'm fighting.
C: I'm sttrugling to to keep this aircraft flying.
F: My god! [FO surprised at how hard it is to trim further nose down]
C: Yeah, the thing is with higher speed the force on the stabilizer will be higher and higher as well.
C: So it becomes almost impossible to move it.
C: So we are now at about 3 degrees.
F: Yup. [FO still tries to continue trimming nose down, the wheels is so difficult to spin]
C: We're still about 3 degrees away from full nose down trim.
C: And I am using everything that I have. [CAPT still holding on to his yoke with both hands]
F: My God ! [the trim wheel barely move for the down trim]
C: This is realistic guys.
C: This is how much of effort it would take to trim the stabilizer at this kind of speed.
C: Umph... [Capt is still trying to hold on to his yoke with his hands]
C: I'm just in control of it, though. But it's getting harder and harder.
C: And remember we're still 2.5 degrees away...
F: My God! [FO still struggles to spin the refused-to-be-spun trim wheel]
C: It's not possible, is it?
C: All right, we stop at that.

C: The reason that we have to try...
C: The reason we have to trim this manually is because the normal trim system wouldn't do this, OK.
C: It would require manual trim to get it away from this.
C: That's fine.
C: Trim it backward. [This time to illustrate the effort to trim the nose back up after "MCAS" brought the AC further nose down]
C: Trim it backward as you can.
F: Oh my God! I couldn't... [FO can't spin the wheel to trim up]
C: OK.
C: Eh...
C: Juan, press the red button! [CAPT called the sim operator...]
C: Press the red button now. [to stop the sim session]
C: This is at 340 knots.
C: And the trim is at...It's still at almost 2.5 degrees.
F: Yeah, 2.5 degrees.



You mentioned, that you saved the video. Is it possible for you to share it?
 
morrisond
Posts: 1394
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 1:14 pm

mjoelnir wrote:
xmp125a wrote:
flybucky wrote:
Here's a video of pilots demonstrating manual wheel trim in a parked 737-800 NG. They set the trim to 0 units (full nose down) and attempted the manual wheel to trim back to normal 5 units.

That was in a parked 737 with no aerodynamic forces on the stabilizer. The FO struggled and was only able to trim 1 unit in 1 minute, and was exhausted afterwards. Using the left hand may have been a factor. There have been several experts that have said that the aerodynamic forces on an out-of-trim stabilizer when the elevator is up (which it was, because the FDR Control Column Position was 5-10º nose up for most of the flight) at high speed would likely make it impossible for the manual wheel to be able to trim up.

Even the Captain in the demo, who had no problems turning the manual trim wheel, took 30 seconds to trim back to normal 5 units. According to the Preliminary Report graph, ET302 crashed ~20 seconds after the final MCAS command.


Ok, I am frankly appalled. Does this effectively mean that a standard part of the 737 controls, the trim wheel, may be inoperable in large parts of the flight envelope by less muscular pilots? For christ sake, we are in 2019! Should I run away if both pilot and copilot are females and the plane is 737MAX or 737NG? I mean, people would say, well, that trim wheel is rarely used, which may be true, but the airplane is certified with the wheel... for all practical purposes the wheel that cannot be turned/cranked easily by the least physically able pilot that is certified to fly 737, does not exist and it should be perhaps removed and plane recertified without it?

Or did I miss something?


That is why I continue to talk about reviewing the complete 737 trim system. The including of "now trim manually with the trim wheel" should be removed from being an acceptable fall back when things go down the drain with the trimming system.

Perhaps there should also be a review of the amount of back pressure the column is able to produce, as part of the forces are produced by the system, being not really an indication of the forces on the elevator.

All that stuff seems to depend on one sensor only and one could start to get philosophically about, how this stuff ever made it through certification. The answer I assume is bit by bit.



Not that it explains what happens at high speed but here is a report from a Captain - 7Boeing7 from the previous page



Having used the manual trim wheels on the ground hundreds of times and watched other pilots (male and female, left hand and right hand), the Captain is much more representative of reality -- 30 seconds might be a little long for the speed I've generally observed myself and others doing. The FO on the other hand is either faking it or is so weak/uncoordinated he has no business in an airplane.

Having also operated the manual trim wheels in flight at various altitudes from 15,000 ft to FL350 at normal operating speeds the loads i observed were not much different than on the ground.
 
morrisond
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 1:22 pm

mjoelnir wrote:
xmp125a wrote:
flybucky wrote:
Here's a video of pilots demonstrating manual wheel trim in a parked 737-800 NG. They set the trim to 0 units (full nose down) and attempted the manual wheel to trim back to normal 5 units.

That was in a parked 737 with no aerodynamic forces on the stabilizer. The FO struggled and was only able to trim 1 unit in 1 minute, and was exhausted afterwards. Using the left hand may have been a factor. There have been several experts that have said that the aerodynamic forces on an out-of-trim stabilizer when the elevator is up (which it was, because the FDR Control Column Position was 5-10º nose up for most of the flight) at high speed would likely make it impossible for the manual wheel to be able to trim up.

Even the Captain in the demo, who had no problems turning the manual trim wheel, took 30 seconds to trim back to normal 5 units. According to the Preliminary Report graph, ET302 crashed ~20 seconds after the final MCAS command.


Ok, I am frankly appalled. Does this effectively mean that a standard part of the 737 controls, the trim wheel, may be inoperable in large parts of the flight envelope by less muscular pilots? For christ sake, we are in 2019! Should I run away if both pilot and copilot are females and the plane is 737MAX or 737NG? I mean, people would say, well, that trim wheel is rarely used, which may be true, but the airplane is certified with the wheel... for all practical purposes the wheel that cannot be turned/cranked easily by the least physically able pilot that is certified to fly 737, does not exist and it should be perhaps removed and plane recertified without it?

Or did I miss something?


That is why I continue to talk about reviewing the complete 737 trim system. The including of "now trim manually with the trim wheel" should be removed from being an acceptable fall back when things go down the drain with the trimming system.

Perhaps there should also be a review of the amount of back pressure the column is able to produce, as part of the forces are produced by the system, being not really an indication of the forces on the elevator.

All that stuff seems to depend on one sensor only and one could start to get philosophically about, how this stuff ever made it through certification. The answer I assume is bit by bit.


Would you rather they take the manual out entirely? How would that make it safer? In the normal part of the Flight Envelope it is quite usable according to 7Boeing7

The A320 has one as well. I would have to guess it is pretty difficult to move outside the envelope as well. Should we remove that to make the 320 safer? What about complete Electric Failure?
 
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scbriml
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 1:27 pm

morrisond wrote:
Having used the manual trim wheels on the ground hundreds of times and watched other pilots (male and female, left hand and right hand), the Captain is much more representative of reality -- 30 seconds might be a little long for the speed I've generally observed myself and others doing. The FO on the other hand is either faking it or is so weak/uncoordinated he has no business in an airplane.

Having also operated the manual trim wheels in flight at various altitudes from 15,000 ft to FL350 at normal operating speeds the loads i observed were not much different than on the ground.


Presumably he wasn't pulling back on the yoke with all his might at the same time as trying to turn the trim wheel? This shouldn't be ignored because that's what the crew of ET302 were faced with.
Time flies like an arrow. Fruit flies like a banana!
There are 10 types of people in the World - those that understand binary and those that don't.
 
morrisond
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 1:48 pm

scbriml wrote:
morrisond wrote:
Having used the manual trim wheels on the ground hundreds of times and watched other pilots (male and female, left hand and right hand), the Captain is much more representative of reality -- 30 seconds might be a little long for the speed I've generally observed myself and others doing. The FO on the other hand is either faking it or is so weak/uncoordinated he has no business in an airplane.

Having also operated the manual trim wheels in flight at various altitudes from 15,000 ft to FL350 at normal operating speeds the loads i observed were not much different than on the ground.


Presumably he wasn't pulling back on the yoke with all his might at the same time as trying to turn the trim wheel? This shouldn't be ignored because that's what the crew of ET302 were faced with.



Good point - but has speculated up above in many posts - it is not quite clear that the Manual trim wheel was even tried.

I think we really are at the point where we need to see the Full Report and the full transcript.

They may have not been speaking English and it may just be translation errors. There are just way too many ways of interpreting what has been presented so far to support any view.

It may have been recoverable - it may not have been. They may have followed the right procedures they might not have. They may have been fully versed on MCAS - they (or possibly either) might not have.

All the pre-lim report has done is created a lot of questions.
 
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PixelFlight
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 2:36 pm

flybucky wrote:
speedbored wrote:
I wonder whether the explanation might just be that the STS/MCAS signals to the FDR are derived from somewhere after the cutout switches, and that the pilot paused momentarily between operating each switch. The STS/MCAS signal from the FCC is only cut off by one of the switches, not both switches like the stab trim control power, and thumb switch power are.

This thought crossed my mind as well. Prior to the MAX, the Stab Trim Cutouts were labelled "MAIN ELECT" and "AUTO PILOT". Apparently, those cutout switches used to serve different purposes in the NG. If you only cutout Stab Trim Auto Pilot, you could still use the Electric Trim. So it's not a stretch to theorize that if you only cutout MAIN ELECT, you would be disabling all electrical trim (Electric Trim and Auto Trim), however, the Auto Trim commands could still be powered and recorded (even though they had no effect).

Even though the Cutouts were relabelled to "PRI" and "B/U" on the MAX, their history may be related. So if the pilots first cutout "PRI (MAIN ELECT)", that might still allow the "B/U (AUTO PILOT)" to be powered and recorded (although with no effect).

The electrons don't know the 737 history. If the published 737 MAX schematic is exact, either of the two switches removes the main power of the stab trim motor. The labels is just a cognitive bias that even Boeing don't car about as pilot only have procedures to manipulate the two switches at the same time. Maybe the individual switches can play a role for the maintenance work to help diagnostic and/or testing, but I can't remember someone writing about that in this forum.

The single "Automatic Trim Commend With No Change In Pitch Trim" without the repetitive pattern of the MCAS is strange because we certainly don't have the information to understand it. But we can elaborate many scenarios that can explain this. For example the FCC have a probe signal to sens the STAB TRIM CUTOUT SW: the 28V signal that pass through both switches and continue to the "B" label to the "CUTOUT SIGNAL" of the "FCC A/B". We can imagine that this "CUTOUT SIGNAL" only disable new MCAS commands but not the one actually under execution.
 
kalvado
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 2:44 pm

scbriml wrote:
morrisond wrote:
Having used the manual trim wheels on the ground hundreds of times and watched other pilots (male and female, left hand and right hand), the Captain is much more representative of reality -- 30 seconds might be a little long for the speed I've generally observed myself and others doing. The FO on the other hand is either faking it or is so weak/uncoordinated he has no business in an airplane.

Having also operated the manual trim wheels in flight at various altitudes from 15,000 ft to FL350 at normal operating speeds the loads i observed were not much different than on the ground.


Presumably he wasn't pulling back on the yoke with all his might at the same time as trying to turn the trim wheel? This shouldn't be ignored because that's what the crew of ET302 were faced with.

As far as I understand, this is directly related to pulling yoke - but it is not that simple.
Yoke (elevator) deflection creates force at the trailing edge of the horizontal stabilizer, where elevator is located - and jackscrew (effectively located at the leading edge of the stabilizer) has to work against that force to turn stabilizer around the axis near the center of the airfoil. So working the trim wheel for small adjustment in stable flight and fighting severe mistrim with a lot of elevator deflection may be two very different scenarios with different amount of force.
What roller-coaster retrim procedure mentioned earlier is about - it is putting the plane into a zero elevator deflection situation, even if that leads to an unusual flight profile (dive or nose up) for some time, using that time to re-trim, and hopefully reversing dangerous pitch before it is too late.
 
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767333ER
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 3:07 pm

morrisond wrote:
mjoelnir wrote:
xmp125a wrote:

Ok, I am frankly appalled. Does this effectively mean that a standard part of the 737 controls, the trim wheel, may be inoperable in large parts of the flight envelope by less muscular pilots? For christ sake, we are in 2019! Should I run away if both pilot and copilot are females and the plane is 737MAX or 737NG? I mean, people would say, well, that trim wheel is rarely used, which may be true, but the airplane is certified with the wheel... for all practical purposes the wheel that cannot be turned/cranked easily by the least physically able pilot that is certified to fly 737, does not exist and it should be perhaps removed and plane recertified without it?

Or did I miss something?


That is why I continue to talk about reviewing the complete 737 trim system. The including of "now trim manually with the trim wheel" should be removed from being an acceptable fall back when things go down the drain with the trimming system.

Perhaps there should also be a review of the amount of back pressure the column is able to produce, as part of the forces are produced by the system, being not really an indication of the forces on the elevator.

All that stuff seems to depend on one sensor only and one could start to get philosophically about, how this stuff ever made it through certification. The answer I assume is bit by bit.


Would you rather they take the manual out entirely? How would that make it safer? In the normal part of the Flight Envelope it is quite usable according to 7Boeing7

The A320 has one as well. I would have to guess it is pretty difficult to move outside the envelope as well. Should we remove that to make the 320 safer? What about complete Electric Failure?

If that’s how you think, you don’t understand much about how electricity works and how reliable it is compared to mechanical things. A complete failure won’t happen because the conditions for it to happen would have to be damage to the airframe that is so bad that it can’t fly anyway. Electrons aren’t going to magically just stop moving just to prove we need planes like the 737. Power goes out in my house once every few years and that’s on a bigger much more vulnerable power grid that’s exposed to the elements and drunk drivers that drive into power lines. If that’s you’re excuse for using the A320 as a counter argument, you’ve lost your case. The 737 is much more likely to have a complete electrical failure as it doesn’t have a RAT and yet it doesn’t even happen on it!

All planes should be designed such that there is enough redundancy and control over the systems that a mechanical backup (not hydraulic) should never have to be used even if the plane has one. There should be no such thing as powered trim on with all the automation or simply powered trim off and nothing in between because it’s obvious that that stupid mechanically connected wheel is there just for certification purposes and not to be used... because you can’t use it! I doesn’t matter if it works for some of or most of the flight envelope, it’s still risky business. Perhaps the crew is a little fatigued and can’t turn it, perhaps they didn’t have their coffee yet, I know I couldn’t turn the thing without coffee. What we’d all rather is have Boeing design a plane like every other plane they build now that doesn’t need these kludge systems and all this mechanical manual stuff that doesn’t always work. There’s a reason all newer planes don’t have junk like this, because technology has progressed and there are newer much better ways of doing things, oh wait if we can’t call it a 737 or pilots have to sim train at all to fly it, WN will have a temper tantrum!
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 3:17 pm

A brief word on "out of trim dive characteristics"

xmp125a wrote:
Ok, I am frankly appalled. Does this effectively mean that a standard part of the 737 controls, the trim wheel, may be inoperable in large parts of the flight
Inoperable in large some parts of the flight envelope, yes.
And presumably progressively more difficult even in more normal aspects of flight

This in itself is bad, but it gets worse.... (p.s. this isn't new news)

EASA 737 Type Certificate; section B-05/MAX wrote:
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, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner.... {and other cases}

In those cases, longitudinal trim is achieved by using the manual stabilizer trim wheel to position the stabilizer. The trim wheel can be used to trim the airplane throughout the entire flight envelope {edit; or maybe not.... :o }
Strictly speaking there is no suggestion that ET302 was in an "aft center of gravity" situation, but the faulty AoA readings may have clouded that fact.

Boeing did not initially consider this to be a compliance issue because trim could always be achieved, even during the conditions where use of the aisle stand trim switch {wheel?} was required.

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 words from Boeing are exactly what we might expect from a manufacturer who is dedicated to making "a safe plane safer", but is this EASA confirming that pilots simply do not use the trim wheel as a regular feature these days?

EASA 737 Type Certificate; section B-05/MAX wrote:
Boeing set the thumb switch limits in order to increase the level of safety for out-of-trim dive characteristics (CS 25.255(a)(1)). The resulting thumb switch limits require an alternative trim method to meet CS 25.161 trim requirements in certain corners of the operational envelope

Boeing set the thumb switch limits....

Ok, so we already know that the thumb switches are not just a simple direct line to the stabiliser jackscrew motor. (e.g. trim rates are different depending on flaps)
But is the software deciding more than just the speed of adjustment? :scratchchin:
Is it for instance disabling the thumb switches at pre-set limits, whilst allowing MCAS full authority in the other direction?

I'm going to need someone to explain this properly to me, because right now I have this uncomfortable feeling that the pilots on ET302 may have run into some as yet unspecified limits regarding the thumb switches, and paid the price with their lives. I hope somebody currently advising the FBI is looking into this.....

Fun fact; Other parts of the EASA type certificate read just like they were crafted by Boeing's PR dept.
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
:rotfl: :rotfl: :rotfl: {hang on; people died}
Out of trim dive characteristics? WTF?
Again, comments are very welcome because I'm very confused.

And this final quote seems to disagree with itself.... or shows careless proof reading by EASA! :o
Furthermore, the additional crew procedures and training material {which Boeing failed to provide} 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 {or do they mean the yoke mounted thumb switches?}

Now, where's that Boeing lawyer dude, the one who is insisting there is no possibility of criminal charges. :box:
I'm seeing EASA as a possible co-defendant :roll:
Nothing to see here; move along please.
 
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 3:20 pm

morrisond wrote:
scbriml wrote:
morrisond wrote:
Having used the manual trim wheels on the ground hundreds of times and watched other pilots (male and female, left hand and right hand), the Captain is much more representative of reality -- 30 seconds might be a little long for the speed I've generally observed myself and others doing. The FO on the other hand is either faking it or is so weak/uncoordinated he has no business in an airplane.

Having also operated the manual trim wheels in flight at various altitudes from 15,000 ft to FL350 at normal operating speeds the loads i observed were not much different than on the ground.


Presumably he wasn't pulling back on the yoke with all his might at the same time as trying to turn the trim wheel? This shouldn't be ignored because that's what the crew of ET302 were faced with.



Good point - but has speculated up above in many posts - it is not quite clear that the Manual trim wheel was even tried.

I think we really are at the point where we need to see the Full Report and the full transcript.

They may have not been speaking English and it may just be translation errors. There are just way too many ways of interpreting what has been presented so far to support any view.

It may have been recoverable - it may not have been. They may have followed the right procedures they might not have. They may have been fully versed on MCAS - they (or possibly either) might not have.

All the pre-lim report has done is created a lot of questions.

It feels like the low hour FO didn’t understand the trimming system. Either he was trying the electrics and they weren’t responding because they were cut out, or he wasn’t turning the wheel more than a turn or two and since it barely has an impact with each turn he gave up on it.

On the Lion flight, the FO was also impatient with the trim and barely did it each time, thinking (probably) that all he had to do was stop the automatic movement rather than “undo” it.

The Captain in these situations isn’t going to notice the FO “not doing it right” until he feels it in the seat of his pants, and then its too late. But there still seems to be CRM issues because the Captains need to make clear what they are doing or what they expect the FO to do when they hand off tasks. The Captain should be confident the FO is capable of everything the Captain is doing but should not assume the FO reads minds and/or doesn't need a reminder or update.

For Lion, before handing off, we should have heard: “When you take over trimming, counteract with at least X seconds. Thats what I’ve been doing.”

For this flight, we don’t know everything said, but do far I’m not hearing any back and forth regarding exactly what they were doing with the trim.
Of all the things to worry about... the Wookie has no pants.
 
kalvado
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 3:32 pm

ikramerica wrote:
It feels like the low hour FO didn’t understand the trimming system. Either he was trying the electrics and they weren’t responding because they were cut out, or he wasn’t turning the wheel more than a turn or two and since it barely has an impact with each turn he gave up on it.


At this point it seems feasible that trim wheel required much more force than usual given severe mistrim. Would you please consider that as a contributing factor in your speculation?
 
mjoelnir
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 3:46 pm

ikramerica wrote:
morrisond wrote:
scbriml wrote:

Presumably he wasn't pulling back on the yoke with all his might at the same time as trying to turn the trim wheel? This shouldn't be ignored because that's what the crew of ET302 were faced with.



Good point - but has speculated up above in many posts - it is not quite clear that the Manual trim wheel was even tried.

I think we really are at the point where we need to see the Full Report and the full transcript.

They may have not been speaking English and it may just be translation errors. There are just way too many ways of interpreting what has been presented so far to support any view.

It may have been recoverable - it may not have been. They may have followed the right procedures they might not have. They may have been fully versed on MCAS - they (or possibly either) might not have.

All the pre-lim report has done is created a lot of questions.

It feels like the low hour FO didn’t understand the trimming system. Either he was trying the electrics and they weren’t responding because they were cut out, or he wasn’t turning the wheel more than a turn or two and since it barely has an impact with each turn he gave up on it.

On the Lion flight, the FO was also impatient with the trim and barely did it each time, thinking (probably) that all he had to do was stop the automatic movement rather than “undo” it.

The Captain in these situations isn’t going to notice the FO “not doing it right” until he feels it in the seat of his pants, and then its too late. But there still seems to be CRM issues because the Captains need to make clear what they are doing or what they expect the FO to do when they hand off tasks. The Captain should be confident the FO is capable of everything the Captain is doing but should not assume the FO reads minds and/or doesn't need a reminder or update.

For Lion, before handing off, we should have heard: “When you take over trimming, counteract with at least X seconds. Thats what I’ve been doing.”

For this flight, we don’t know everything said, but do far I’m not hearing any back and forth regarding exactly what they were doing with the trim.


Barely turning the trim wheel can also be based upon it being impossible to turn the trim wheel.

The points to the crash are very simple. Boeing laid a deadly trap for pilots with designing MCAS. Boeing offered a solution, a checklist ending on an impossible task, turning the trim wheel when there is a load on the tailplane. Boeing compounded all of the above by sabotaging the possibility for training or even only trying out the failure mode of MCAS on a simulator.
 
MSPNWA
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 3:58 pm

mjoelnir wrote:
Barely turning the trim wheel can also be based upon it being impossible to turn the trim wheel.

The points to the crash are very simple. Boeing laid a deadly trap for pilots with designing MCAS. Boeing offered a solution, a checklist ending on an impossible task, turning the trim wheel when there is a load on the tailplane. Boeing compounded all of the above by sabotaging the possibility for training or even only trying out the failure mode of MCAS on a simulator.


Can also be based? Yes. Supported by the evidence we have so far? No. The evidence - very incomplete at this stage - points the other way.

It's very clear that you've made your conclusions based on speculation and before we have all the evidence. That's not the behavior of someone concerned about improving safety in this industry.
 
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 4:06 pm

jollo wrote:
Of course this is only speculation at this point, but would gain much credibility with an additional bit of information, asked several times in this and in the grounding thread but never AFAIK answered: in the 737 NG, does switching AUTOPILOT STAB TRIM CUTOFF to CUTOFF disable just the A/P, of does it disable STS also? If it does disable STS too (as would be logical), then piggy-backing MCAS on STS would mean AUTOPILOT STAB TRIM CUTOFF would have disabled MCAS too. And as an "MCAS inop" mode would have called for additional sim training, that would be the trigger for the chain of bad decisions.

So I will again put forward the question for members with actual NG experience: in the 737 NG switching only AUTOPILOT STAB TRIM CUTOFF to CUTOFF disables STS or not?

I have read this about the STS: https://www.pprune.org/tech-log/614997- ... st10300036
Originally Posted by Jwscud 
I can’t find my old 737 FCOM but my understanding of the system was that trim was inhibited the moment you applied the control column in the opposite direction, and that included speed trim.
No, it will happily continue to trim even if you push (you need to) forward on the controls after take off when you accelerate. It will stop when you apply opposite trim. Which I do on every take off since STS trims the aircraft out of trim. It normally takes 2-3 turns of trim, depending on when you start to trim.
Weird system!

This is a hint that the MCAS is really a piggy-bake on the STS. More finding required.
 
dragon6172
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 4:43 pm

mjoelnir wrote:
dragon6172 wrote:
mjoelnir wrote:
Perhaps there should also be a review of the amount of back pressure the column is able to produce, as part of the forces are produced by the system, being not really an indication of the forces on the elevator.

That force comes from certification requirements though correct? 1lb for every 6 knots out of trim?


How can you be knots out of trim? And do the forces also depend on an AoA sensor and again only on one?

And what use is it, if the force overwhelms the pilot?

For example, the aircraft is trimmed straight and level at 300 knots, the pilot pulls aft on the controls with no adjustment to trim or engines and slows to 250 knots, this would require just over 8 lb of force on the control column. Letting the control column return to zero and the aircraft should return to the trimmed airspeed of 300 knots.

For the accident aircraft, the stab is probably trimmed for a speed way up around 500-600 knots straight and level. The aircraft is only going around 300-350 knots. That's where the 35-50 lb of force on the control column comes from.

If you want to make the argument that there should be a limit on the amount of force artificially added I'd agree, but that to me seems to be a regulatory item and not Boeing.
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mjoelnir
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:07 pm

dragon6172 wrote:
mjoelnir wrote:
dragon6172 wrote:
That force comes from certification requirements though correct? 1lb for every 6 knots out of trim?


How can you be knots out of trim? And do the forces also depend on an AoA sensor and again only on one?

And what use is it, if the force overwhelms the pilot?

For example, the aircraft is trimmed straight and level at 300 knots, the pilot pulls aft on the controls with no adjustment to trim or engines and slows to 250 knots, this would require just over 8 lb of force on the control column. Letting the control column return to zero and the aircraft should return to the trimmed airspeed of 300 knots.

For the accident aircraft, the stab is probably trimmed for a speed way up around 500-600 knots straight and level. The aircraft is only going around 300-350 knots. That's where the 35-50 lb of force on the control column comes from.

If you want to make the argument that there should be a limit on the amount of force artificially added I'd agree, but that to me seems to be a regulatory item and not Boeing.


There is no feedback on an Airbus, so I doubt it being a hard regulatory point and rather related to the Boeing design.
 
dragon6172
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:10 pm

ikramerica wrote:
It feels like the low hour FO didn’t understand the trimming system. Either he was trying the electrics and they weren’t responding because they were cut out, or he wasn’t turning the wheel more than a turn or two and since it barely has an impact with each turn he gave up on it.

I dont know about not understanding the system. I think the dire situation wasnt known though. During the 2.5 minutes the stab trim was cut off the trim actually moved more nose down from 2.3 to 2.1. So not only could the trim not be moved towards nose up, it couldn't be held in place. Or was moved in the wrong direction. Also during this time the target altitude and target heading settings were changed. As I said earlier these seem like things that aren't real important if you're struggling to control the aircraft
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:16 pm

kalvado wrote:
What roller-coaster retrim procedure mentioned earlier is about - it is putting the plane into a zero elevator deflection situation, even if that leads to an unusual flight profile (dive or nose up) for some time, using that time to re-trim, and hopefully reversing dangerous pitch before it is too late.


But, even if the crew of ET302 knew about this procedure (it seems to have been consigned to the history books), they'd be pretty unlikely to try it at low altitude.
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dragon6172
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:27 pm

mjoelnir wrote:
dragon6172 wrote:
mjoelnir wrote:

How can you be knots out of trim? And do the forces also depend on an AoA sensor and again only on one?

And what use is it, if the force overwhelms the pilot?

For example, the aircraft is trimmed straight and level at 300 knots, the pilot pulls aft on the controls with no adjustment to trim or engines and slows to 250 knots, this would require just over 8 lb of force on the control column. Letting the control column return to zero and the aircraft should return to the trimmed airspeed of 300 knots.

For the accident aircraft, the stab is probably trimmed for a speed way up around 500-600 knots straight and level. The aircraft is only going around 300-350 knots. That's where the 35-50 lb of force on the control column comes from.

If you want to make the argument that there should be a limit on the amount of force artificially added I'd agree, but that to me seems to be a regulatory item and not Boeing.


There is no feedback on an Airbus, so I doubt it being a hard regulatory point and rather related to the Boeing design.

By Boeing design you mean conventional controls and not fly by wire then yes. It's part of the stability requirements. Fly by wire still has to show that the aircraft returns to trimmed condition when their is a disturbance.
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aaexecplat
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:30 pm

mwmav8r01 wrote:
rheinwaldner wrote:
mwmav8r01 wrote:
In my opinion... I think thats what happened. The Captain asked the FO if it was working and he tried his yoke switch. But there is absolutely no way to know.

Why? Check the FDR plot.
Hint: there is no electric trim input visible so we absolutely know that he did not continue using the yoke switch.

[/list]

Noted. If my life depended on it I would try to get the other guy to try too. I have used the trim wheel in the MAX at a decent speed and had no issue. This part is a red flag to me. But I want to see the full report.


What speed? And were you significantly out of trim when you moved the trim wheel manually?
 
kalvado
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:35 pm

scbriml wrote:
kalvado wrote:
What roller-coaster retrim procedure mentioned earlier is about - it is putting the plane into a zero elevator deflection situation, even if that leads to an unusual flight profile (dive or nose up) for some time, using that time to re-trim, and hopefully reversing dangerous pitch before it is too late.


But, even if the crew of ET302 knew about this procedure (it seems to have been consigned to the history books), they'd be pretty unlikely to try it at low altitude.

I am just trying to connect the dots, no blame. If that procedure - and why such procedure - can be used to describe what happened in flight; it makes sense to bring it up as a piece of a puzzle.
 
WIederling
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:36 pm

mjoelnir wrote:
There is no feedback on an Airbus, so I doubt it being a hard regulatory point and rather related to the Boeing design.


But could you overwhelm the trim system in a similar way?
( IMU Airbus slow trims the tailplane to meet the average of the elevator position? )

Even if the tailplane is balanced for neutral elevator low trim forces
the moment you introduce large elevator deflection the reaction force will load the trim spindle.
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SwissCanuck
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:43 pm

speedbored wrote:
flybucky wrote:
I'm leaning towards that when Stab Trim is Cutout, Electric Trim is not recorded in the FDR.

Looking at the schematic, it's clear that power to the pilot's trim switches is disconnected when either of the cutout switches are set to cutout. Without power, I fail to see how either of the trim switches could possibly send a signal to be recorded in the FDR.


Because you are thinking too "simply" - that's not an insult, just a fact. For example, I grew up in Canada where almost every wall switch physically cut power, by flipping the switch, to the lamp in the room. Now I live in Switzerland, and in almost all new construction here, wall switches are low-voltage-DC-supplied control surfaces. Its the breaker back in the panel that is actually doing the on/off/dimming.

Now the 737 is very very clearly a mix of these two worlds. I VERY highly doubt the actual physical switch on the control column feeds the actual power to the motor that turns the trim wheel. Its a control surface, which is connected to a box inside (or not!) of the motor that accepts control signals. It might be even more complicated than that - I've no actual idea. There might be a box that takes all sorts of inputs and then outputs them to the devices involved. So the control box still has power, and thus so does the switch, and can report it back to the DFDR. But the power supply to the motor is cut, so the messages are sent but never received.

I very rarely comment here on something so technical, because aircraft (like my own industry) are "different" than the norm, but I'd be willing to bet a nice 5-star dinner that the yoke controls do NOT directly control the motor, and thus can be interrupted/captured.
 
hivue
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:49 pm

jollo wrote:
I believe the AUTOPILOT cutout switch had to go because, by logic, it should have disabled all automatic control augmentations as well, MACS included.


MCAS does not function with AP on. I think AP cutout (the presence of which implies the possibility for automated trim other than MCAS) had to go due to the creation of MCAS. Two pieces of software in a food fight is pointless. Either all electric trim is gone or none.
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dakota123
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:49 pm

mjoelnir wrote:
dragon6172 wrote:
mjoelnir wrote:
Perhaps there should also be a review of the amount of back pressure the column is able to produce, as part of the forces are produced by the system, being not really an indication of the forces on the elevator.

That force comes from certification requirements though correct? 1lb for every 6 knots out of trim?


How can you be knots out of trim? And do the forces also depend on an AoA sensor and again only on one?

And what use is it, if the force overwhelms the pilot?


Every trim condition is associated with a speed to which the plane will return if pitch is disturbed. If the plane is trimmed for level flight at 250 knots, but you're doing 225 and holding backpressure, and you let go, the plane will descend until hitting 225 knots -- in reality, it will overshoot and eventually climb, oscillating in phugoid fashion until finally settling within an allowable margin (control friction being the driver as to why it doesn't always return to the theoretical speed). The acceptable minimum force value is 1 lb / 6 knots * 25 knots = 4 lbs backpressure to hold 225 knots when trimmed for 250, for example. Again, that's a minimum. There are other requirements for gradient stability when at different flight regimes.

The max under conditions that to me are the most similar to 302 are predicated on the out-of-trim condition produced by hitting the electric trim switch for three seconds while at VMO, in which case when at Vdf a 125-lb control force application is to give a 1.5g recovery. The 125-lb control force application may include use of trim, in which case the trim is to be capable of contributing to the required force with the elevator loaded as required to produce the 1.5g recovery (or loaded per some other limits which wouldn't seem applicable since 1.5g is the required recovery.) i.e. motor not stalled out.
“And If I claim to be a wise man, well surely it means that I don’t know”
 
OEMInsider
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:52 pm

SheikhDjibouti wrote:
A brief word on "out of trim dive characteristics"...
EASA 737 Type Certificate; section B-05/MAX...


Wow. If I'm reading this right (not guaranteed!), this is really serious isn't it?

Here is a link to the EASA document in question (page 15 of 114): EASA TCDS for 737 MAX.

It seems to say that the 737MAX does not meet part of the Certification requirement for out-of-trim behaviour (CS 25.161(a) page 34 of 617) by using the thumb switches alone. This was to guarantee that they met 25.255(a), which says that at Vmo (i.e. high speeds) you need to have controllability of the aircraft when out of trim. The amount of out-of-trim is defined as how far the stabiliser moves at its normal (unloaded) speed in 3 seconds. Boeing "set the thumb switch limits in order to increase the level of safety for out-of-trim dive characteristics". Presumably this is to limit how far out of trim you can go and hence avoid the nasty dive characteristics. Boeing have said (and EASA/FAA accepted) that there are others methods of trimming to cover requirements beyond these imposed limits.

However...the other trim methods do not work at these conditions (i.e. high speeds), so the "Equivalent Safety" level determined by EASA is not met. (Equivalent Safety findings are not uncommon btw). Does the 737MAX meet the certification requirements for trim? Boeing can't rely on the manual wheel (it won't move due to aero loads) and can't rely on the thumb switches (to protect for the dive characteristics). What do they do?

Furthermore...the out-of-trim dive characteristics is also concerning given what we know about the ET302 crash. The requirement is:
the aeroplane must have satisfactory manoeuvring stability and controllability with the degree of out-oftrim in both the aeroplane nose-up and nose-down directions
. Boeing had to impose (it seems) artificial limits on the thumb switch trim specifically for the MAX in order to meet this requirement. To me this implies that the if you apply normal (737NG/Classic) amounts of nose-down trim to a MAX flying at Vmo it would go into an uncontrollable dive. This, of course, is exactly what MCAS did.

Boeing deliberately limited a critical flight control system to (rightly) prevent a dangerous input. But then created another system (MCAS) which went and did that anyway (without telling the pilots, without training them etc etc). This isn't getting any better...
Last edited by OEMInsider on Mon Apr 08, 2019 5:54 pm, edited 1 time in total.
 
dakota123
Posts: 233
Joined: Wed Aug 30, 2006 11:03 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 5:52 pm

WIederling wrote:
mjoelnir wrote:
There is no feedback on an Airbus, so I doubt it being a hard regulatory point and rather related to the Boeing design.


But could you overwhelm the trim system in a similar way?
( IMU Airbus slow trims the tailplane to meet the average of the elevator position? )

Even if the tailplane is balanced for neutral elevator low trim forces
the moment you introduce large elevator deflection the reaction force will load the trim spindle.


For general interest, came across A-380 Special Conditions dealing with many of these issues: https://www.federalregister.gov/documen ... ystems-and
“And If I claim to be a wise man, well surely it means that I don’t know”
 
flybucky
Posts: 175
Joined: Tue Apr 17, 2018 7:44 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:02 pm

dragon6172 wrote:
During the 2.5 minutes the stab trim was cut off the trim actually moved more nose down from 2.3 to 2.1. So not only could the trim not be moved towards nose up, it couldn't be held in place. Or was moved in the wrong direction.

From the PR: "From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units."

This is disturbing. There were no Auto Trim commands (STS, MCAS, etc) or Electric Trim commands during that period. So that means either the pilots manually turned the trim wheel in the nose down direction, or the stabilizer was slipping on its own.

It's possible that if the pilots were not able to trim the wheel nose up, they might try if it will move nose down to see if the wheel is just stuck, or if they only could not overcome the nose up aerodynamic forces. However, if that were the case, I would expect to see a short jump from 2.3 to 2.1 units when they tried that, not gradually over 2.5 minutes.

Is it even possible for the stabilizer to slip gradually? With the elevator up, that puts pressure on the stabilizer to nose down. Could it gradually slip 0.2 units over 2.5 mins?
 
WIederling
Posts: 8888
Joined: Sun Sep 13, 2015 2:15 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:06 pm

dakota123 wrote:
For general interest, came across A-380 Special Conditions dealing with many of these issues: https://www.federalregister.gov/documen ... ystems-and

reading.

apropos:
could one press the observation that the 737 trim system ( electric, manual ) is underpowered for the MAX?

another thing: blowback was brought up by some sources ( Björn Ferm for example )
is this still a strong observation or do the things apparent now on trim overlay that view?
Murphy is an optimist
 
User avatar
speedbored
Posts: 2207
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Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:11 pm

SwissCanuck wrote:
speedbored wrote:
flybucky wrote:
I'm leaning towards that when Stab Trim is Cutout, Electric Trim is not recorded in the FDR.

Looking at the schematic, it's clear that power to the pilot's trim switches is disconnected when either of the cutout switches are set to cutout. Without power, I fail to see how either of the trim switches could possibly send a signal to be recorded in the FDR.


Because you are thinking too "simply" - that's not an insult, just a fact. For example, I grew up in Canada where almost every wall switch physically cut power, by flipping the switch, to the lamp in the room. Now I live in Switzerland, and in almost all new construction here, wall switches are low-voltage-DC-supplied control surfaces. Its the breaker back in the panel that is actually doing the on/off/dimming.

Now the 737 is very very clearly a mix of these two worlds. I VERY highly doubt the actual physical switch on the control column feeds the actual power to the motor that turns the trim wheel. Its a control surface, which is connected to a box inside (or not!) of the motor that accepts control signals. It might be even more complicated than that - I've no actual idea. There might be a box that takes all sorts of inputs and then outputs them to the devices involved. So the control box still has power, and thus so does the switch, and can report it back to the DFDR. But the power supply to the motor is cut, so the messages are sent but never received.

I very rarely comment here on something so technical, because aircraft (like my own industry) are "different" than the norm, but I'd be willing to bet a nice 5-star dinner that the yoke controls do NOT directly control the motor, and thus can be interrupted/captured.

No, I am not "thinking simply". I'm well aware of how the circuits work - the switches are low voltage, and, yes, the real power is switched by relays or sold-state switches elsewhere.

I can also read circuit diagrams, and there is no way that power could be supplied back down the signal cable from the trim motor control box and return back to anywhere as a switch signal - if the trim cutouts have been used, one side of those switches is floating - they simply cannot provide any indication of their state to anything.
 
XRAYretired
Posts: 632
Joined: Fri Mar 15, 2019 11:21 am

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:20 pm

SwissCanuck wrote:
speedbored wrote:
flybucky wrote:
I'm leaning towards that when Stab Trim is Cutout, Electric Trim is not recorded in the FDR.

Looking at the schematic, it's clear that power to the pilot's trim switches is disconnected when either of the cutout switches are set to cutout. Without power, I fail to see how either of the trim switches could possibly send a signal to be recorded in the FDR.


Because you are thinking too "simply" - that's not an insult, just a fact. For example, I grew up in Canada where almost every wall switch physically cut power, by flipping the switch, to the lamp in the room. Now I live in Switzerland, and in almost all new construction here, wall switches are low-voltage-DC-supplied control surfaces. Its the breaker back in the panel that is actually doing the on/off/dimming.

Now the 737 is very very clearly a mix of these two worlds. I VERY highly doubt the actual physical switch on the control column feeds the actual power to the motor that turns the trim wheel. Its a control surface, which is connected to a box inside (or not!) of the motor that accepts control signals. It might be even more complicated than that - I've no actual idea. There might be a box that takes all sorts of inputs and then outputs them to the devices involved. So the control box still has power, and thus so does the switch, and can report it back to the DF, .DR. But the power supply to the motor is cut, so the messages are sent but never received.

I very rarely comment here on something so technical, because aircraft (like my own industry) are "different" than the norm, but I'd be willing to bet a nice 5-star dinner that the yoke controls do NOT directly control the motor, and thus can be interrupted/captured.
.

Yes. Thumb Switch will undoubtably initiate a procedure in software (assuming other initiation conditions are met) as MCAS and STS are software procedures.

Ray
 
smartplane
Posts: 1024
Joined: Fri Aug 03, 2018 9:23 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:20 pm

SheikhDjibouti wrote:
Fun fact; Other parts of the EASA type certificate read just like they were crafted by Boeing's PR dept.
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
:rotfl: :rotfl: :rotfl: {hang on; people died}
Out of trim dive characteristics? WTF?
Again, comments are very welcome because I'm very confused.

So the thumb switches aren't operable throughout the flight envelope. When MCAS prevails for instance?
 
dragon6172
Posts: 1072
Joined: Sat Jul 14, 2007 9:56 am

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:24 pm

speedbored wrote:
SwissCanuck wrote:
speedbored wrote:
Looking at the schematic, it's clear that power to the pilot's trim switches is disconnected when either of the cutout switches are set to cutout. Without power, I fail to see how either of the trim switches could possibly send a signal to be recorded in the FDR.


Because you are thinking too "simply" - that's not an insult, just a fact. For example, I grew up in Canada where almost every wall switch physically cut power, by flipping the switch, to the lamp in the room. Now I live in Switzerland, and in almost all new construction here, wall switches are low-voltage-DC-supplied control surfaces. Its the breaker back in the panel that is actually doing the on/off/dimming.

Now the 737 is very very clearly a mix of these two worlds. I VERY highly doubt the actual physical switch on the control column feeds the actual power to the motor that turns the trim wheel. Its a control surface, which is connected to a box inside (or not!) of the motor that accepts control signals. It might be even more complicated than that - I've no actual idea. There might be a box that takes all sorts of inputs and then outputs them to the devices involved. So the control box still has power, and thus so does the switch, and can report it back to the DFDR. But the power supply to the motor is cut, so the messages are sent but never received.

I very rarely comment here on something so technical, because aircraft (like my own industry) are "different" than the norm, but I'd be willing to bet a nice 5-star dinner that the yoke controls do NOT directly control the motor, and thus can be interrupted/captured.

No, I am not "thinking simply". I'm well aware of how the circuits work - the switches are low voltage, and, yes, the real power is switched by relays or sold-state switches elsewhere.

I can also read circuit diagrams, and there is no way that power could be supplied back down the signal cable from the trim motor control box and return back to anywhere as a switch signal - if the trim cutouts have been used, one side of those switches is floating - they simply cannot provide any indication of their state to anything.

That being said, the diagram on page 94 of this thread is not the actual circuit diagram and wouldn't be used to shoot wires during troubleshooting. Which means its possible there are parts missing.
Phrogs Phorever
 
SimonL
Posts: 41
Joined: Tue Mar 12, 2019 3:38 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:57 pm

OEMInsider wrote:
SheikhDjibouti wrote:
A brief word on "out of trim dive characteristics"...
EASA 737 Type Certificate; section B-05/MAX...


Wow. If I'm reading this right (not guaranteed!), this is really serious isn't it?

Here is a link to the EASA document in question (page 15 of 114): EASA TCDS for 737 MAX.

It seems to say that the 737MAX does not meet part of the Certification requirement for out-of-trim behaviour (CS 25.161(a) page 34 of 617) by using the thumb switches alone. This was to guarantee that they met 25.255(a), which says that at Vmo (i.e. high speeds) you need to have controllability of the aircraft when out of trim. The amount of out-of-trim is defined as how far the stabiliser moves at its normal (unloaded) speed in 3 seconds. Boeing "set the thumb switch limits in order to increase the level of safety for out-of-trim dive characteristics". Presumably this is to limit how far out of trim you can go and hence avoid the nasty dive characteristics. Boeing have said (and EASA/FAA accepted) that there are others methods of trimming to cover requirements beyond these imposed limits.

However...the other trim methods do not work at these conditions (i.e. high speeds), so the "Equivalent Safety" level determined by EASA is not met. (Equivalent Safety findings are not uncommon btw). Does the 737MAX meet the certification requirements for trim? Boeing can't rely on the manual wheel (it won't move due to aero loads) and can't rely on the thumb switches (to protect for the dive characteristics). What do they do?

Furthermore...the out-of-trim dive characteristics is also concerning given what we know about the ET302 crash. The requirement is:
the aeroplane must have satisfactory manoeuvring stability and controllability with the degree of out-oftrim in both the aeroplane nose-up and nose-down directions
. Boeing had to impose (it seems) artificial limits on the thumb switch trim specifically for the MAX in order to meet this requirement. To me this implies that the if you apply normal (737NG/Classic) amounts of nose-down trim to a MAX flying at Vmo it would go into an uncontrollable dive. This, of course, is exactly what MCAS did.

Boeing deliberately limited a critical flight control system to (rightly) prevent a dangerous input. But then created another system (MCAS) which went and did that anyway (without telling the pilots, without training them etc etc). This isn't getting any better...


Ive asked that question earlier. and im quite sure that they are going to dig into that as well..
 
L1049L1011
Posts: 15
Joined: Tue Sep 06, 2016 6:54 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 6:57 pm

flybucky wrote:
speedbored wrote:
I wonder whether the explanation might just be that the STS/MCAS signals to the FDR are derived from somewhere after the cutout switches, and that the pilot paused momentarily between operating each switch. The STS/MCAS signal from the FCC is only cut off by one of the switches, not both switches like the stab trim control power, and thumb switch power are.

This thought crossed my mind as well. Prior to the MAX, the Stab Trim Cutouts were labelled "MAIN ELECT" and "AUTO PILOT". Apparently, those cutout switches used to serve different purposes in the NG. If you only cutout Stab Trim Auto Pilot, you could still use the Electric Trim. So it's not a stretch to theorize that if you only cutout MAIN ELECT, you would be disabling all electrical trim (Electric Trim and Auto Trim), however, the Auto Trim commands could still be powered and recorded (even though they had no effect).

Even though the Cutouts were relabelled to "PRI" and "B/U" on the MAX, their history may be related. So if the pilots first cutout "PRI (MAIN ELECT)", that might still allow the "B/U (AUTO PILOT)" to be powered and recorded (although with no effect).


Quote from BOEING 737-7/-8 System Differences manual - see http://www.b737.org.uk/mcas.htm
... the pilots move the STAB TRIM PRI (primary) switch to the CUT OUT position. This removes power to the STAB TRIM B/U (backup) switch and these:
    - Captain and F/O stabilizer trim switches
    - R64 (de-energizes)
    - R1192 contact
    - Signals to the FCCs
 
morrisond
Posts: 1394
Joined: Thu Jan 07, 2010 12:22 am

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 7:06 pm

flybucky wrote:
dragon6172 wrote:
During the 2.5 minutes the stab trim was cut off the trim actually moved more nose down from 2.3 to 2.1. So not only could the trim not be moved towards nose up, it couldn't be held in place. Or was moved in the wrong direction.

From the PR: "From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units."

This is disturbing. There were no Auto Trim commands (STS, MCAS, etc) or Electric Trim commands during that period. So that means either the pilots manually turned the trim wheel in the nose down direction, or the stabilizer was slipping on its own.

It's possible that if the pilots were not able to trim the wheel nose up, they might try if it will move nose down to see if the wheel is just stuck, or if they only could not overcome the nose up aerodynamic forces. However, if that were the case, I would expect to see a short jump from 2.3 to 2.1 units when they tried that, not gradually over 2.5 minutes.

Is it even possible for the stabilizer to slip gradually? With the elevator up, that puts pressure on the stabilizer to nose down. Could it gradually slip 0.2 units over 2.5 mins?


I would have to guess that when they were well over Vmo almost anything is possible.
 
dragon6172
Posts: 1072
Joined: Sat Jul 14, 2007 9:56 am

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:07 pm

smartplane wrote:
SheikhDjibouti wrote:
Fun fact; Other parts of the EASA type certificate read just like they were crafted by Boeing's PR dept.
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
:rotfl: :rotfl: :rotfl: {hang on; people died}
Out of trim dive characteristics? WTF?
Again, comments are very welcome because I'm very confused.

So the thumb switches aren't operable throughout the flight envelope. When MCAS prevails for instance?

The thumb switches can trim down to around 4 units of trim, and up to around 14.5 units of trim with flaps retracted. Autopilot, MCAS, STS and thumb switches when flaps extended can trim down to 0.05 and up to 14.5 units. Manual trim can go from -0.20 to 16.9.

So with flaps retracted, pilots can only trim nose down to around 4 units of trim. Any further nose down trim required would have to come from using the manual trim wheel.

Note that "exceeding" a limit in one direction does not prevent the trim from being used in the other direction. For example, with flaps retracted if MCAS trims nose down to 1.0 units of trim, the pilot trim switches will not work in the nose down direction, they will work in the nose up direction. This can be seen in the schematic on page 94 of this thread. The up and down limit switches and commands follow different paths.
Phrogs Phorever
 
User avatar
PixelFlight
Posts: 650
Joined: Thu Nov 08, 2018 11:09 pm

Re: Ethiopian Airlines 737MAX crashes enroute to Nairobi

Mon Apr 08, 2019 8:14 pm

SwissCanuck wrote:
speedbored wrote:
flybucky wrote:
I'm leaning towards that when Stab Trim is Cutout, Electric Trim is not recorded in the FDR.

Looking at the schematic, it's clear that power to the pilot's trim switches is disconnected when either of the cutout switches are set to cutout. Without power, I fail to see how either of the trim switches could possibly send a signal to be recorded in the FDR.


Because you are thinking too "simply" - that's not an insult, just a fact. For example, I grew up in Canada where almost every wall switch physically cut power, by flipping the switch, to the lamp in the room. Now I live in Switzerland, and in almost all new construction here, wall switches are low-voltage-DC-supplied control surfaces. Its the breaker back in the panel that is actually doing the on/off/dimming.

Now the 737 is very very clearly a mix of these two worlds. I VERY highly doubt the actual physical switch on the control column feeds the actual power to the motor that turns the trim wheel. Its a control surface, which is connected to a box inside (or not!) of the motor that accepts control signals. It might be even more complicated than that - I've no actual idea. There might be a box that takes all sorts of inputs and then outputs them to the devices involved. So the control box still has power, and thus so does the switch, and can report it back to the DFDR. But the power supply to the motor is cut, so the messages are sent but never received.

I very rarely comment here on something so technical, because aircraft (like my own industry) are "different" than the norm, but I'd be willing to bet a nice 5-star dinner that the yoke controls do NOT directly control the motor, and thus can be interrupted/captured.

I have published several times in this thread the link to the 737 MAX stab trim electric schematic. The 28V low power DC is used to power the circuits that pass through the various switches implementing the pilot interface and logic, because obviously the triple phase high power 115V is too dangerous and complex to wire everywhere into the cockpit. In the above discussion the "power" to the switches is the 28V DC low power. At one point, one of the 28V signal that pass trough the two cutoff switch is connected to the thumb switches to "power" them and the schematic very clearly show that there are no other power to that switches. Without electrical power source there is no physical way to sens the thumb switches. The same 28V low power DC signal energize the R64 replay that commute the high power triple phase feeding the stab trim electric motor. So if any of the two cutoff switches cut the 28V signal, the electrical source is removed from the thumb switches, preventing FDR recording, and from the stab trim motor (via R64 relay),

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