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3356 posts

Uber Geek

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  #2210686 4-Apr-2019 12:28
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Technofreak:

 

What computer are you talking about? The 737 isn't a Fly By Wire aircraft. There is no computer for the trim system.

 

 

There *is* a computer for the trim system... it provides Mach Trim, Speed Trim, and of course MCAS. These are active while the pilot is flying manually. It is called control augmentation rather than fly-by-wire because it can be overpowered/disabled by the pilots.

 

And I guess the altitude hold of the autopilot, which is another computer, would also want to control pitch via the trim, and therefore would be interested to know if the trim had been disabled.

 

 


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  #2210956 4-Apr-2019 21:20
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frankv:

There *is* a computer for the trim system... it provides Mach Trim, Speed Trim, and of course MCAS. These are active while the pilot is flying manually. It is called control augmentation rather than fly-by-wire because it can be overpowered/disabled by the pilots.


And I guess the altitude hold of the autopilot, which is another computer, would also want to control pitch via the trim, and therefore would be interested to know if the trim had been disabled.


 



OK, I see what you're getting at.

The way I understand it is the MCAS is separate from the trim system as is the auto pilot, but they both have outputs that can control the trim system. I expect the mach trim may be the same.

It my understanding the Cut Out switches remove power from the trim mechanism. Then the only way to move the trim is by the wheel on the central control pedestal via cables to the trim jackscrew. With the power removed, none of the items like the autopilot etc can move the trim jackscrew, as they can only control the motor that operates the jackscrew when there is power available.




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  #2211072 5-Apr-2019 07:52
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Technofreak:
frankv:

There *is* a computer for the trim system... it provides Mach Trim, Speed Trim, and of course MCAS. These are active while the pilot is flying manually. It is called control augmentation rather than fly-by-wire because it can be overpowered/disabled by the pilots.


And I guess the altitude hold of the autopilot, which is another computer, would also want to control pitch via the trim, and therefore would be interested to know if the trim had been disabled.


 



OK, I see what you're getting at.

The way I understand it is the MCAS is separate from the trim system as is the auto pilot, but they both have outputs that can control the trim system. I expect the mach trim may be the same.

It my understanding the Cut Out switches remove power from the trim mechanism. Then the only way to move the trim is by the wheel on the central control pedestal via cables to the trim jackscrew. With the power removed, none of the items like the autopilot etc can move the trim jackscrew, as they can only control the motor that operates the jackscrew when there is power available.

 

 

Saw a diagram online... the trim switch does indeed control power to the stabiliser jackscrew, which is what the speed trim and MCAS control. Mach trim is done on the elevator trim. There's a position sensor on the jackscrew, so the computer will know if the jackscrew isn't responding to the computer's signals, in which case warning is lit.

 

 


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Uber Geek


  #2211361 5-Apr-2019 14:32
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https://assets.documentcloud.org/documents/5793877/Preliminary-Report-B737-800MAX-Ethiopia.pdf

 

Some interesting bits in there:

 

"Six seconds after the autopilot engagement, there were small amplitude roll oscillations
accompanied by lateral acceleration, rudder oscillations and slight heading changes. These
oscillations continued also after the autopilot was disengaged"

 

"The left Indicated Airspeed increased, eventually reaching approximately 458 kts and the right
Indicated Airspeed reached 500 kts at the end of the recording. The last recorded pressure altitude
was 5,419 ft on the left and 8,399 ft on the right."

 

"The crew performed runaway stabilizer checklist and put the stab trim cutout switch to
cutout position and confirmed that the manual trim operation was not working."

 

 

 

"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"

 

So if the cutout was in place how did an automatic AND occur some nearly 3 minutes later? I can't see anything to indicate that the trim cutout switches were touched after the above

 

At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic
trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in
approximately 5 seconds





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  #2211372 5-Apr-2019 14:48
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https://www.seattletimes.com/business/boeing-aerospace/boeings-emergency-procedure-for-737-max-may-have-failed-on-ethiopian-flight/

 

In this article it is mentioned that the pilot manual for the early 737s had a special procedure to deal with a runaway trim situation but because of improvements it was not needed from the 737-300 and later variants.  The article discusses whether the MCAS on the MAX has reintroduced this long fixed problem.

 

You need to read the whole article but the runaway trim procedure on the 737-100 and 737-200 was called 'roller coaster' where the pilot would lift the nose and then push it down.   While this was going on the trim wheel could be moved.  If they didn't do this the trim wheel couldn't be moved.

 

The article also mentions a simulator situation where pilots were unable to recover from this problem.

 

I am very glad to hear that it was fixed in the later versions of the 737.   I am old enough to have had many flights in 737-200s and glad I never heard about this until now!

 

 


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Ultimate Geek

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  #2211423 5-Apr-2019 16:30
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amiga500:

 

the runaway trim procedure on the 737-100 and 737-200 was called 'roller coaster' where the pilot would lift the nose and then push it down.   While this was going on the trim wheel could be moved.  If they didn't do this the trim wheel couldn't be moved.

 

I am very glad to hear that it was fixed in the later versions of the 737.   I am old enough to have had many flights in 737-200s and glad I never heard about this until now!

 

 

That problem wasn't "fixed in the later versions of the 737" it just wasn't mentioned any more.

 

It looks as though, after switching the automated trim system off, the pilots in the Ethiopian Airlines crash were unable to manually wind the trim up because of excess aerodynamic pressure on the trim surfaces - due to the aircraft's speed (apparently travelling at close to 500 knots).

 

They tried turning the switches back on to regain assisted trim control, but the automated system then added more down trim.

 

It appears that the recommended 'runaway trim' method in earlier manuals was switch the motorised trim system off, and - at higher speeds that made manual trimming difficult -  to porpoise the aircraft, manually winding the trim wheel as the pressure eased on the dip and holding on the climb.

 

The Ethiopian Airlines pilots didn't have that option as they were at low altitude.


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  #2211426 5-Apr-2019 16:50
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Initial findings for Ethiopian crash clear the pilots.

Link here

 
 
 
 


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  #2211647 6-Apr-2019 11:07
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The evidence so far seems to be supporting the blowback theory discussed earlier in this thread.


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  #2211651 6-Apr-2019 11:18
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alasta:

The evidence so far seems to be supporting the blowback theory discussed earlier in this thread.



I think that is a by product of what happened




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  #2211657 6-Apr-2019 11:28
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dafman: Initial findings for Ethiopian crash clear the pilots.

Link here


Not really. The fact they carried out the instructions repeatedly indicates they likely didn't carry out the instructions correctly or they misunderstood the instructions.

The Boeing instructions mention the possibility of high stick forces and the possible subsequent difficulty in trimming the aircraft manually with a MCAS fault. The instruction provides information on how to handle this situation.




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  #2211696 6-Apr-2019 12:58
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Technofreak:
dafman: Initial findings for Ethiopian crash clear the pilots.

Link here


Not really. The fact they carried out the instructions repeatedly indicates they likely didn't carry out the instructions correctly or they misunderstood the instructions.

The Boeing instructions mention the possibility of high stick forces and the possible subsequent difficulty in trimming the aircraft manually with a MCAS fault. The instruction provides information on how to handle this situation.


The article states the findings are that the pilots correctly followed the emergency procedures provided by Boeing. It can't be much clearer than that.

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  #2211799 6-Apr-2019 15:51
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dafman: 
The article states the findings are that the pilots correctly followed the emergency procedures provided by Boeing. It can't be much clearer than that.

 

Yes the article does say that, in fact it says, and I quote;

 

The country’s transport minister, Dagmawit Moges, did not cite the aircraft’s controversial anti-stall system by name, but said: “The crew performed all the procedures repeatedly provided by the manufacturer but was not able to control the aircraft.

 

The final part of Boeing's Operating Instructions state the TRIM CUTOUT switches are to moved to the CUTOUT position and stay there for the rest of the flight.

 

The only way for the pilots could have repeatedly performed the instructions was to have repeatedly moved the TRIM CUTOUT switches back to the operating position in contravention of the Boeing instructions. No doubt you see the incongruity of what is in that article to what is in the Boeing Instructions? Therefore the pilots cannot have performed the Boeing instructions repeatedly and also done them correctly.

 

That brings me back to what I said in my other post, the pilots either didn't understand the instructions or didn't follow the instructions correctly.





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  #2211800 6-Apr-2019 15:55
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What else were they supposed to do when the cutout switches were engaged but they were unable to operate the manual trim wheel? Does Boeing even have a procedure for that situation?


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  #2211802 6-Apr-2019 16:13
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alasta:

 

What else were they supposed to do when the cutout switches were engaged but they were unable to operate the manual trim wheel? Does Boeing even have a procedure for that situation?

 

 

Yes Boeing does, and that is covered in the bulletin Boeing issued with the Operating Instructions I referred to in my other post. This procedure is carried out prior to operating the CUTOUT switches. 





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  #2211917 6-Apr-2019 20:11
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according to this report, pilots followed all the Boeing protocols including cutting power to trim control

 

https://www.nzherald.co.nz/world/news/article.cfm?c_id=2&objectid=12219937





Involuntary autocorrect in operation on mobile device. Apologies in advance.


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