planecane
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Physial design of control of 737 jackscrew

Sat Jun 08, 2019 1:55 pm

While researching documents and watching videos about runaway stabilizer on the 737, I came across the information that the manual wheel has the highest priority and will override any electric trim (automatic or manual). I even saw a video of a pilot in a real 737NG cockpit hold the thumb switch and grab the trim wheel and the trim stopped moving.

I am curious how this is actually implemented in the design of the motor and cable interface with the jack screw. I don't think they can be connected in parallel with each other because I can't imagine a human grip is strong enough to stop an electric motor which is powerful enough to easily move the stabilizer under flight loads. Does anybody have a picture or diagram showing the actual interface of the cable from the manual wheel and the electric motor with the jackscrew?

Please do not turn this into a discussion of MCAS or runaway trim.
 
Lpbri
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Re: Physial design of control of 737 jackscrew

Sat Jun 08, 2019 2:46 pm

I have a good schematic but I don't know how to post it
 
planecane
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Re: Physial design of control of 737 jackscrew

Sat Jun 08, 2019 3:51 pm

Lpbri wrote:
I have a good schematic but I don't know how to post it


maybe a moderator can help you. It would be very interesting (to me, at least) to see it.
 
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Starlionblue
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Re: Physial design of control of 737 jackscrew

Sat Jun 08, 2019 11:02 pm

Lpbri wrote:
I have a good schematic but I don't know how to post it


If it is an image file, you can upload it to a free service like Imgur and then embed it here using the "insert image" button. If it is a PDF you can put it on DropBox or the like and link to it.
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Balerit
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Re: Physial design of control of 737 jackscrew

Mon Jun 10, 2019 7:40 pm

Image
Licensed Aircraft Maintenance Engineer (retired).
 
GalaxyFlyer
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Re: Physial design of control of 737 jackscrew

Mon Jun 10, 2019 8:18 pm

Is it really a single drive motor assembly with a single channel electrical actuator path?
 
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fr8mech
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Re: Physial design of control of 737 jackscrew

Mon Jun 10, 2019 10:15 pm

I can’t imagine it’s much different than a B727. The motor had a clutch that engaged when the trim was engaged electrically. When electrically inactive, the clutch was ‘off’ and the manual trim wheel could be used. I can’t recall how manually trimming, using the wheel, disengaged the electric clutch.
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GalaxyFlyer
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Re: Physial design of control of 737 jackscrew

Mon Jun 10, 2019 10:54 pm

In 1979, the Challenger has dual channels controlling two motors, it seems more primitive than I imagined.


GF
 
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CALTECH
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Re: Physial design of control of 737 jackscrew

Tue Jun 11, 2019 12:15 am

planecane wrote:
While researching documents and watching videos about runaway stabilizer on the 737, I came across the information that the manual wheel has the highest priority and will override any electric trim (automatic or manual). I even saw a video of a pilot in a real 737NG cockpit hold the thumb switch and grab the trim wheel and the trim stopped moving.

I am curious how this is actually implemented in the design of the motor and cable interface with the jack screw. I don't think they can be connected in parallel with each other because I can't imagine a human grip is strong enough to stop an electric motor which is powerful enough to easily move the stabilizer under flight loads. Does anybody have a picture or diagram showing the actual interface of the cable from the manual wheel and the electric motor with the jackscrew?

Please do not turn this into a discussion of MCAS or runaway


Moving the column in the opposite direction of Stab Trim stops the motion, then the Stab Trim cutout switches kill Stab Trim. If the manual wheels are moving in main trim, they'll break your wrists. Nothing to really grab on to, as the handles are usually stowed. Autopilot Stab Trim, is a slower rate, one might be able to grab the wheel but stab trim should be stopped electrically, then pop the handles out and adjust stab trim manually.
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planecane
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Re: Physial design of control of 737 jackscrew

Tue Jun 11, 2019 2:06 am

CALTECH wrote:
planecane wrote:
If the manual wheels are moving in main trim, they'll break your wrists. Nothing to really grab on to, as the handles are usually stowed. Autopilot Stab Trim, is a slower rate, one might be able to grab the wheel but stab trim should be stopped electrically, then pop the handles out and adjust stab trim manually.


I will try to find the video but a pilot in a real cockpit of a 737 (not a simulator) held down the thumb switch with his left hand and grabbed the wheel between his thumb and fingers of his right hand and the wheel stopped. As soon as he let go it started spinning again. There is some mechanism that allows the manual wheel to override the electric motor. I can't remember which manual but I read in a Boeing published manual that the trim wheel overrides all other trim.
 
milhaus
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Re: Physial design of control of 737 jackscrew

Tue Jun 11, 2019 7:44 pm

I tried it some time ago. I had extended trim wheel handle, held it and then momentarily applied trim command. It is possible to hold it but force is very high and I think it is possible to catch it and hold when it turns.
 
strfyr51
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Re: Physial design of control of 737 jackscrew

Tue Jun 11, 2019 7:51 pm

planecane wrote:
CALTECH wrote:
planecane wrote:
If the manual wheels are moving in main trim, they'll break your wrists. Nothing to really grab on to, as the handles are usually stowed. Autopilot Stab Trim, is a slower rate, one might be able to grab the wheel but stab trim should be stopped electrically, then pop the handles out and adjust stab trim manually.


I will try to find the video but a pilot in a real cockpit of a 737 (not a simulator) held down the thumb switch with his left hand and grabbed the wheel between his thumb and fingers of his right hand and the wheel stopped. As soon as he let go it started spinning again. There is some mechanism that allows the manual wheel to override the electric motor. I can't remember which manual but I read in a Boeing published manual that the trim wheel overrides all other trim.

that's also a physical maintenance ground test when ops checking Stab trim. you have to know it can be overridden
 
planecane
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Re: Physial design of control of 737 jackscrew

Tue Jun 11, 2019 8:19 pm

Here is the video. I want to find out what in the design makes this possible.
https://youtu.be/cQirIH_DuAs
 
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7BOEING7
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Re: Physial design of control of 737 jackscrew

Tue Jun 11, 2019 9:39 pm

milhaus wrote:
I tried it some time ago. I had extended trim wheel handle, held it and then momentarily applied trim command. It is possible to hold it but force is very high and I think it is possible to catch it and hold when it turns.


Just to be clear, there is a technique for grabbing and stopping the trim wheel if you have a runaway and the previous checklist items don’t stop it. The last thing you want to do is try and grab the wheel with the handles extended — you will suffer extreme pain.
 
strfyr51
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Re: Physial design of control of 737 jackscrew

Wed Jun 12, 2019 4:25 am

GalaxyFlyer wrote:
Is it really a single drive motor assembly with a single channel electrical actuator path?

The Jackscrew motor has a Main and Standby Motor. the standby is considerably slower than the main. If either fails It will Cancel the flight.
 
milhaus
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Re: Physial design of control of 737 jackscrew

Wed Jun 12, 2019 3:01 pm

I am just engineer not pilot . I just wanted to feel power of the system..
 
jakubz
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Re: Physial design of control of 737 jackscrew

Wed Jun 12, 2019 4:53 pm

Mods, is it possible/warranted to correct the title to "Physical"? I'm pretty sure that is what OP meant.

Either way, feel free to delete this comment after you consider the change.
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