Stab trim switches on yoke

[Flying_Fox]

Hi All,

There are two stab trim switches on the 737 yoke. The manual says:

Stabilizer Trim Switches (spring-loaded to neutral)
Push (both) -
- electrically commands stabilizer trim in desired direction
- autopilot disengages if engaged.

Why there are two buttons (and switches) if they should be pushed both in the same direction?
I've seen yoke replicas designs with one switch (only imitation of buttons separation) and with two switches under buttons.

Should there be two switches  -separate ones for each button?   

Nick

[Mach7]

Nick,

The up/down trim switch is a split switch arrangement as one half of the switch operates the electrical power to the trim motor, and the other half engages the clutch mechanism to allow operation. Therefore both left and right halves must be operated together for trim operation.

It is designed as a safety feature to prevent a trim runaway in the event one half of the switch shorts out.

There is a first flight test of the trim system where the pilot engages only one half of the switch at a time to ensure the trim wheel does not move. He then operates  both at the same time (both switches together) and confirms operation.

Jim

[sagrada737]

Great comments on this Jim.  The Boeing Yoke has a "gazillian" wires coming out of its think harness.

As for accomplishing what Jim describes...   You can wire the TRIM as follows:

NOSE UP = White/Red wire (Common) + White/Black/Orange wire

NOSE DOWN = White/Red wire (same Common wire) + White/Orange wire

This results in requiring "both" TRIM switches needing to be pushed (either UP or DOWN) in order to have electrical continuity, which would allow the desired function. However, when wired as above, if you push either TRIM switch separately, nothing will happen (no continuity).

Of course, this assumes that you have the same Boeing Yoke wire harness, but I believe they should be the same.

Mike

[Flying_Fox]

Thank you Jim for the detailed explanation!
Now it is clear how to build it. So, I understand it's more like a safety feature - if the pilot accidentally touches only one of switches then nothing happens.

Mike: thanks for input - I will install two switches then.

Nick

[sluyt050]

Thanks Jim for your explanation. It's completely clear now and I will wire my switches as Mike suggested.
Another topic to bookmark.

[sagrada737]

Hello Edward,

I want to suggest that if you follow the "color coding" that I mentioned, please bench check that this is the exact same for you Yoke, and perform a continuity check of wiring to ensure it indeed is the same.   As I said, it should be, but you need to double check.

Mike

[sluyt050]

Hello Edward,

I want to suggest that if you follow the "color coding" that I mentioned, please bench check that this is the exact same for you Yoke, and perform a continuity check of wiring to ensure it indeed is the same.   As I said, it should be, but you need to double check.

Mike

Hello Mike,

I wrote down the color coding so it should be easy. One disadvantage, I have open my flight deck again (removing J-rails etc.) to be able to cut and splice wires before my Leo Bodnar card. Lucky for you as I could easily make additional measurements and/or pictures if you wish. Clever hint Mike!

Cheers,

[sagrada737]

Hello Edward,

Sorry you have to dis-assemble everything, but it will be a good chance to double check things down there.  If you do have a chance to take more photos and measurements, that would be great.  Have fun!

Mike

[Aerosim Solutions]

Hi guys, I recently fitted real Boeing yokes and I have the wiring code documented, I will post it here tomorrow when I get home!
Cheers Gwyn

[sagrada737]

Thanks Gwyn...   It will be interesting to compare the color coding that I have on my real 737 Yoke.  If they are the same, then it indeed bodes well for the above discussion.

The following from the FDS Forum is what I referenced, and I confirmed that it is the same as my real 737 Yokes for the functions that I needed, which are:  TRIM; A/P Disconnect;  and PPT for COMM.

It turns out that within the wire harness, there are "extra wires" that could be used for other purposes if desired, eg. LED Map Light, or whatever.  So you may want to make provisions for future access while the Yoke is "open".

http://www.flightdecksolutions.com/forum/viewtopic.php?t=2315&sid=d166678a339b0085849c49a95480b917

Mike

[Aerosim Solutions]

Hi all, my yokes are from a B737-300 series and the wiring code is as follows -

ELEVATOR TRIM
White/Red = Common
White/Orange = Trim DOWN
White/Black/Orange = Trim UP
White/Violet = not used but it does make with White/Red in both directions (probably for the clutch as mentioned in an earlier post) Not needed in FS
[smg id=1009]

PUSH TO TALK ROCKER SWITCH
White/Black/Brown = Common
White/Grey = Intercom
White/Red/Black = Mic

AUTOPILOT DISENGAGE (This is a three pole switch ie: 3 pairs of wires)
White/Blue + White/Black/Yellow = A/P Disengage (This pair make when button is pressed)
White/Yellow + White/Green is normally closed, (This pair opens when button pressed)
White/Black/Green + White/Brown is normally closed, (This pair opens when button pressed)

Cheers Gwyn

[sluyt050]

Hello Edward,

Sorry you have to dis-assemble everything, but it will be a good chance to double check things down there.  If you do have a chance to take more photos and measurements, that would be great.  Have fun!

Mike

Hi Mike,

I have managed to reroute my wires right at the trim switches so opening up my flight deck won't be necessary anymore saving me a lot of work. Consequently I cannot produce additional pictures.

[sagrada737]

Hi Edward,

No problem...  I'm glad you didn't have to go to extra trouble with your Yoke system - good news!  Hopefully, the wiring comments will work for you.

Mike