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Boeing switches

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woxofswa

Joined: 12 Aug 2008
Posts: 349
Location: AZ

Posted: Tue Jul 02, 2013 9:50 am Post subject: Boeing switches

Greetings.  I am trying to infuse a couple of little touches of my day job into my RV-10 project.  To that end, I have purchased the yoke of a B737 and cut off the uprights to use as the stick grips of my -10.



I would obviously like to use the original Boeing pitch trim, PTT, and A/P disconnect switches if possible for the left upright.  I will just wire in off the shelf switches for the right one.



The PTT, and A/P disco should be easy, but the Pitch trim might be a problem.  Boeing uses a two part switch mechanism and there are four leads that come from the module.  Here is the wiring diagram from the trim module itself.







What I am trying to do is interface these four leads into a Safety Trim three lead connection.  Keeping the split switch capability would be okay, but not necessary.



Any comments or suggestions would be greatly appreciated.

_________________
Myron Nelson
Mesa, AZ
Flew May 10 2014

peter(at)sportingaero.com
Guest

Posted: Tue Jul 02, 2013 11:36 am Post subject: Boeing switches

Myron,



Without the logic backing up how this is connected its difficult to 

suggest how to use both switches. I would connect common ground (pin 7 I 

think) to WHT/Vxx (connected to A1 & A3) and the trim up/down inputs to 

B3 (WHT/GRN I think) and to B1 (WHT/BLU/ORN?), leaving the WHT/RED not 

connected. You would be using the top switch only, but I don't think 

that's a problem in service.



Regards, Peter



On 02/07/2013 18:50, woxofswa wrote:

 	  Quote: 	
		   



 Greetings.  I am trying to infuse a couple of little touches of my day job into my RV-10 project.  To that end, I have purchased the yoke of a B737 and cut off the uprights to use as the stick grips of my -10.



 I would obviously like to the Boeing pitch trim, PTT, and A/P disconnect switches if possible.



 The latter two should be easy, but the Pitch trim might be a problem.  Boeing uses a two part switch mechanism and there are four leads that come from the module.



   (http://s125.photobucket.com/user/myronnelson/media/photo_zpsfd9c6ca8.jpg.html)



 What I am trying to do is interface these four leads into a Safety Trim three lead connection.  Keeping the split switch capability would be okay, but not necessary.



 Any comments or suggestions would be greatly appreciated.



 --------

 Myron Nelson

 Mesa, AZ

 Emp completed, QB wings completed, legacy build fuse on gear.  Finishing kit and FWF kit in progress.





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=403817#403817

edpav8r(at)yahoo.com
Guest

Posted: Tue Jul 02, 2013 6:12 pm Post subject: Boeing switches

Myron,



What is the pinout of the Safety Trim 3-lead connection (i.e. what inputs does it require from the switch)?  I presume there's a common ground and one pin each for trim nose up and trim nose down.  Can you confirm?



Can we get a clearer photo of the switch module legend?



It may be possible to accomplish what you want with relays, or some 4000-series logic ICs, or a simple micro-controller.



I presume your preferred outcome is to retain the "both switches must be closed" logic for the trim switches, as in the Boeing.



Eric

On Jul 2, 2013, at 12:50 PM, "woxofswa" <woxof(at)aol.com> wrote:

 	  Quote: 	
		   Greetings.  I am trying to infuse a couple of little touches of my day job into my RV-10 project.  To that end, I have purchased the yoke of a B737 and cut off the uprights to use as the stick grips of my -10.

 

 I would obviously like to the Boeing pitch trim, PTT, and A/P disconnect switches if possible.

 

 The latter two should be easy, but the Pitch trim might be a problem.  Boeing uses a two part switch mechanism and there are four leads that come from the module.

 

 (http://s125.photobucket.com/user/myronnelson/media/photo_zpsfd9c6ca8.jpg.html)

 

 What I am trying to do is interface these four leads into a Safety Trim three lead connection.  Keeping the split switch capability would be okay, but not necessary.

 

 Any comments or suggestions would be greatly appreciated.

 

 --------

 Myron Nelson

 Mesa, AZ

 Emp completed, QB wings completed, legacy build fuse on gear.  Finishing kit and FWF kit in progress.

edpav8r(at)yahoo.com
Guest

Posted: Tue Jul 02, 2013 7:11 pm Post subject: Boeing switches

Myron,



After looking at this a little more, I have to agree with Peter.  I was definitely over-thinking it!



The "both switches must be closed" logic is internal to the module.  They simply have the two thumb switches wired in series.



Peter's advice below should work.



Eric

On Jul 2, 2013, at 2:35 PM, Peter Pengilly <peter(at)sportingaero.com> wrote:

 	  Quote: 	
		   

 Myron,

 

 Without the logic backing up how this is connected its difficult to suggest how to use both switches. I would connect common ground (pin 7 I think) to WHT/Vxx (connected to A1 & A3) and the trim up/down inputs to B3 (WHT/GRN I think) and to B1 (WHT/BLU/ORN?), leaving the WHT/RED not connected. You would be using the top switch only, but I don't think that's a problem in service.

 

 Regards, Peter

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 03, 2013 7:26 am Post subject: Boeing switches

At 10:11 PM 7/2/2013, you wrote:

 	  Quote: 	
		  



Myron,



After looking at this a little more, I have to agree with Peter.  I 

was definitely over-thinking it!



The "both switches must be closed" logic is internal to the 

module.  They simply have the two thumb switches wired in series.



Peter's advice below should work.



Eric




	


   The split trim switch and the recommended wiring

   is a prophylactic against trim runaway caused by

   failure of a single switch - i.e. both switches

   CAN be operated as a single switch for normal

   operations but BOTH switches have to fail to

   produce a runaway condition. This technique has

   been used on a host of electric trim systems.



   Further the switches can be operated independently

   of each other for pre-flight check  of integrity

   for the opposite switch.  Wiring as recommended

   honors the legacy failure mode effects analysis

   which I've discussed in detail here.



http://tinyurl.com/lpvth8d



   Bob . . .

woxofswa

Joined: 12 Aug 2008
Posts: 349
Location: AZ

Posted: Wed Jul 03, 2013 9:47 am Post subject: Re: Boeing switches

Thanks for the input. I'll try the suggested method. The Safety Trim gives runaway protection so I'm not so concerned about maintaining that from the switch itself. 



I haven't delved into the PTT or A/P disco yet, but hopefully those should interface easily.

_________________
Myron Nelson
Mesa, AZ
Flew May 10 2014

Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu Jul 04, 2013 4:29 am Post subject: Re: Boeing switches

My two cents:



Using two parallel switches to prevent trim runaway seems boneheaded, and wouldn't stand a thorough engineering fault-tree review. Furthermore I'd bet that it never prevented ANY problem from occurring. 



Sure, I understand the intention, but I don't think it's good engineering.

_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jul 04, 2013 6:29 am Post subject: Boeing switches

At 07:29 AM 7/4/2013, you wrote:

 	  Quote: 	
		  



My two cents:



Using two parallel switches to prevent trim runaway seems 

boneheaded, and wouldn't stand a thorough engineering fault-tree 

review. Furthermore I'd bet that it never prevented ANY problem from occurring.


	


   Whether or not a problem was demonstrably

   prevented is irrelevant to the reliability

   study. Good FMEA mitigation assumes that the

   part WILL fail. Working out a logical means

   by which single failures might go undetected

   for no more than a single tank full of fuel

   has been a time-honored legacy design goal.



 	  Quote: 	
		  Sure, I understand the intention, but I don't think it's good engineering.


	


   they are not used in PARALLEL. In the Boeing

   case, they are in series with one side controlling

   direction, the other side in SERIES controlling power.

   Each side can be explored for functionality

   in pre-flight, neither side capable of causing

   a runaway-by-sticking.



   Beech (and many others) uses the two spdt

   switches to handle one half of a PM motor UP-OFF-

   DN switching duty. Again, both sides easily

   evaluated in pre-flight, neither side capable

   of causing a runaway-by-sticking.



   Lear (and many others) uses the 'coolie hat' to

   drive relays  that DID offer a possibility for runaway-by-

   sticking. But their systems architecture included

   a wheel-master-disconnect that removed power from

   all trim systems along with the autopilot. I AM

   aware of trim runaways in aircraft fitted with

   wheel master disconnects that would not have happened

   if the airplane had been fitted with the split

   trim switch where the WMD switch provided the

   mitigation.



   Bob . . .

jay(at)horriblehyde.com
Guest

Posted: Thu Jul 04, 2013 10:05 am Post subject: Boeing switches

Good argument Bob....  I use a switch on the panel to power the trim, but

that may be a bit far away for a quick reaction, before the trim lurks all

the way to the end.



Jay

--

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jul 04, 2013 2:22 pm Post subject: Boeing switches

At 01:05 PM 7/4/2013, you wrote:

 	  Quote: 	
		  



Good argument Bob....  I use a switch on the panel to power the trim, but

that may be a bit far away for a quick reaction, before the trim lurks all

the way to the end.


	


   You would do well to deduce the severity of a

   trim runaway. Is the airplane controllable with

   full trim either up or down?  How fast does the

   trim run with respect to pilot reaction times

   needed to deal with a runaway? Are there stops

   or limits to trim excursion necessary for the full

   range of CG envelopes or will the system produce

   more pitch authority than would ever be necessary

   for normal operations?



   In a Lear, at Mach cruise, the system is manually

   failed and the pilot is obligated to keep hands in

   the lap for 3 seconds. Only then can he move to

   deal with the situation. After achieving a stable,

   safe condition, recorded data is analyzed for

   busting limits on attitude, altitude and airspeed

   excursions.



   The exercise is repeated and trim speeds then

   adjusted until a sweat-free, runaway recovery

   is assured.



   The trim systems I've seen in some TC aircraft

   are so slow that unexpected trims under worst

   case conditions are no big deal. But what's the

   story for YOUR airplane?



   Yes, if trim speeds are brisk, trim authority high

   and you have good reason for not reducing them, then some

   form of inarguable override is indicated. Instructing

   the pilot to pull a breaker for taming a trim runaway

   is generally not acceptable . . . even panel mounted

   switche situated amongst other switches will get

   you frowns from the human factors guys.



   At the same time, find out just how much risk is

   associated with an uncontrolled trim event and

   see if that can be engineered out as opposed to

   stacking emergency ops hardware on top of the

   design.

   Bob . . .

jay(at)horriblehyde.com
Guest

Posted: Mon Jul 08, 2013 12:33 am Post subject: Boeing switches

That is exactly what I will be doing once the aircraft is flying; testing

how heavy the control forces are with the trim at either limit; on a

'step-by-step' basis until full travel is reached- then I can decide if any

further circuitry is merited, or perhaps a change in the mechanical trim

arrangement.  That is what I like so much about OBAM aircraft; with a bit of

careful observation and rational thinking we can determine and apply the

solutions ourselves.. 



Jay



--

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