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Runaway trim
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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Sat Jun 09, 2007 7:14 am Post subject: Re: Runaway trim

A couple years ago I designed a domino-size circuit that forced the trim motor to neutralize the trim flap upon loss of power. This loss of power could have been caused either by short circuit, a blown fuse or the pilot pulling a breaker in a runaway-trim event. It is easy (maybe easier) to design a trim box circuit that reverts the trim to the "where it was some short time before" position. This seems better.



It does not, however, solve a possible failure inside the MAC/RAC trim box, or linkages, or the control surface, or the tiny control circuit itself. 



I believe there exists a simple solution for those who are really worried about a stuck trim airfoil and would be satisfied with a disconnected trim airfoil--explosive bolts. 



A less exciting alternative would be to arrange a cotter spring pin so that it can be extracted remotely via fishing line or actuating cable from the clevis pin that attaches the trim flap arm to the mechanism.



I agree that keeping it simple is the best approach. Backups for backups leads to a system that has unpredictable failure modes. Making sure the airplane still flies with a jammed trim is good design. Well-designed aircraft should fly with a failure of any single control or trim surface. Otherwise wear a parachute.



"...Beans for supper tonight, six o'clock. 

Navy beans cooked in Oklahoma ham...

Got to eat 'em with a spoon, raw onions

and cornbread; nothing else...."

 --Will Rogers

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

rv9jim(at)juno.com
Guest

Posted: Sat Jun 09, 2007 8:33 am Post subject: Runaway trim

Hi Eric,

        You can also just put in physical limits to the trim travel. That

would give you max min. travel that could be controlled if the system

went to an extreme.

Jim Nelson

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khorton01(at)rogers.com
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Posted: Sat Jun 09, 2007 10:09 am Post subject: Runaway trim

On 9 Jun 2007, at 12:29, James H Nelson wrote:



 	  Quote: 	
		   

 <rv9jim(at)juno.com>



 Hi Eric,

         You can also just put in physical limits to the trim  

 travel. That

 would give you max min. travel that could be controlled if the system

 went to an extreme.


	


There is certainly no point to having more trim travel than is needed  

for the extreme cases in the normal flight envelope.  The nose down  

trim extreme case is VNE at aft CG.  The nose up trim extreme case is  

approach speed at forward CG.  If the trim travel is greater than is  

needed for these two extreme cases, it just makes the runaway  

emergency even more serious, without providing any benefit during  

normal operations.



Kevin Horton         RV-8 (finishing kit)

Ottawa, Canada

http://www.kilohotel.com/rv8

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N1DeltaWhiskey(at)comcast
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Posted: Sun Jun 10, 2007 5:56 am Post subject: Runaway trim

Kevin,



Good point.  Normally, I would tweak the trim into takeoff position which 

would verify proper operation.  I have a checklist item for that purpose, 

but it is easy to see how actual operation of the trim setting could be 

overlooked (e.g., already set at takeoff trim position)..



However, even overlooking this step would not be the equivalent of the trim 

going to extreme and should be totally controllable for slow a go-around and 

landing to address the problem.



Regards, Doug



---

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khorton01(at)rogers.com
Guest

Posted: Sun Jun 10, 2007 8:04 am Post subject: Runaway trim

I'm not sure if I've misunderstood your intended design, or if you  

have misunderstood my point.



I understood the intended design to require two switches to be  

activated to make the trim move.  One switch controls power to the  

whole system, and the other controls the signal to the servo.  The  

idea is that both switches need to fail to trigger a runaway trim  

(and, a wiring fault can't trigger a runaway trim, as there is no  

power in the wires unless switch #1 is selected).



My concern is that if you just use the system normally, you can't  

detect the failure case where switch #1 has failed in the hot  

position.  In this failure case, the trim will still work normally -  

it will move in the selected direction, and it will only move when  

switch #2 is selected.  But, this failure has negated the protections  

provided by this design.  If you are concerned enough about runaway  

trim to install this design architecture, then you should devise some  

sort of periodic test to confirm that switch #1 has not failed in the  

hot position.  E.g., leave switch #1 in the neutral position, and  

push switch #2 - the trim should not move.  If the trim moves, this  

tells you that switch #1 has failed in the hot position.



Kevin



On 10 Jun 2007, at 09:55, Doug Windhorn wrote:



[quote] 

 <N1DeltaWhiskey(at)comcast.net>



 Kevin,



 Good point.  Normally, I would tweak the trim into takeoff position  

 which would verify proper operation.  I have a checklist item for  

 that purpose, but it is easy to see how actual operation of the  

 trim setting could be overlooked (e.g., already set at takeoff trim  

 position)..



 However, even overlooking this step would not be the equivalent of  

 the trim going to extreme and should be totally controllable for  

 slow a go-around and landing to address the problem.



 Regards, Doug



 ---

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Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Sun Jun 10, 2007 9:42 am Post subject: Runaway Trim

I have a Ray Allen Company (RAC) trim servo on each of the three trim tabs  on my Lancair Legacy.  When I was doing my electrical design, I talked with  one of the principals at RAC, who claimed that the design of the servo makes  "runway" within the servo impossible.  Since almost anything is possible, I  interpreted his statement to mean that it was very unlikely.  

  

 I think the most likely way to get runaway trim in my airplane is a stuck  trim switch.  I expect that physically forcing the switch to the  opposite position would likely unstick the switch and allow control of the trim  tab until landing.  

  

 Another, less likely, cause of runway trim would be shorted  wires between the electrical bus and the servo.  I've tried to  minimize that risk by careful termination of the wires and attention  to supporting the wire bundles and preventing chafing.  

  

 I've also tried to simplify the trim systems to keep the parts count  down.  For example, I do not have anything to defeat the copilot's  trim switch; both stick grips are "hot" all the time.   I brief passengers  on what the buttons on the copilot stick grip do and the need to avoid  inadvertent actuation during the preflight passenger briefing.  I do not  have electro/mechanical relays in the system.

   

 For my system, I feel runaway trim is a very low probability risk and  that my time and energy is better focused on other, higher probability  risks.  However, this discussion has reminded me that I need to  complete my flight testing plan to fully explore the edges of the envelope  related to trim travel and controllability.

  

 Best,

 Dennis Johnson

 Lancair Legacy, now  flying     

    [quote][b]

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N1DeltaWhiskey(at)comcast
Guest

Posted: Tue Jun 12, 2007 6:48 am Post subject: Runaway trim

Kevin,



I think you understand the design arrangement just fine.  I understood your 

concern to be if the power switch failed to make contact, not get stuck in 

contact as you describe below.  I would agree that a simple test of the trim 

switch alone should result in no action and could easily be accomplished by 

quickly testing each switch individually - both should result in no trim 

response.



Doug



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