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GAMarr(at)charter.net
Guest

Posted: Fri Sep 28, 2007 6:12 am Post subject: FAA Policy on Circuit Breakers

Bob



You may find this FAA Designee Newsletter of interest.



http://www.faa.gov/other_visit/aviation_industry/designees_delegations/desig

nee_news/designee_updates/



Giff Marr

Rhino

Joined: 01 Aug 2007
Posts: 56
Location: Xenia, Ohio

Posted: Fri Sep 28, 2007 7:03 am Post subject: Re: FAA Policy on Circuit Breakers

 	  GAMarr(at)charter.net wrote: 	
		  Bob



You may find this FAA Designee Newsletter of interest.



http://www.faa.gov/other_visit/aviation_industry/designees_delegations/desig

nee_news/designee_updates/



Giff Marr 	
This link goes directly to the newsletter in question.



http://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_news/designee_updates/media/2007/2007_10_Update.pdf

nuckolls.bob(at)cox.net
Guest

Posted: Fri Sep 28, 2007 9:18 am Post subject: FAA Policy on Circuit Breakers

At 09:07 AM 9/28/2007 -0500, you wrote:



 	  Quote: 	
		  



Bob



You may find this FAA Designee Newsletter of interest.


	




http://tinyurl.com/2hpesb



  Yes, it was interesting in a disappointing sort of

  way. Here's an analysis of the document:



http://aeroelectric.com/articles/Testing_in_Aircraft_with_EFID.pdf



  Bob . . .



        ----------------------------------------

        ( "Problems are the price of progress. )

        ( Don't bring me anything but trouble. )

        ( Good news weakens me."               )

        (              -Charles F. Kettering-  )

        ----------------------------------------

hooverra(at)verizon.net
Guest

Posted: Fri Sep 28, 2007 4:06 pm Post subject: FAA Policy on Circuit Breakers

I must agree with Bob on his response to this but an even simpler 

analysis is that the writer missed half of the breaker definition and 

assumed design intent!



"AC 23-17B, System and Equipment Guide for Certification of Part 23

Airplanes, (Amendment 23-49 and Subsequent) For part 23 applications,

the definitions of a switch and a circuit breaker are as follows: 1) A

switch is a device for opening and closing or for changing the

connection of a circuit; 2) A circuit breaker is a device designed to

open and close a circuit by non-automatic means and to open the circuit

automatically at a predetermined overload of current, without injury to

itself when properly applied within its rating. Thus, circuit breakers

used for operational functions are not acceptable because they are not

performing their intended function, which is protection against

overloads. "



According to the above definition the functions of a breaker are *"open 

and close a circuit by non-automatic means" *and *"to open the circuit

automatically at a predetermined overload of current"*. To conclude 

using the breaker as a switch is not acceptable assumes that only the 

second part of the definition is intended. If the breaker is the 

pullable type then switching is an intended function. If the designers 

intent was to restrict the use of the breaker as a switch then a 

non-pullable breaker could be applied. I would assume that if the intent 

was to restrict the switching function of a pullable breaker then a 

caution should be applied near the breaker or at least in the operation 

instructions.



Just my two cents.



-- 

Ralph C. Hoover

RV7A 

hooverra at verizon dot net

nuckolls.bob(at)cox.net
Guest

Posted: Fri Sep 28, 2007 6:06 pm Post subject: FAA Policy on Circuit Breakers

At 08:06 PM 9/28/2007 -0400, you wrote:



 	  Quote: 	
		  



I must agree with Bob on his response to this but an even simpler analysis 

is that the writer missed half of the breaker definition and assumed 

design intent!



"AC 23-17B, System and Equipment Guide for Certification of Part 23

Airplanes, (Amendment 23-49 and Subsequent) For part 23 applications,

the definitions of a switch and a circuit breaker are as follows: 1) A

switch is a device for opening and closing or for changing the

connection of a circuit; 2) A circuit breaker is a device designed to

open and close a circuit by non-automatic means and to open the circuit

automatically at a predetermined overload of current, without injury to

itself when properly applied within its rating. Thus, circuit breakers

used for operational functions are not acceptable because they are not

performing their intended function, which is protection against

overloads. "



According to the above definition the functions of a breaker are *"open 

and close a circuit by non-automatic means" *and *"to open the circuit

automatically at a predetermined overload of current"*. To conclude using 

the breaker as a switch is not acceptable assumes that only the second 

part of the definition is intended. If the breaker is the pullable type 

then switching is an intended function. If the designers intent was to 

restrict the use of the breaker as a switch then a non-pullable breaker 

could be applied. I would assume that if the intent was to restrict the 

switching function of a pullable breaker then a caution should be applied 

near the breaker or at least in the operation instructions.



Just my two cents.



--

Ralph C. Hoover


	


    Interesting take. I missed that. I'm trying to put the

    gray-matter around:



    "Thus, circuit breakers used for operational functions are

    not acceptable because they are not performing their

    intended function, which is protection against overloads."



    Does this mean that IF you use a breaker for any purpose,

    it's first function is for protection and switching is

    secondary? Therefore, the antithesis would suggest that

    if one uses a breaker primarily for it's capabilities

    as a switch and intent for protection is claimed,

    then the designer is to be shot at dawn?



    Bob . . .

kuffel(at)cyberport.net
Guest

Posted: Sat Sep 29, 2007 5:56 am Post subject: FAA Policy on Circuit Breakers

The FAA sez:



"A circuit breaker is a device designed to open and close a 

circuit by non-automatic means and to open the circuit 

automatically at a predetermined overload of current, without 

injury to itself when properly applied within its rating. Thus, 

circuit breakers used for operational functions are not 

acceptable because they are not performing their intended 

function, which is protection against overloads."



This is a typical case of bureaucratic blindness.  It ignores the 

existence of breakers *designed* to also be used as switches. 

The hydraulic/magnetic series from Airpax and Carling cost $15 - 

$20 each.  I claim their trip point is more stable than the 

ambient temperature sensitive thermal types we typically use. 

And the lower parts count of combining the switch and breaker 

function simplifies wiring which almost by definition increases 

reliability.



This ability to use technologies more modern than W.W.II is yet 

another advantage of OBAM aircraft.



Tom Kuffel

handainc(at)madisoncounty
Guest

Posted: Sat Sep 29, 2007 7:50 am Post subject: FAA Policy on Circuit Breakers

Which part numbers do you recommend for aircraft 12 VDC use?



M. Haught



The Kuffels wrote:

 	  Quote: 	
		   

 <kuffel(at)cyberport.net>



 The FAA sez:



 "A circuit breaker is a device designed to open and close a circuit by 

 non-automatic means and to open the circuit automatically at a 

 predetermined overload of current, without injury to itself when 

 properly applied within its rating. Thus, circuit breakers used for 

 operational functions are not acceptable because they are not 

 performing their intended function, which is protection against 

 overloads."



 This is a typical case of bureaucratic blindness.  It ignores the 

 existence of breakers *designed* to also be used as switches. The 

 hydraulic/magnetic series from Airpax and Carling cost $15 - $20 

 each.  I claim their trip point is more stable than the ambient 

 temperature sensitive thermal types we typically use. And the lower 

 parts count of combining the switch and breaker function simplifies 

 wiring which almost by definition increases reliability.



 This ability to use technologies more modern than W.W.II is yet 

 another advantage of OBAM aircraft.



 Tom Kuffel

aerobubba(at)earthlink.ne
Guest

Posted: Sat Sep 29, 2007 8:14 am Post subject: FAA policy on circuit breakers

Hi All-

  >  Interesting take. I missed that. I'm trying to put the

  >  gray-matter around:

 	  Quote: 	
		  

    "Thus, circuit breakers used for operational functions are

    not acceptable because they are not performing their

    intended function, which is protection against overloads."



    Does this mean that IF you use a breaker for any purpose,

    it's first function is for protection and switching is

    secondary? Therefore, the antithesis would suggest that

    if one uses a breaker primarily for it's capabilities

    as a switch and NO intent for protection is claimed,

    then the designer is to be shot at dawn?


	




Why, yes- of course!



My take on the FedWords is that CB's are not to be used as switches as a matter of course.  However, there have been times when an electrical problem has caused smoke / fire without drawing adequate current to trip a CB.  For example, a low power component inside a high powered box could fail dramatically, causing fire / smoke without tripping the protection for the box.  (think Swissair)  SOP for these situations is to make ones way through the electrical system, pulling breakers until the faulty circuit is isolated and depowered.  Hence, the breaker is being used for an emergency, as opposed to operational, function.



regardz- 

Glen Matejcek

nuckolls.bob(at)cox.net
Guest

Posted: Sat Sep 29, 2007 8:47 am Post subject: FAA Policy on Circuit Breakers

At 10:49 AM 9/29/2007 -0500, you wrote:



 	  Quote: 	
		  

<handainc(at)madisoncounty.net>



Which part numbers do you recommend for aircraft 12 VDC use?


	


   ANY breaker you find in the catalogs will function

   as expected for the purpose of keeping your airplane

   from catching fire. It's a matter of how much $time$

   and panel space you are willing to invest in procuring,

   installing, wiring and carrying them around for the

   lifetime of the airplane.



   The breakers of choice for most builders is the same

   device used in the majority of type certified aircraft.

   Exemplar devices look like this:



   http://www.aeroelectric.com/Pictures/Breakers/circuitbreakers.jpg



   and are offered by



http://www.bandc.biz/cgi-bin/ez-catalog/cat_display.cgi?7X358218#cb25



http://aircraftspruce.com/menus/el/circuitbreakers.html



http://steinair.com/circuitbreakers.htm



http://www.wicksaircraft.com/catalog/product_detail.php/pid=1826~subid=629/index.html

   The greatest return on investment is to not use fuses

   in lieu of breakers:



http://www.aeroelectric.com/Pictures/Fuses/fuseblks.jpg



http://www.aeroelectric.com/Pictures/Fuses/s889.jpg



   Bob . . .

AV8ORJWC

Joined: 13 Jul 2006
Posts: 1149
Location: Aurora, Oregon "Home of VANS"

Posted: Sat Sep 29, 2007 10:36 am Post subject: FAA policy on circuit breakers

Glen - thanks for clear language to help in "To use or not use" CBs.  In

our air transport aircraft, a flight crewmember can "Open"  closed

circuit but only on rare occasion close a CB which is provided for its

primary purpose - to protect the wire.  No one should assume the CB is

installed to protect the end component(s).



Trained maintenance personnel operating on the ground with the flight

formally terminated may then troubleshoot and close an errant CB.



The CBs are designed and constructed to aid in Operator assisted

"opening".  They are assumed to always be in the ready to serve their

primary purpose "to protect the wire run" when Closed.  The Fed requires

that our technical department brief the pilots not to close them except

in isolated and pre-approved in-flight conditions.



We are briefed that they are only used as switches and moved between

"Off (open) / ON (closed)" while on the ground, in maintenance mode and

with other personnel present.



To second a comment that was projected earlier, I do not consider

installation of CBs on the primary flight instrument panel to be a

prudent use of valuable real estate. Nor are they particularly

aesthetically pleasing to look at for the life of the panel.  Leaves a

conundrum as to where to place them, how easy to read/see which one pops

Open and how easy to open them when necessary as a procedural switch

during that rare occurrence.



John Cox



--

handainc(at)madisoncounty
Guest

Posted: Sat Sep 29, 2007 1:03 pm Post subject: FAA Policy on Circuit Breakers

I guess I wasn't clear, as I was referring to the breakers that are also 

designed as switches, re: the Airpax and Carling units.  I googled a 

site that has them, and was totally boggled by the variety and number of 

possibilities.  Makes sense to me to cut down on parts count, and every 

where I need a switch and  a circuit break, I would considered using one 

of those units.  However, I can visualized HOURS poring through that 

site trying to find a suitable 12 volt unit.  I'm very familiar with 

aircraft circuit breakers and have several surplus breakers in my stock 

of parts for replacement of units in my certified Pacer.  Just thought 

it was interesting to be able to cut down on complexity, connections and 

number of parts by using the dual purpose breakers / switches. 

I'm building a Bearhawk and am primarily interested in reliability and 

simplicity (I believe both of those characteristics go hand in had from 

my personal experience with airplane electrical systems!). 

However, Bob, I certainly appreciate you taking the time and making the 

effort to educate a "newby".  By the way - I attended one of your 

seminars nearly 20 years ago now, and that is where I obtained my basic 

education on aircraft wiring.  I was glad to find this site as I am 

starting to plan my electrical system for my Bearhawk.

M. Haught

Robert L. Nuckolls, III wrote:

 	  Quote: 	

 <nuckolls.bob(at)cox.net>

 At 10:49 AM 9/29/2007 -0500, you wrote:

> 

> <handainc(at)madisoncounty.net>

>

> Which part numbers do you recommend for aircraft 12 VDC use?

   ANY breaker you find in the catalogs will function

   as expected for the purpose of keeping your airplane

   from catching fire. It's a matter of how much $time$

   and panel space you are willing to invest in procuring,

   installing, wiring and carrying them around for the

   lifetime of the airplane.

   The breakers of choice for most builders is the same

   device used in the majority of type certified aircraft.

   Exemplar devices look like this:

   http://www.aeroelectric.com/Pictures/Breakers/circuitbreakers.jpg

   and are offered by

 http://www.bandc.biz/cgi-bin/ez-catalog/cat_display.cgi?7X358218#cb25

 http://aircraftspruce.com/menus/el/circuitbreakers.html

 http://steinair.com/circuitbreakers.htm

 http://www.wicksaircraft.com/catalog/product_detail.php/pid=1826~subid=629/index.html 

   The greatest return on investment is to not use fuses

   in lieu of breakers:

 http://www.aeroelectric.com/Pictures/Fuses/fuseblks.jpg

 http://www.aeroelectric.com/Pictures/Fuses/s889.jpg

   Bob . . .

khorton01(at)rogers.com
Guest

Posted: Sat Sep 29, 2007 3:42 pm Post subject: FAA policy on circuit breakers

On 29 Sep 2007, at 14:34, John W. Cox wrote:



 	  Quote: 	
		   To second a comment that was projected earlier, I do not consider

 installation of CBs on the primary flight instrument panel to be a

 prudent use of valuable real estate. Nor are they particularly

 aesthetically pleasing to look at for the life of the panel.  Leaves a

 conundrum as to where to place them, how easy to read/see which one  

 pops

 Open and how easy to open them when necessary as a procedural switch

 during that rare occurrence.


	




I agree that CBs should be located so that they can be confirmed as  

IN during pre-flight checks.  But once you are in flight, I am not  

convinced that we need easy view or access.  Typically the first sign  

of a problem is when you notice that some piece of equipment is no  

longer functioning.  At that point, does it really matter whether the  

failure is caused by a popped CB, a wire that has come off a  

terminal, or a failure of the item itself?  Why do you need to be  

able to easily see the CB?



If a piece of equipment has a potential failure mode that would  

require removing its power supply, it should probably be wired  

through a switch, or you should plan on killing power to the bus that  

powers it (only acceptable if you can accept the loss of all other  

items on that bus).  You shouldn't plan on pulling the CB, as they  

can sometimes be very difficult to grab on to.



--

Kevin Horton         RV-8 (finishing kit)

Ottawa, Canada

http://www.kilohotel.com/rv8

Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Sat Sep 29, 2007 7:13 pm Post subject: FAA policy on circuit breakers

Hi John,

Remember that a number of type certificated aircraft are equiped with 

circuit breakers such as the P&B W31 series that are intended to be 

routinely used for switching devices on and off, as well as providing 

circuit protection. Of course that is from the CAA days, when the feds 

actually had some folks that knew something.  Lets see, my TC aircraft 

has boost pump, pitot heat, rotating beacon, nav lights and landing 

lights on such breakers on its type certificate, and a couple more added 

later for STC approved equipment.The only failure mode I have seen is 

the internal springs get weak with age and they can nuisance trip, at 

which point they are due for replacement.



Kelly



John W. Cox wrote:

[quote] 



 Glen - thanks for clear language to help in "To use or not use" CBs.  In

 our air transport aircraft, a flight crewmember can "Open"  closed

 circuit but only on rare occasion close a CB which is provided for its

 primary purpose - to protect the wire.  No one should assume the CB is

 installed to protect the end component(s).



 Trained maintenance personnel operating on the ground with the flight

 formally terminated may then troubleshoot and close an errant CB.



 The CBs are designed and constructed to aid in Operator assisted

 "opening".  They are assumed to always be in the ready to serve their

 primary purpose "to protect the wire run" when Closed.  The Fed requires

 that our technical department brief the pilots not to close them except

 in isolated and pre-approved in-flight conditions.



 We are briefed that they are only used as switches and moved between

 "Off (open) / ON (closed)" while on the ground, in maintenance mode and

 with other personnel present.



 To second a comment that was projected earlier, I do not consider

 installation of CBs on the primary flight instrument panel to be a

 prudent use of valuable real estate. Nor are they particularly

 aesthetically pleasing to look at for the life of the panel.  Leaves a

 conundrum as to where to place them, how easy to read/see which one pops

 Open and how easy to open them when necessary as a procedural switch

 during that rare occurrence.



 John Cox



 --

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

nuckolls.bob(at)cox.net
Guest

Posted: Sun Sep 30, 2007 11:00 am Post subject: FAA policy on circuit breakers

At 08:11 PM 9/29/2007 -0700, you wrote:



 	  Quote: 	
		  



Hi John,

Remember that a number of type certificated aircraft are equiped with 

circuit breakers such as the P&B W31 series that are intended to be 

routinely used for switching devices on and off, as well as providing 

circuit protection. Of course that is from the CAA days, when the feds 

actually had some folks that knew something.  Lets see, my TC aircraft has 

boost pump, pitot heat, rotating beacon, nav lights and landing lights on 

such breakers on its type certificate, and a couple more added later for 

STC approved equipment.The only failure mode I have seen is the internal 

springs get weak with age and they can nuisance trip, at which point they 

are due for replacement.


	


   The W31 switch breakers don't exercise the

   components that calibrate circuit breaker

   trip when the switch portion is operated.

   See:



http://www.aeroelectric.com/Pictures/Breakers/W31_3.jpg

http://www.aeroelectric.com/Pictures/Breakers/W31_2.jpg

http://www.aeroelectric.com/Pictures/Breakers/W31_1.jpg



   Note that the trip latch and heater strut

   maintain a constant, non-moving relationship

   with each other as the switch side transitions

   the contacts from closed to open.



   Here's an excerpt from a posting I did about 2 years

   ago on this product:



---------------



These are the switch breakers I've mentioned on several occasions

in the past and featured in the comic book at:



http://aeroelectric.com/articles/Breakers/Breakers.html



The failure mode cited is NOT a short but an open that causes

the load path to shift. In the picture at:



http://www.aeroelectric.com/Pictures/Breakers/W31_3.jpg



you can see the two copper wire jumpers across moving

joints made up of little ropes of exceedingly fine strands

of wire. These are RATED at many thousands of cycles on the

data sheet . . . but it seems that the sum total of environmental

stresses as-installed are such that these critters fail in a few

thousand cycles. The failure begins as a breaking (or fusing)

of the last remaining strands of jumper.



This happens in the upper jumper. When that pathway is lost,

the switch-breaker's current still flows through the spring

that is intended to open the switch contacts when the lever

is moved to OFF. This spring is capable of carrying current

in the switch-breakers of smaller size . . . 5A and 10A but

in the Prop DeIce circuit of a Bonanza or Baron, the current is

so great as to cause the spring to glow cherry red and cause

surrounding plastic cases to char. This is what produces the

smoke. The amount of smoke was not nearly worthy of the doom

and gloom inferred by the AD.



The "fix" was to insert the plastic insulating sheet you can

see in the inside, upper left corner of the metal frame. This

prevents the spring from becoming a secondary load path and

the breaker fails passively by simply allowing the downstream

load to go OFF.



Cessna's decision to use this product as a master switch-breaker

was unfortunate. There was no need for this device to be a breaker too.

Selection of this part drove parts count up and drove reliability

down. If you must have an avionics master switch, the plain-jane

toggle is the device of choice. Better yet, leave it out entirely.

-------------------------



    Switch=breakers are an entirely different breed of

    cat. They're not vulnerable to the adverse performance

    conditions demonstrated by routinely using a pull-type

    circuit breaker as an operating switch. This is also

    a separate issue from the discussion about adverse

    effects on a switched system whether by pulling a breaker

    or interrupting power by any other means.



    Bob . . .

MaxNr(at)aol.com
Guest

Posted: Sun Sep 30, 2007 9:19 pm Post subject: FAA Policy on Circuit Breakers

I have been lurking on this list for a while trying to learn a little about electrical systems, but I cannot bite my tongue any longer. As stated in AC 120-80, the manufacturer many times in the Flight Manual includes the use of C.B.s to disable systems to check systems on run up or to deal with in flight malfunctions or emergencies. Some C.B.s have colored collars to help in locating in emergencies. Available at ACS, Sporties and other fine shops. The last A/C (Sikorsky 76 series) that I flew before retiring was such an A/C. Most C.B.s don't even have a corresponding on/of switch. I have a lot of war stories where we got things back under control by finding and pulling the right C.B. Chip lights for example. As for resetting in flight: About 25 years ago, I was single pilot, had passengers on board and trying to land in heavy rain. The Pilot's Windshield Wiper C.B. repeatedly popped. I used my emergency options as PIC to hold the C.B. in and chance a fire in order to land (one handed) safely. You got to do what you got to do. There was no fire, maintenance readjusted the wiper tension and all was fine. As for simulating EFIS failure, they all have a brightness knob and you can make it go dark that way. It will then go into reversion mode for you. I discussed the subject bulletin this evening with my friend who is a Maint Inspector/AI for my old company. He lost it. The only thing that he said that was printable on a family web site was something about "know nothing white shirts hiding in the FAA office." Sorry for pouring gas on a fire. I'm retired remember.

 

 RTD

**************************************

 See what's  [quote][b]

wjoke(at)shaw.ca
Guest

Posted: Sun Sep 30, 2007 10:17 pm Post subject: FAA policy on circuit breakers

Kevin Horton wrote:

 	  Quote: 	

 <khorton01(at)rogers.com>

 On 29 Sep 2007, at 14:34, John W. Cox wrote:

> To second a comment that was projected earlier, I do not consider

> installation of CBs on the primary flight instrument panel to be a

> prudent use of valuable real estate. Nor are they particularly

> aesthetically pleasing to look at for the life of the panel.  Leaves a

> conundrum as to where to place them, how easy to read/see which one pops

> Open and how easy to open them when necessary as a procedural switch

> during that rare occurrence.

 I agree that CBs should be located so that they can be confirmed as IN 

 during pre-flight checks.  But once you are in flight, I am not 

 convinced that we need easy view or access.  Typically the first sign 

 of a problem is when you notice that some piece of equipment is no 

 longer functioning.  At that point, does it really matter whether the 

 failure is caused by a popped CB, a wire that has come off a terminal, 

 or a failure of the item itself?  Why do you need to be able to easily 

 see the CB?

 If a piece of equipment has a potential failure mode that would 

 require removing its power supply, it should probably be wired through 

 a switch, or you should plan on killing power to the bus that powers 

 it (only acceptable if you can accept the loss of all other items on 

 that bus).  You shouldn't plan on pulling the CB, as they can 

 sometimes be very difficult to grab on to.

 -- 

 Kevin Horton         RV-8 (finishing kit)

 Ottawa, Canada

 http://www.kilohotel.com/rv8

One example of the situation Kevin describes in his second paragraph is 

the aileron trim on the FAR 25 aircraft I fly in my 'day' job. If it 

quits dead due to motor burn out somewhere near the neutral position 

that's one thing. If the operating rocker switch fails so as to drive it 

to full travel in one or the other direction that's another and probably 

more demanding situation with obvious aircraft handling implications. 

The manufacturer's procedure for a runaway trim is to attempt to use the 

operating switch to drive the trim to neutral and then pull the 

associated circuit breaker to stop the trim in something close to the 

neutral position. In this case there are lots of circuit breakers 

relatively easy to access from the pilot seats so not really an issue to 

add one more for the aileron trim for the once in 10,000 (or whatever) 

flying hour that it needs to be pulled to disable the trim. The 

alternative would be to add a dedicated trim disable switch that would 

add parts and likely decrease overall reliability.

Of note, just about all the other 120 or so CBs on the airplane are 

treated as "never reset except in dire emergency" devices.

Jim Oke

RV-6A

Wpg., MB

nuckolls.bob(at)cox.net
Guest

Posted: Mon Oct 01, 2007 8:03 am Post subject: FAA policy on circuit breakers

 	  Quote: 	
		  >--

>Kevin Horton         RV-8 (finishing kit)

>Ottawa, Canada

>http://www.kilohotel.com/rv8

One example of the situation Kevin describes in his second paragraph is 

the aileron trim on the FAR 25 aircraft I fly in my 'day' job. If it quits 

dead due to motor burn out somewhere near the neutral position that's one 

thing. If the operating rocker switch fails so as to drive it to full 

travel in one or the other direction that's another and probably more 

demanding situation with obvious aircraft handling implications. The 

manufacturer's procedure for a runaway trim is to attempt to use the 

operating switch to drive the trim to neutral and then pull the associated 

circuit breaker to stop the trim in something close to the neutral 

position. In this case there are lots of circuit breakers relatively easy 

to access from the pilot seats so not really an issue to add one more for 

the aileron trim for the once in 10,000 (or whatever) flying hour that it 

needs to be pulled to disable the trim.


	




 	  Quote: 	
		  The alternative would be to add a dedicated trim disable switch that would 

add parts and likely decrease overall reliability.


	


   Yes, when parts count goes up, probability of a failure in that

   system or sub-system goes up . . . but let us take care that we

   do not equate high "system reliability" with low "cost of ownership".

   A runaway trim with poor mitigation features translates to lower

   system reliability because the pilot is likely to "break a sweat"

   or worse, wrinkle the airplane and occupants.



   A failure in a rapid response shutdown system (wheel master disconnect)

   is pre-flight detectable and has a minor effect on pilot workload

   for the various failure mode effects. And yes, the presence of extra

   electro-whizzies has a calculable if not demonstrable impact on cost

   of ownership . . . but better the owner curses designers, suppliers

   and mechanics in the maintenance hangar than while airborne.



   I have to laugh when I read some of the "approved" runaway trim

   procedures in various operating manuals. The often ignored fundamental

   requirement for comfortable outcome of the event is: "Perceive, interpret,

   refer to procedure, run the troubleshooting matrix and ultimately find

   and pull a breaker . . . in four seconds or less."



   A/p and pitch trim runaway qualification requirements for the last

   program I worked called for a pilot to put hands in lap, test engineer

   generates a simulated runaway, pilot WAITS three seconds minimum before

   moving to mitigate the situation. When order is restored, the airplane

   shall not have departed from stable flight conditions by specified

   maximums."



   I'm aware of ONLY two ways to make this work within the 3-second window.

   (1) automatic failure sensing and control - VERY expensive and level

   A software or (2) slow the motors down such that the 3-second delay

   + action time to hit the wheel master disconnect produces a scenario

   acceptable to everyone involved. If anyone has another, simpler and/or

   more elegant idea, I'd be pleased to know of it and so would my customers!



   Bob . . .

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