Installation of an Avionics Master Switch - Major or Minor?

[GlennAB1]

True. They (Advisory Circulars) aren't mandatory.

 

[bluerooster]

I do believe the addition of a AV-master would be a minor alteration, but should be documented for later reference. I see no need for the FAA to run the whole gambit of a field approval. the IA should be allowed to make this decision as to airworthiness of the minor change.
in cases like this. JMHO

What about a case where there is a single buss feeding all systems, through a single copper bar. The need to seperate the systems invokes the need to modify the copper buss bar, so that the avionics buss can be seperate from all other systems, and fed through the AV master switch, which is fed from the main buss.
I consider it to be a minor alteration. (but a major PITA ) But it doesn't matter what I think, what would the FAA think?

 

[weirdjim]

What about a case where there is a single buss feeding all systems, through a single copper bar. The need to seperate the systems invokes the need to modify the copper buss bar, so that the avionics buss can be seperate from all other systems, and fed through the AV master switch, which is fed from the main buss.
I consider it to be a minor alteration. (but a major PITA ) But it doesn't matter what I think, what would the FAA think?

Yes, but each system comes off the bus with a separate feed from the copper bar, so all you are doing is inserting a switch and breaker/fuse in series with the wire feed. What is the major in that?

I've found, at least with a REASONABLE PMI (Principal Maintenance Inspector) that you have established a relationship with, if you've got a good plan backed up with well thought out calculations and data that they will approve damn near anything reasonable short of a cast iron bathtub drawn by a flock of black swans.

Jim

 

[Tom-D]

For a part 91, single engine piston unpressurized airplane built under CAR 3 and some early amendments of 23?

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 43.13-2B.pdf

Just because it is acceptable data, does not mean it applies to this application. This AC does not show you how a buss is supposed to be split, Nor does it tell you that splitting a buss is acceptable. Just because some routine is easy to do does not mean it is a minor alteration.

 

[Tom-D]

People,, When the FAA made the policy that the 43-13 could be used as acceptable data for alterations, they specified that it would not be used when the aircraft has support in the form of a manufacturer's manuals. they farther specified that the page and paragraph of the applying data would be quoted.

So... some one quote the acceptable data from the 43-13 that would be used to gain approval of the field approval.

 

[Tom-D]

Yes, but each system comes off the bus with a separate feed from the copper bar, so all you are doing is inserting a switch and breaker/fuse in series with the wire feed. What is the major in that?
Jim

You know well enough that is not how it is done.

 

[bluerooster]

Yes, but each system comes off the bus with a separate feed from the copper bar, so all you are doing is inserting a switch and breaker/fuse in series with the wire feed. What is the major in that?

I've found, at least with a REASONABLE PMI (Principal Maintenance Inspector) that you have established a relationship with, if you've got a good plan backed up with well thought out calculations and data that they will approve damn near anything reasonable short of a cast iron bathtub drawn by a flock of black swans.

Jim

Yes, there are seperate circuit breakers, fed by a single buss bar. (in the scenerio I posted, 1969 PA28-140) But, there is no way to seperate the avionics breakers from the rest, without removing the buss bar, and either replacing it, or modifying it, so that the avionics are fed independently of the rest of the electrical system. Then (after the modification) there must be a feed line from the main buss to the AV master switch, then back to the new avionics buss.

 

[bnt83]

People,, When the FAA made the policy that the 43-13 could be used as acceptable data for alterations, they specified that it would not be used when the aircraft has support in the form of a manufacturer's manuals. they farther specified that the page and paragraph of the applying data would be quoted.

So... some one quote the acceptable data from the 43-13 that would be used to gain approval of the field approval.

Your understanding of 41.13 is much different than mine. That doesn't explain how a note on a drawing references 43.13 practices for both structural fabrication and wiring that alters a 5 year old still in production jet. They STC holder wrote a drawing that covers the critical details needed and then just left it up to the installer to use 41.13 or OEM data to fill in the noncritical gaps.

 

[bnt83]

Says right there we can use it as approved if appropriate.

 

[kontiki]

I really appreciate everyone's insights into my OP. When I quit messing with this thing and start flying it again, I need to start getting around and have a beer with you people. I'm really slow.

I decided that maybe Major/Minor partition isn't really the right way to approach the original question. The answer is likely different for different airplanes, because you start out with different approved configurations. Mine is very simple. My airplane has a simple engine driven rectified alternator with a 60A current limiting breaker and a bus. Installing the switch doesn't even change the the load.

One of the things I don't like about the master, besides the breaker being too big and in the wrong location, is the single point failure possibility. The trade I keep hearing about is single point failure vs electrical spike.

I did look through RTCA DO-160, Environmental Conditions and Test Procedures for Airborne Equipment. The Power Input and Voltage spike test sections necessary for TSO approval aren't really extreme.

I also participate on an Engineer's social network, and I did ask the folks in there if anyone knows of an accurate study that describes the electrical conditions on an old aircraft electrical system like mine. I didn't find anything Googling. Who's profiled the electrical environment on an old ford? I'd like to see what we're all protecting against.

There have been a lot of equipment changes on this airplane and and after seeing what some folks did, I don't trust all those entries on the 337 that say, "within limits." I did workup a complete Electrical Load Analysis to understand what's on every breaker, every bus, and to model the current draw for every phase of flight, just to see how it looked. The 43-13 says continuous load shouldn't exceed 80% of the capacity, it doesn't.

I also wanted to see what modes of flight actually do draw more than the 60A rating of the ALT. I'll workup some ALT FAIL load shed procedures based on it, it's an old alternator! Barring ALT failure, I really don't expect a problem unless I continuously operate the flaps with a hot mike while the landing light is on.

In my case, I only have so many physical positions near the bus, so I'll have to double up the loads on a couple breakers. I did a little matrix workup to make sure that no one CB/circuit failure could take out a primary device and the backup. I'm a little unsure how I'll label those breakers, because I'm keeping the smaller breakers to protect the 22 AWG wiring to the individual radio/device in most cases. I just don't want to deal with trying to shoehorn an 18 AWG wire into a little bitty pin. The bit parts all have a research and lead time.

It's going to be too much work, but I have it apart, I'm probably gonna start Sat if I don't find out something really ugly happens on a power system like this one.

 

[GlennAB1]

I don't see how anyone could think that splitting a bus, and adding a bus tie with a switch or switch/breaker, isn't a change to basic design.

 

[kontiki]

In this case bus is unchanged. In reality is is something of a hodgepodge under there because the original bus bar couldn't be used to connect the breakers with a different profile. All the breakers I'm using are rated to an aviation mil spec, with a manual trip feature.

Loads are either connected directly to a bus via circuit breakers (original) or connected by a large wire and switch/circuit breaker that feeds power to same breakers and same loads. All the loads in question have an on off power switch on the face. Electrically it's the same circuit and it's all optional equipment from the original certification standpoint. Topographically it's a little different, but it's invisible to the electrical system.

 

[Tom-D]

Your understanding of 41.13 is much different than mine. That doesn't explain how a note on a drawing references 43.13 practices for both structural fabrication and wiring that alters a 5 year old still in production jet. They STC holder wrote a drawing that covers the critical details needed and then just left it up to the installer to use 41.13 or OEM data to fill in the noncritical gaps.

What does a STC have to do with a field approval ?
Or do you believe that the 43,13 applies to all aircraft?
The methods in the AC are allowed to be used as acceptable data, that may or may not be approved on the request for a field approval.

 

[GlennAB1]

One of the things I don't like about the master, besides the breaker being too big and in the wrong location, is the single point failure possibility.

With that last thought, I think you hit the nail on the head as to why it should be a major.

 

[Tom-D]

I'm soliciting opinions.

I'm looking at the undocumented installation of an Avionics Master switch. It's a simple 35A current protected SPST switch in series with smaller breakers that power to all the Com, Nav, GPS, and XPDR gear on the airplane.

It does happen to be a 35A switch/breaker installed at the load end of a 12 AWG wire from the power bus. It is too big a breaker to protect that wire. At the load end, it protects only about the last 2"-3" of wire between the switch breaker and the other breakers. The 2ft of wire between the switch breaker and the power bus are completely unprotected.

I'm getting ready to pull it out, but I thought I'd see what consensus is. It's my own airplane, I'm still getting acclimated to the wild west world of GA coming from an airline perspective. I do appreciate the ability to share the experienced insights others.

You don't tell us much, the make and model aircraft would be good info. knowing the skit is to know what is normal wiring and what is not.

 

[kontiki]

You don't tell us much, the make and model aircraft would be good info. knowing the skit is to know what is normal wiring and what is not.

Apologies Tom, it's a Grumman Tiger AA-5B. I didn't assume anyone would know specifics on any particular Type.

 

[kontiki]

With that last thought, I think you hit the nail on the head as to why it should be a major.

Glenn, Outside of the description in Part 43 Appendix A, I understand Major changes as those things likely to impact certification requirements and data specifically approved by the FAA, like flying qualities, engine performance, limitations etc. I don't have a feel for what the structures side of Major/Minor is.

I think reliability is important and electrical system reliability (and engine reliability) especially so in an airplane capable of IFR. I haven't specifically looked for a part 23 or part 25 certification reliability requirements.

 

[Tom-D]

Apologies Tom, it's a Grumman Tiger AA-5B. I didn't assume anyone would know specifics on any particular Type.

It makes a big difference knowing the type of system you are starting with, some aircraft have a simple stud that is powered up by one fused wire from the master relay. others have copper strip that feeds a set of fuses or C/Bs.

Removing a few wires from a single stud type, and placing them on second stud controlled by a switch, would be simply adding a circuit. You did not modify the original stud. (buss) that would be a minor modification.
Cutting a copper strip type and controlling part of it with a switch will be a modification of the original design of the copper strip (buss) and a major alteration.
Adding a Fuse or C/B to to the copper strip type, then adding a second copper strip controlled by a switch is simply adding a circuit which in my opinion would be a minor alteration.
Neither of these will require a load analysis to be approved on a field approval.
Were I asked to approve the return to service in either methods given above, I'd be looking for proper wire size IAW the charts given in the 43,13, and aircraft quality hardware, bundling, and proper tie offs.

 

[Tom-D]

Glenn, Outside of the description in Part 43 Appendix A, I understand Major changes as those things likely to impact certification requirements and data specifically approved by the FAA, like flying qualities, engine performance, limitations etc. I don't have a feel for what the structures side of Major/Minor is.

I think reliability is important and electrical system reliability (and engine reliability) especially so in an airplane capable of IFR. I haven't specifically looked for a part 23 or part 25 certification reliability requirements.

Really, Car 3 or FAR 23 and 21 have no bearing on later modifications. (That is covered by 43-A) Car 3, FAR 21 & 23 only apply to certification of new product seeking a production certificate.

 

[GlennAB1]

Back to my first post; as long as nothing bad happens, it probably doesn't matter. If something bad happens, it matters, and if your name is on it it better be right. Is that why there's no documentation of the alteration?

 

[kontiki]

Really, Car 3 or FAR 23 and 21 have no bearing on later modifications. (That is covered by 43-A) Car 3, FAR 21 & 23 only apply to certification of new product seeking a production certificate.

Tom, I think they do if you try to generalize what requires FAA involvement beyond application of part 43 Appendix A by a mechanic or IA.

I've worked the last 20 years or so as an airline engineer. Major/Minor has been the first consideration in every airline EA or EO I've issued for over 20 years. In one airline we had to attach a logic flow work sheet highlighting our logic flow to every document we released. It's a different environment for similar decisions and limited authority. All I can approve in an airline is FAA Minor. Determining you have the authority is always the first question to answer.

Because of the operator certification rules, we get pulled into more things than is encompassed by Part 43, for example, we regularly see requests to change wording in the MELs. We can never alter anything in an MEL because the airlines MEL is FAA approved, based on the MMEL (91.313). We may call the FAA 24x7 and can release on a verbal, if it's really minor and obvious, but we go through our company FAA liaison office and the paper has to be in their office within 24 hours.

Everyone FAA certified to operate under some specific set of operator rules has different privileges and authority and their own Major/Minor guidelines. There different FAA operating certificates. For a General Aviation A&P I agree, look no further than Part 43.

 

[Tom-D]

Tom, I think they do if you try to generalize what requires FAA involvement beyond application of part 43 Appendix A by a mechanic or IA.

Think about it, did the load change simply by modifying the control of it.

 

[Tom-D]

I've worked the last 20 years or so as an airline engineer.

Totally different set of rules your experiences don't apply here, don't cloud the issue by trying to applying them here.

 

[kontiki]

No, as I stated earlier (maybe I was too verbose, I try to avoid that) I think the answer to my original question on my airplane is that it's probably FAA minor.

 

[kontiki]

Totally different set of rules your experiences don't apply here, don't cloud the issue by trying to applying them here.

I was trying to respond to Glenn's question.

 

[Tom-D]

Back to my first post; as long as nothing bad happens, it probably doesn't matter. If something bad happens, it matters, and if your name is on it it better be right. Is that why there's no documentation of the alteration?

Read your insurance policy, many say the aircraft must be in an airworthy status when you fly, If it isn't there may be an issue collecting the money.

 

[Tom-D]

No, as I stated earlier (maybe I was too verbose, I try to avoid that) I think the answer to my original question on my airplane is that it's probably FAA minor.

Have your A&P-IA inspect it, see what paper they think it needs if any.

 

[Tom-D]

I was trying to respond to Glenn's question.

And when you did that, you brought many issues into the conversation that have no bearing on your question.

 

[Tom-D]

(Advisory Circulars) aren't mandatory.

BUT.... they do give you acceptable methods to comply with FAR 43.13.

 

[kontiki]

And when you did that, you brought many issues into the conversation that have no bearing on your question.

Yes, I was responding to his statement or question. If the editor was friendlier, I'd show you but is isn't.

 

[kontiki]

I think all experiences help build judgement. It is a regulated industry every environment has specific rules.

It always distills down to responding to the requirements of the task at hand as dictated by physical laws of nature, operating environment and maybe human factors. The rules are hopefully well suited to the task and environment at hand.

When the underlying facts change it's necessary to identify that and adjust. Interpretation of the grey areas in the rules can be dicey. That's where experience and judgement come in. That's also why I think it's good to work with others and get perspective.

I've worked in military, large airline, some general, some commuter, some military flight test, some component repair environments. Spelling out rules that are unambiguous for everything you encounter in any one environment or for any specific type of operation may not be possible.

 

[weirdjim]

Single point failure ... hmmm. Single alternator goes out you have maybe two or three hours of reserve power on board before you have to land ... single point failure. Carburetor (or injector) goes TU you have a few minutes because of single point failure. Only one transponder with a thousand components with a MTBF calculated on a per thousand basis? SIngle point failure. Two navcoms, one on the essential avionics bus with a single switch, one on the standby bus with a single switch. Several thousand components in each radio each with their individual MTBF but what are the odds of BOTH switches or BOTH radios going TU at the same time? There are literally tens of thousands of individual components flying in the loose formation we call an aircraft.

Now, what are the switch failure modes? Mostly the mechanical actuators. Once the switch is turned ON the failure rate for turning OFF by itself is down in the noise of the odds. Same for OFF to ON, and if this is the failure mode, you find it on the ground before you depart and can deal with it.

Lots of people sell crappy switches and to be quite honest, if you are building aircraft you go for the optimum price/performance sweet spot. The best quality? Probably not. The worst quality? Probably not, but your odds for switch failure rest somewhere between the two. YOU have the choice when installing a new switch to pick the quality that you want ... and the odds that you are willing to live with.

Never forget what John Glenn said was on his mind just before they lit the fuse to throw him up in the 1960s. HIs comment to reporters when he got back was his thought just before liftoff was that this whole damned rocket and everything on it was built by the lowest bidder.

To Tom-D, I wouldn't for the moment approve somebody splitting the main bus in two (or three, or four), but once it comes off the breaker already split and you add a simple switch between that wire and the device being powered, I don't understand how that could be construed as a major alteration of the electrical system? Now, since there is not "approval" required for part 91 radios, and there is a switch inside the radio for on/off, I can't argue that a switch OUTSIDE the radio is a major problem.

But I could be wrong, and that's what makes horse races.

Jim

 

[bnt83]

Car 3 or FAR 23 and 21 have no bearing on later modifications.

Car 3, FAR 21 & 23 only apply to certification of new product seeking a production certificate.

They do Tom. Anytime you get an FAA form 8100-11, 8100-9 or 8110-3 approving data to support a major alteration or major repair guess what that form is signifying? It is saying the data listed (drawing numbers etc) has met the regulations (almost always the regs listed on it are pulled directly from the ship's TCDS). We cannot (A&P or A&P IA) modify an airplane under part 43 in any way that would be contrary to regulations listed on that TCDS.





The difference between minor and major is simple, amount of oversight required.

Minor - Mechanic does the alteration such as disconnected wire A from avionics bus relay and connected it to new circuit breaker/switch. Disconnects wire B from avionics bus relay and connected it to new circuit breaker/switch then removes the relay and documents it in the logbook.

Major - The exact same physical work is performed on the aircraft but now the data is formally drawn up and given a drawing # and revision then given to a DER and they review the applicable regulations (found on the ship's TCDS, possibly others) that the alteration must meet and fills out the approval.

Major part 2 - Or a person can create the data and write it on the back of the 337 and submit it to the FSDO for approval. They too review the ship's TCDS and the data on the 337 before granting approval.

The regulations listed in the TCDS is the master key that the aircraft must conform to, even when being altered 35 years later by an A&P IA trying to decide major vs minor.

 

[3393RP]

Back to my first post; as long as nothing bad happens, it probably doesn't matter. If something bad happens, it matters, and if your name is on it it better be right. Is that why there's no documentation of the alteration?

"I was wrong again."

 

[Tom-D]

They do Tom. Anytime you get an FAA form 8100-11, 8100-9 or 8110-3 approving data to support a major alteration or major repair guess what that form is signifying? It is saying the data listed (drawing numbers etc) has met the regulations (almost always the regs listed on it are pulled directly from the ship's TCDS). We cannot (A&P or A&P IA) modify an airplane under part 43 in any way that would be contrary to regulations listed on that TCDS.

Have you forgotten what airworthiness is? or the phrase " or it's properly altered condition" what does that mean?

OBTW the forms you mentioned only insure the data used to make certain repairs were in compliance with approved methods, they do not require the requirements of FAR 21/23 are complied with.

 

[bnt83]

Have you forgotten what airworthiness is? or the phrase " or it's properly altered condition" what does that mean?

You just said :

Car 3 or FAR 23 and 21 have no bearing on later modifications.

If that were true, how is anyone supposed to know what a proper altered condition is?

 

[Tom-D]

You just said :

Car 3 or FAR 23 and 21 have no bearing on later modifications.

If that were true, how is anyone supposed to know what a proper altered condition is?

That's the part the IA is supposed to know, by researching the documentation in the aircraft history records.

 

[bnt83]

If a customer came in with a Cessna 525C with a .5 inch wide painted band around the exterior of an emergency exit, is it legal?

Can you legally paint it that wide or omit it?

Those who know how to research this only the TCDS and the historical FARs.

 

[bnt83]

Have you forgotten what airworthiness is? or the phrase " or it's properly altered condition" what does that mean?

OBTW the forms you mentioned only insure the data used to make certain repairs were in compliance with approved methods, they do not require the requirements of FAR 21/23 are complied with.

I'm not sure what you are trying to say really. Those forms are used for approving alteration and repair data.

 

[Tom-D]

I'm not sure what you are trying to say really. Those forms are used for approving alteration and repair data.

I see no connection between repair data and Certification requirements of new product. FAR 21&23. The forms you mentioned are used by DAR and FSDO to insure the data you are requesting to use is the proper method to do the repair.