MX lesson...what is needed

[davidm767]

Got into a discussion in class today (decided to start MX school as a "third" of life crisis lol)). I know and agree that a 337 is necessary for changing out a fuse for a CB in a certified aircraft. Question is, what else may need to be changed? Can a bus that originally operate with fuses handle the operation of CBs, assuming the aircraft came with mixed fuses/CBs and was designed and built in mid 70s. Does this alteration require a STC? What would be required for paperwork on back of 337? I really appreciate input, as its likely an exercise ill do on my own aircraft.

Thanks

Dave

 

[bnt83]

Got into a discussion in class today (decided to start MX school as a "third" of life crisis lol)). I know and agree that a 337 is necessary for changing out a fuse for a CB in a certified aircraft. Question is, what else may need to be changed? Can a bus that originally operate with fuses handle the operation of CBs, assuming the aircraft came with mixed fuses/CBs and was designed and built in mid 70s. Does this alteration require a STC? What would be required for paperwork on back of 337? I really appreciate input, as its likely an exercise ill do on my own aircraft.

Thanks

Dave

Stop. The #1 lesson you really need is how to determine major/minor. This is something A&P schools don't cover well and thousands of mechs don't do well either, even some FAA inspectors and FSDO's. I don't know everything and never will, but here is scratching the surface:

I look at this way. DER (designated engineering rep) is a more educated version of an IA with an engineering backround of some sort and holds an authority to do certain things. How does a DER do their job? Where do they get the rules to follow when altering a Cessna 150 or Falcon 900LX?

I see all the drawings and documents that come out of engineering. It's a wonderful experience to see what really goes into "in house" issued STC's etc. The stuff I am unable to comprehend (uneducated) is structure. I do not know how to compute, design and show compliance for structural mods like, building a shelf for a new 406 ELT to meet installation manual specs. I've seen structural analysis reports and the math is what makes my head hurt.

I've also learned that the alteration proposed may be minor, but a DER will be more than happy to whip out easy approved data that was unnecessary, in exchange for $$$. They need business, meaning they will approve any minor mod as a major.

 

[bnt83]

So unless you know how to determine major from minor you always go off of assumptions from passed experiences like "this is STC'ed on aircraft X, so that must need an STC on aircraft Y" stuff which is completely wrong.

I could change the seat cover (fabric over the foam) on a Cessna 170 without approved data (operated under part 91, if operated under 135 it must be burn tested to 25.853), but on an aircraft where the seats are TSO'd or are rated for 12G impacts or some new fancy helicopter, that seat fabric change must be approved, as the texture and slickness of the fabric could impact the ability of the altered (reupholstered) seat's ability to meet the rules it was originally certified to in addition to burn testing.

 

[bnt83]

Got into a discussion in class today (decided to start MX school as a "third" of life crisis lol)). I know and agree that a 337 is necessary for changing out a fuse for a CB in a certified aircraft. Question is, what else may need to be changed? Can a bus that originally operate with fuses handle the operation of CBs, assuming the aircraft came with mixed fuses/CBs and was designed and built in mid 70s. Does this alteration require a STC? What would be required for paperwork on back of 337? I really appreciate input, as its likely an exercise ill do on my own aircraft.

Thanks

Dave

The answer: Depends. First source of info, how was the electrics certified in the original design. What rules did the airplane have to meet when built? The answer is: the aircraft's Type Certificate Data Sheet (TCDS).

Look at FAA form 8100-11 and the 8110-3 which is a few forms (of several) that is used to document the FAA approval of design data. Block 5 "Airworthiness Requirements" will be the rules that the propsed alteration (listed in block 6) the data has shown compliance with, from the aircraft TCDS (and possibly other regulatory documents like an AD).

 

[bnt83]

They key here is, STC's, Field Approvals, MRA (major repair or alteration) projects are all designed and used to make certain that the alteration or repair DOES NOT change the aircraft's ability to meet the rules it was originally certified to (airworthiness requirements).

The difference between substituting a non-insulated ring terminal with an insulated ring terminal VS changing the floor plan, installing an entertainment system on a Cessna 525 is the amount of oversight required to ensure the airplane still meets the airworthiness requirements in the TCDS, any applicable AD's or other regulatory documents.

If it's a minor alteration no APPROVED data is needed. Only acceptable data (manufacturer's unapproved installation manuals/drawings, industry standards, advisory circulars, etc) are needed to complete the alteration which would be then returned to service by a log entry.

 

[bnt83]

They may laugh at you in school today, but this is how it's really done and they don't know **** if they do.

 

[Skylane81E]

They key here is, STC's, Field Approvals, MRA (major repair or alteration) projects are all designed and used to make certain that the alteration or repair DOES NOT change the aircraft's ability to meet the rules it was originally certified to (airworthiness requirements).

This part is key, three years from graduation you may want to take the test for two more letters to put after your name.

Big part of that is the ability to do annuals, and a big part of an annual is conformity. The airplane must be in compliance with its original type certificate, OR in a properly altered state

 

[Silvaire]

The 337 form has got to be THE most over-used and abused document in the entire system. The key is found in Part 43 Appendix A in which major repairs and alterations are specifically outlined. I swear I see 337's all the time that convince me the person who filed it didn't even bother to look at that rule.

The system is also like a chain of command. Your A&P, your IA have certain authority and it is on them to interpret the rules and make decisions regarding the applicability. In many cases the filing of a 337 is just a CYA action to bump the responsibility of the decision to the next level.

In regards to replacing fuses with circuit breakers for instance: Are you going to consider it a change to the basic design of the electrical system as per 43 Appendix A(a)(xii)?

Some may, some may not...

 

[bnt83]

The 337 form has got to be THE most misunderstood over-used and abused document in the entire system. The key is found in Part 43 Appendix A in which major repairs and alterations are specifically outlined. I swear I see 337's all the time that convince me the person who filed it didn't even bother to look at that rule.

The system is also like a chain of command. Your A&P, your IA have certain authority and it is on them to interpret the rules and make decisions regarding the applicability. In many cases the filing of a 337 is just a CYA action to bump the responsibility of the decision to the next level.

In regards to replacing fuses with circuit breakers for instance: Are you going to consider it a change to the basic design of the electrical system as per 43 Appendix A(a)(xii)?

Some may, some may not...

FTFY but exactly. The Appendix is also a good tool and also used in the process by all.


The Advisory Circulars are further guidance on how to show compliance with the airworthiness requirements that any alteration would be held to.

 

[bnt83]

Lets look at a different mod. Say a part 135 Cessna 750 (Citation X) came with provisions (the wiring, rack, and circuit breaker) installed for a flight data recorder. Would removing the rack and the circuit breaker be major?

There is no requirements in the regulations that say this airplane must be equipped with one. There is nothing in the 135 operator's op specs that says they must have flight data recorders. The rack removal adds no additional load to the radio shelf it's mounted to.

How do we do it? Bag and stow the connectors, remove the rack, remove the circuit breaker and plug the hole and cap the wires. Log entry. Done.