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FRUIT SALAD


To begin with, STCs are born and raised as an airworthiness standard under “Part 21: Certification Procedures for Products and Articles.” Whereas aircraft alterations, a performance standard, are found under “Part 43: Maintenance, Preventative Maintenance, Rebuilding, and Alteration.”


These are two separate regulations that control two separate regulatory processes.


Nowhere in Part 21 does it mention major/minor alteration. And while there is a single mention of “STC” in Part 43.2, it’s in specific context to records of overhaul and rebuilding. So, why the widespread belief all STCs are major alterations?


Perhaps it’s because of limited personal experience or just a misunderstanding of the regulations. Or maybe another explanation is that it’s generational learning that’s to blame, where methods and practices are passed down as fact (like the misconception that placing batteries on concrete floors causes them to discharge).


While I probably can’t convince everyone that putting a battery on concrete is no longer an issue, I can provide a better understanding on how STCs interact with aircraft alterations.


DEFINITIONS


Let’s start with the easy side: aircraft alterations. Part 1 defines a major alteration as:


“An alteration not listed in the aircraft, aircraft engine, or propeller specifications — that might appreciably affect strength,


weight, balance, structural


flight characteristics, or other qualities affecting airworthiness; or that


performance, powerplant operation, is not done according


to accepted practices or cannot be done by elementary operations.”


This definition is further supported by Part 43 Appendix A(a) which contains an itemized list of parts and alterations that are considered major alterations. And, as seen in the definition above, there is no reference to STCs.


Conversely, a minor alteration is defined as an “alteration other than a major alteration.” It’s all pretty straightforward.


On the STC side, it’s not as simple. Neither Part 1 nor Part 21 define “Supplemental Type Certificate.” And most attempts to delve into existing guidance for a proper definition push us closer to that rabbit hole than to the answer we are looking for: namely, whether an STC is a major or minor alteration.


So, let’s look at an STC from a different angle. rotorcraftpro.com 69


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