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STC 411


As an owner/operator or mechanic, we usually encounter an STC in one of two ways: when we purchase an article or kit for installation, or when we physically modify our aircraft from its original type design. Though these two conditions appear similar, they each go through a different process.


For example, when a person wants to offer/sell an article or product for use on a type-certificated aircraft, they must obtain two approvals: a design approval and a production approval. The latter provides the regulatory method to produce/manufacture the item listed in the design approval.


Part 21 provides design or production approval procedures as separate certifications like a TC or a PC, `or as a combined design and production approval like a PMA or TSOA.


And as luck would have it, “design approval” is defined in Part 21:


“Design approval means a type certificate (including amended and supplemental type certificates) or the approved design under a PMA, TSO authorization, letter of TSO design approval, or other approved design.”


Bingo! Simply put, an STC is a specific type of design approval. Nothing more.


In fact, there are actually several variants of STCs. But due to limited space, I’ll stick to two types: a standard STC and an AML-STC (AML stands for Approved Model List.)


The difference between these types is defined in Order 8110.4 and, more recently, in AC 20-180:


“A standard STC is an approval method for a change in type design that is limited to a single type certificate (TC). An AML-STC is a multi-model approval method that allows a set of compliance data (i.e., type design data and substantiating data) to be designated as “baseline data” that is applicable to various aircraft models.”


In some cases, vendors will use AML- STCs to streamline their design approval certification tests and costs. An AML-STC is like a blue-light special when it comes to design approvals: one approval for multiple aircraft or type certificates.


But before we continue, let’s turn to the other STC condition we mentioned.


70


Jan/Feb 2019


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