What Is an FAA STC?

If you've followed our posts on PMA and the two ways to obtain one, you know a PMA is about producing a replacement part. A Supplemental Type Certificate is a different tool for a different job: it's how you get FAA approval to modify a product that's already type-certified.

An STC approves a major modification to a type-certified product — an aircraft, engine, or propeller — and documents how that modification affects the original design. An STC can be issued for a single aircraft ("one only") or for multiple, and most are written for multiple so the approval isn't tied to one tail number. One thing an STC is not: it's not a manufacturing approval. It approves the change to the design, not the production of parts.

What Counts as a "Major" Modification

The whole concept of an STC turns on the word "major" — so it helps to define it by its opposite. Per 14 CFR 21.93, a minor change is one with no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other qualities affecting the airworthiness of the product. A major modification is simply a change that rises above that threshold.

For Part 33 engines, that line gets crossed by changes like increasing fuel flow, adding horsepower, or changing or adding hardware. For aircraft, the range is much broader — new avionics, a new engine, an increase in installed engine power (which also drives an STC on the engine itself), passenger-to-cargo conversions, and many more. In our experience, aircraft STCs are the most common type. Propeller STCs are less common, but they show up for things like lighter materials or an increased allowable RPM range.

The reason the major/minor distinction matters so much in practice: it determines whether you're looking at an STC project at all, and getting that call wrong early sends a program down the wrong path.

When an STC Is Not the Answer

It's just as useful to know when an STC doesn't apply. The most obvious case is a minor change — STCs exist only for major modifications to a type-certified product. Technical Standard Order (TSO) design changes also aren't approved through the STC path. And an STC won't be granted at all if the applicant can't show the modification meets the applicable FAA regulations — the FAA, or an ODA acting on its behalf, won't issue an approval the data doesn't support.

How the Process Works

The FAA lays out the path from concept to approval in Advisory Circular 21-40, the application guide for obtaining a Supplemental Type Certificate. The AC walks through what's required at each stage, and it's a useful roadmap. But the work underneath it — defining the modification, establishing the certification basis, building the substantiation that proves the changed product is still airworthy — is where STC projects are actually won or lost. That's the part experience matters for.

Where Prime Propulsion Fits

Most STC activity in the industry is on the airframe side — avionics, interiors, cargo conversions. Our focus is the propulsion end of it: engine and propeller STCs, and the airframe STCs that change how an installed engine operates. Those modifications have to demonstrate that the change doesn't compromise the airworthiness of a type-certified product, and that substantiation is where these projects succeed or stall.

Our in-house DER has propulsion certification experience from inside the FAA and in private industry, and can help you take an engine or propeller modification from concept through the AC 21-40 process to an approved STC.

If you have a modification in mind and want to know what an STC path looks like for it, contact us — there's no cost to talk it through. You can also learn more about our STC certification services.

Prime Propulsion delivers expert FAA certification solutions for small to medium-sized aerospace firms. Our DER-led team specializes in PMA, STC, Test Cell Correlations, and Repair Specification support.