Major Repair vs. STC

This is a question we get asked often, and it usually comes up the same way: a repair scheme that started out clearly as a repair has grown, and somewhere along the line it started to look less like fixing the part and more like changing it. The person asking already knows what a major repair is and what an STC is. What they're not sure about is which side of the line their particular project has landed on.

It's a fair question, because the line is real but it isn't always obvious — and getting it wrong has consequences. So here's how to think about where a repair ends and a modification begins.

The Core Distinction: Restore vs. Change

Strip everything else away and the distinction comes down to one thing: intent relative to the original type design.

A major repair restores a product to its original type-designed condition. The part was airworthy as designed, something happened to it — damage, wear, corrosion — and the repair brings it back to conforming with that original design. The type design is the destination. You're returning the part to what it was supposed to be.

An STC changes the product away from its original type design. It's a major modification — the product ends up as something the original type design didn't describe. The type design is the starting point you're departing from, not the destination you're returning to.

That's the whole framework: a repair restores, a modification changes. Every hard case is really a question of which of those two things your scheme is actually doing.

Why a Repair Scheme Starts to Look Like a Design Change

The reason this question comes up isn't usually that someone set out to do a modification and called it a repair. It's that repair schemes evolve.

A repair gets developed for a damaged part. Then the damage encountered in practice is a little different, so the scheme gets extended to cover it. Then it's adapted again. Each individual step still feels like "repairing the part" — but the cumulative result can be a scheme that adds material, alters geometry, introduces features, or changes how the part behaves in ways the original type design never contemplated. At some point the scheme isn't restoring the part to the type design anymore. It's defining a new configuration.

That's the threshold. And the useful question to ask of any repair scheme that's grown is: if I compare the end state of this repair to the original type design, am I looking at the same part restored — or a different part? If a repaired part still conforms to the original type design, it's a repair. If the scheme produces something that deviates from that design in a way that affects form, fit, function, or airworthiness characteristics, it has crossed into modification territory, and that points toward an STC.

A Note on Repair Specifications

It's worth being clear where a Repair Specification fits in all this, because it sometimes gets pulled into the same conversation. A Repair Specification is still a major repair — it's on the repair side of the line, restoring parts to the original type design, not modifying them. What makes it distinct is that it isn't tied to a single serial number. Rather than approving a one-off repair for one specific part, a Repair Specification approves a repair the station can perform repeatedly, across parts, as a standing capability.

So a Repair Specification doesn't change the repair-versus-modification question — it's firmly a repair. It just changes the form the repair approval takes. If your scheme is genuinely a repair and you expect to perform it again and again, a Repair Specification may be the right vehicle for it. If your scheme has crossed into modifying the type design, you're back to the STC question regardless of how many times you intend to do it.

Why Getting the Call Right Matters

It's tempting to treat this as a paperwork distinction. It isn't. The repair and modification paths have different approval mechanisms, different data requirements, and different substantiation expectations. A scheme processed as a major repair when it's actually a modification is a scheme approved on the wrong basis — and that's the kind of problem that surfaces at the worst possible time, often well after the work is done.

The cost of getting it right is a conversation early. The cost of getting it wrong is rework, delay, and questions about the validity of an approval you thought you had. That asymmetry is the whole reason this question is worth taking seriously the moment a repair scheme starts to feel like it's growing past a repair.

Where Prime Propulsion Fits

The honest reality is that the repair-versus-modification line has genuine gray area, and the cases that reach it are rarely simple — that's exactly why people ask. Working through it well takes someone who has seen where that line actually falls in practice, not just in principle.

Our in-house DER has propulsion certification experience from inside the FAA and in private industry, with delegation across Parts 23, 25, and 33. We help manufacturers and repair facilities look at a repair scheme honestly, determine which side of the line it's on, and take the right path — whether that's substantiating it as a major repair, structuring it as a Repair Specification if it's a recurring capability, or pursuing an STC for what is, in fact, a modification.

If you have a repair scheme that's started to look like a design change and you want a clear read on it, contact us — there's no cost to talk it through. You can also learn more about what an STC is and our STC certification services.