When installing an engine not on the aircraft’s type certificate, you need STC or FAA field approval.

When you hear about upgrading an airplane with a different engine, the big question isn’t just “will it fit?” It’s “how do I prove it’s safe and legal?” That’s where FAA approvals come in. If the new engine isn’t on the aircraft’s Type Certificate, you don’t just bolt it on. You need a formal path that shows the modification won’t compromise airworthiness. The correct route is either a Supplemental Type Certificate (STC) or an FAA field approval. Let me unpack what that means and why it matters.

Two roads, one clear goal: safety and compliance

Think of it like this: the Type Certificate is about the airplane model as designed by the manufacturer. It’s the blueprint that says, “This airplane frame, engine interface, and systems setup are approved.” When you want to swap in an engine that isn’t included in that original certificate, you’re entering a new territory. You can choose between two generally accepted FAA routes:

• Supplemental Type Certificate (STC): This is a formal, thorough alteration approval. An STC shows the FAA that the proposed engine installation has been evaluated, tested if needed, and meets safety and regulatory standards. It’s a complete package: design data, installation drawings, wiring diagrams, performance data, and often some flight-test results. Once the STC is issued, the installation is authorized under that certificate, and owners can install the engine in compliance with the STC’s terms.

• FAA field approval: This is a more on-site, less formal path used in specific, often smaller-scale situations. A field approval lets you install non-type-certificated equipment under conditions laid out by the FAA inspector who handles the field approval. It’s a practical option when the modification doesn’t require a full-blown STC or when the circumstances fit the FAA’s policy for field approvals. It’s generally quicker, but it can carry more limitations than an STC.

What this isn’t

• Type certification alone doesn’t cover a post-production engine swap. The aircraft’s Type Certificate confirms the airframe and its original engine lineup; it doesn’t automatically authorize new, non-included engines to be installed.

• Part 145 repair station approval isn’t the tool for approving new installations. Part 145 covers maintenance, repair, and overhaul organizations. It governs who can do certain work, not whether a new engine installation is legally permitted.

A closer look at what each path involves

STC — the formal, in-depth route

• The design package: You or the applicant submit a complete installation package. That includes mechanical drawings, mounts, engine interfaces, fuel and electrical integration, and a clear assessment of how the new engine affects weight, balance, and performance.

• Compliance data: The FAA evaluates whether the installation meets applicable airworthiness standards. That often means showing how the engine complies with safety requirements, plus any new environmental or emissions considerations.

• Testing and validation: Depending on the case, flight testing or bench testing may be part of the approval to verify performance and handling characteristics.

• Documentation and issuance: Once the FAA is satisfied, an STC certificate is issued. With that certificate, the installation can be performed on aircraft of the same model family under the terms of the STC.

Field approval — speed with constraints

• Scope and conditions: Field approvals are granted by FAA personnel at the local Field Office and are intended for specific, limited circumstances. The approval will spell out what can be installed, under what conditions, and what must be done to maintain airworthiness.

• On-site, practical path: This route is useful when the modification is relatively straightforward, doesn’t require extensive testing, or when the operator can demonstrate that compliance goals will be met through the given conditions.

• Trade-offs: Field approvals can be faster, but they’re typically more constrained than an STC. They may limit operation, require additional maintenance steps, or call for future actions if circumstances change.

Putting it into context: how it shows up on the hangar floor

Imagine you’re evaluating a modern turbocharged engine that isn’t part of the aircraft’s original design. If you want to keep everything tidy and future-proof, an STC is often the preferred path. It gives you a documented, repeatable standard for multiple airplanes and simplifies future maintenance or resale because the installation has a recognized, public certification basis.

On the other hand, if you’re working within a tight schedule or a limited scope—perhaps a single-aircraft customization for a specific mission—a field approval might be the sensible workaround. It buys time and gets you moving, but you’ll want to keep the installation within the approved bounds and be ready for inspections or follow-up actions if the operator’s use changes.

A quick note on how the process touches real-world flight operations

• Airworthiness is the headline. Any engine swap that isn’t on the Type Certificate is, by design, subject to extra scrutiny to ensure safety for the skies you’ll fly in.

• Documentation matters. Whether you go STC or field approval, the paperwork has to align with FAA expectations. Clear installation instructions, performance data, and maintenance guidance aren’t optional extras; they’re the backbone of the approval.

• Compliance is ongoing. After approval, operators must adhere to the approved configuration and inspect and maintain the installation as specified. If anything whispers “that’s not in the data,” it’s time to revisit the agreement with the FAA.

A few practical tips that keep the process sane

• Start with the TCDS and the STC catalog. The Type Certificate Data Sheet (TCDS) for your aircraft model will tell you what is and isn’t allowed, and it can guide your discussion with the FAA.

• Talk early with the FAA. A straightforward conversation up front can save a lot of back-and-forth later, especially if you’re considering a field approval.

• Be honest about performance impacts. Extra power, weight shifts, or altered fuel economics aren’t just numbers; they affect handling, stability, and operating cost. The more complete your data, the smoother the path.

• Think long-term. If you anticipate multiple engines or future upgrades, an STC can deliver a cleaner, more scalable solution than piecemeal field approvals.

In the real world, these approvals aren’t just paperwork hoops

They’re part of a larger commitment to safety, reliability, and accountability. Aircraft owners, operators, and technicians all know that a solid approval path makes maintenance less of a guessing game and more of a clear, defendable choice. It’s not just about ticking boxes; it’s about ensuring that every bolt and interface plays nicely with the rest of the airplane.

If you’re exploring this topic from a learning perspective, you’ll notice a pattern: whenever a change isn’t covered by the Type Certificate, the FAA’s two main avenues—STC and field approval—provide a structured way to maintain airworthiness while still allowing meaningful upgrades. The distinction isn’t just academic. It translates into how quickly you can responsibly implement a change and how robust the subsequent maintenance and oversight will be.

So, what should you remember?

• When an engine isn’t on the aircraft’s Type Certificate, you don’t just install it. You pursue an STC or a field approval.

• STCs are thorough, formal, and designed for repeatable installations across multiple aircraft of the same model family.

• Field approvals are faster and more limited, issued on a case-by-case basis at the local FAA office.

• Type Certification covers the base design, not every possible modification, and Part 145 isn’t the path for approving new engine installations.

• Good documentation, early FAA communication, and a clear plan for compliance keep the process moving smoothly.

If you’re curious to see how the pieces fit together, you can browse FAA resources and Type Certificate Data Sheets online. They’re not bedtime reading, but they do demystify how the FAA views engine-to-airframe interfaces and what data counts toward a legitimate, safe modification. The goal is simple: you want that engine upgrade to work in harmony with the airplane, not in spite of it.

In the end, the choice between STC and field approval comes down to scale, risk, and future-proofing. Both paths are legitimate, both keep safety at the forefront, and both require a clear, well-documented story about why the change makes sense and how it’s going to behave in the air. And that’s the kind of rigor that helps pilots, mechanics, and operators sleep better at night.