A supplemental type certificate (STC) is an FAA-issued approval that allows a specific modification to an aircraft, engine, or propeller that changes the original type design. Think of it as a legal stamp of approval for upgrading or altering an aircraft beyond what the manufacturer originally built. The STC doesn’t replace the aircraft’s original type certificate. Instead, it works alongside it, approving both the modification itself and how that modification affects the original design.
How an STC Differs From the Original Type Certificate
Every certified aircraft rolls off the production line with a type certificate (TC), which is the FAA’s approval of the aircraft’s core design. That certificate covers everything from the wing shape to the engine rating to the cockpit instruments. An STC builds on top of that foundation. Under 14 CFR Part 21, Subpart E, an STC formally consists of two things: the FAA’s approval of a change to the product’s type design, and the original type certificate previously issued for that product. The two documents function together, meaning an STC is always tied to a specific aircraft type and a specific modification.
This distinction matters because an STC isn’t a blanket permission to do whatever you want to a plane. It’s a precisely scoped approval for a defined change to a defined product.
What Kinds of Modifications Need an STC
STCs cover a wide range of changes, from minor capability upgrades to dramatic overhauls. Common examples include:
- Engine changes: increasing horsepower, swapping a piston engine for a turbine
- Aerodynamic modifications: STOL (short takeoff and landing) wing kits, propellers with increased diameter
- Landing gear changes: installing floats, skis, or heavy-duty landing gear
- Avionics upgrades: GPS/WAAS navigators, satellite communication systems, glass cockpit panels
- Structural changes: enlarged rear windows, increased baggage area, gross weight increases
Some aircraft models have pages of approved STCs available. Piston-to-turbine engine conversions, for instance, are common enough that they’ve created entire sub-markets within general aviation.
STC vs. Field Approval
Not every modification requires an STC. Simpler alterations can sometimes be approved through a one-time field approval, documented on FAA Form 337 (Major Repair and Alteration). The deciding factor is generally the complexity of the modification. The FAA’s Flight Standards Information Management System specifies which types of alterations qualify for field approval under certain conditions, and which must go through the full STC process or be evaluated by FAA engineering.
The key difference: a field approval covers one specific aircraft. An STC, once issued, can be applied to every aircraft of the same type and model, making it far more valuable commercially. If you’re a company developing a modification you want to sell to hundreds of aircraft owners, you need an STC.
The Approval Process
Getting an STC is not quick or simple. The FAA lays out a multi-step certification process that can take months or years depending on the complexity of the modification. The major steps include:
The applicant submits a formal STC application, followed by familiarization meetings and a preliminary type certification board (TCB) meeting with the FAA. The FAA then develops a certification program plan and establishes the certification basis, which defines exactly what regulations the modification must meet. From there, the applicant submits engineering data for approval, and the FAA conducts design evaluations, conformity inspections, and compliance reviews. Ground inspections, ground tests, and flight tests follow, with the FAA reviewing the results and performing its own certification flight tests. The process wraps up with functional and reliability testing, approval of any flight manual supplements, a final TCB meeting, and a continuing airworthiness determination before the FAA officially issues the STC.
Each of those steps can involve back-and-forth between the applicant and FAA specialists. For general aviation modifications, the investment in engineering, testing, and certification fees can be substantial, which is why STCs are typically developed by companies that plan to sell the modification to many aircraft owners rather than by individual operators.
Who Owns the STC
This is a detail that surprises many aircraft owners: the STC and all its related information (drawings, data, specifications) are the property of the STC holder, not the FAA and not the aircraft owner. If you want to install an STC modification on your aircraft and it’s not your STC, you must contact the holder and obtain written permission. The FAA will not release STC information without the holder’s authorization. This requirement is codified in 14 CFR ยง 21.120.
In practice, this usually means purchasing a kit or installation package from the STC holder, which includes the engineering data, installation instructions, and the permission statement your mechanic needs to do the work legally.
Instructions for Continued Airworthiness
Every STC comes with Instructions for Continued Airworthiness (ICA), which are maintenance instructions that must be folded into the aircraft’s ongoing maintenance program. These instructions cover inspections, service intervals, and any special maintenance procedures related to the modification. Your mechanic or maintenance shop needs these documents to keep the aircraft in compliance after the STC is installed. Without them, the modification could technically render the aircraft unairworthy.
How STCs Affect Aircraft Value
The relationship between STCs and resale value is more nuanced than most owners expect. While certain modifications clearly enhance capability (a turbine conversion on a piston airframe, for example), the financial return on avionics upgrades can be disappointing. According to VREF Aircraft Values & Appraisals, aircraft with original steam gauges and those with glass panels have “almost identical markets in terms of overall buying populations.” Their advice: be careful upgrading avionics specifically for resale purposes, because you likely won’t recoup the investment in a higher sales price.
What does matter for value is documentation. As aircraft age and accumulate modifications, it becomes complicated for appraisers to establish a baseline value. Keeping thorough records of every STC installation, including the approval data and maintenance history, protects your investment far more than the modification itself might.
International Recognition
An FAA-issued STC doesn’t automatically carry legal weight in other countries, but bilateral agreements make the process of gaining foreign acceptance much smoother. The FAA maintains Bilateral Aviation Safety Agreements (BASAs) with numerous countries, each with Implementation Procedures for Airworthiness that define how design approvals like STCs are validated for import and export. The agreement between the U.S. and the European Union uses a similar but slightly different framework called Technical Implementation Procedures (TIP), which accounts for the fact that the EU represents many sovereign states, some of which retain limited regulatory responsibility.
If you’re operating an aircraft internationally or purchasing one with STCs from another country’s authority, the relevant bilateral agreement determines whether those modifications are recognized or require additional validation.
Looking Up Existing STCs
The FAA maintains a searchable public database of all current STCs through its Dynamic Regulatory System (DRS). You can filter by aircraft model, STC holder, product type, CFR part reference, and other criteria. This is useful if you’re shopping for available modifications for your aircraft, verifying the legitimacy of an STC claimed on an aircraft you’re considering buying, or researching what upgrades exist for a particular airframe. The database is free and accessible at drs.faa.gov.