When the FAA signs off on that process, it issues a type certificate, which is the agency’s formal acknowledgment that the aircraft design meets the applicable airworthiness standards under 14 C.F.R. Part 21, Subpart B. Once that approval is in place, the type design is essentially frozen. You can’t make major changes to the airframe, engine, or propeller without additional FAA authorization.
That’s where Supplemental Type Certificates—STCs—enter the picture. An STC is FAA approval to make a major design change to an existing type certificate (14 C.F.R. § 21.113). To obtain one, the applicant has to provide data, engineering analysis, and in many cases, ground or flight testing to show the change maintains the same level of safety required under the original type certificate. The FAA looks at whether the modification complies with the certification basis and applicable airworthiness standards and does not introduce any unsafe condition.
STCs cover a wide range of enhancements: avionics retrofits like glass panels and ADS-B upgrades, performance improvements like STOL kits or vortex generators, engine and propeller swaps, float conversions, wingtip fuel tanks, or control surface modifications. They give owners a legal path to modernize older aircraft without pursuing a brand-new type certificate, and the practical benefits often include better performance, mission flexibility, or resale value.
If you're considering a modification, the first step is to determine if an STC already exists for your make and model. The FAA maintains a publicly accessible STC database, and type clubs, maintenance shops, and engineering firms often know what’s available. If an STC exists, you have to contact the holder to obtain permission to use it (usually for a fee). Once you acquire license to use that STC, you must ensure it's properly installed and documented. That means including the STC number in the logbooks, following the installation instructions precisely, and placing the corresponding supplement in the AFM or POH as required by 14 C.F.R. §§ 91.417. And because STCs involve major repair or alterations, 14 C.F.R. § 43.9 requires documentation in the aircraft’s maintenance records with a reference to the specific STC number using FAA Form 337. Without that paperwork, the aircraft isn’t in a legally modified configuration—even if the physical work has been completed.
If no STC exists and you want to pursue one, you should know that the process is not light work. The FAA generally views the STC process through five phases: conceptual design, requirements definition, compliance planning, implementation, and post-certification activity. Formally, the process starts by submitting FAA Form 8110-12 (Application for Supplemental Type Certificate), the process really begins by coordinating with your regional Aircraft Certification Office. At that early stage, you describe the modification, identify the certification basis, and lay out how you intend to show compliance under 14 C.F.R. § 21.115. Depending on the scope, the FAA may require structural substantiation reports, electrical load analyses, flight or ground tests, and systems safety assessments before moving forward. This may require hiring a designated engineering representative (DER).
If the FAA finds that the modification meets airworthiness standards and does not reduce safety, it will issue the STC under 14 C.F.R. § 21.117. The certificate comes with approved drawings, limitations, placards, instructions for continued airworthiness, and any required flight manual supplements. The STC belongs to the holder, who can use it or license it to others.
There’s a related but distinct tool called an amended type certificate. Unlike an STC, which supplements the original design, an amended type certificate under 14 C.F.R. § 21.101 modifies the existing type design itself. Manufacturers or type certificate holders typically pursue amended type certificates for large-scale changes or production-level alterations. Most individual operators won’t go that route, but it's part of the same regulatory landscape.
From a pilot or operator perspective, adding an STC brings real obligations. If performance, systems, or limitations change, the AFM or POH must be supplemented and actually followed, as required by 14 C.F.R. § 91.9. Changes in gross weight, CG envelope, climb rates, or stall speeds will affect flight planning and performance calculations. If the modification involves new systems—like an engine upgrade or new autopilot—you’ve got to learn the normal and emergency procedures tied to that system. Insurance shouldn’t be overlooked either. Some carriers expect additional hours, training, or checkouts following major changes. Assuming that your current policy covers significant modifications can be a risky move.
An STC is not a one-page permission slip—it is a full approval package covering engineering data, operating limitations, installation procedures, and continued airworthiness requirements. To stay legal and safe, pilots and owners need to understand how the modification affects performance, documentation, and maintenance. Whether you're installing an existing STC or pursuing a new one, compliance and safety drive the entire process. STCs allow aircraft owners to keep aging airframes relevant and capable, but they come with responsibilities that shouldn’t be overlooked.
