An STC, or Supplemental Type Certificate, is an FAA-issued approval that allows someone to modify an aircraft from its original design. Every aircraft is manufactured under a Type Certificate (TC), which certifies that the design meets federal airworthiness standards. When you want to make a major change to that certified design, such as installing a new engine, upgrading avionics, or replacing fuel cells, you need an STC to prove the modification is safe and legal to fly.
How an STC Relates to a Type Certificate
When a manufacturer designs and builds an aircraft, the FAA issues a Type Certificate confirming that the entire design meets safety standards. That TC covers the aircraft as it rolls off the production line. But aircraft don’t stay stock forever. Owners install new navigation systems, swap engines for more powerful models, add camera systems, or modify fuel tanks. If any of these changes count as a “major change in type design,” the modification needs its own approval.
That approval is the STC. Legally, an STC consists of two things: the FAA’s approval of the design change, and the original Type Certificate for the product being modified. Think of it as a certified amendment layered on top of the original design approval. The aircraft still flies under its original TC, but the STC documents exactly what was changed and confirms those changes meet airworthiness requirements.
Who Needs to Apply for One
Federal regulations draw a clear line here. If you hold the Type Certificate for a product (meaning you’re the original manufacturer or the current TC holder) and you want to make a major design change, you can either apply for an STC or amend the original Type Certificate. You get to choose. But if you don’t hold the TC, which covers virtually every aftermarket shop, avionics installer, and third-party engineering firm, you must apply for an STC. There’s no option to amend someone else’s Type Certificate.
This distinction matters because it’s what makes the aftermarket modification industry possible. A company that has never built an airplane can still design, certify, and sell a modification for one, as long as they go through the STC process.
What Kinds of Modifications Use an STC
STCs cover a wide range of changes. Common examples in general aviation include installing radar altimeters, replacing wing fuel cells, adding surveillance and thermal imaging camera systems, fitting supplemental cooling systems, and installing data transmission equipment. In the commercial world, STCs are used for cabin reconfigurations, engine upgrades, winglet installations, and cockpit modernization programs.
Not every aircraft change requires an STC. Minor alterations, like replacing a part with an FAA-approved equivalent, typically don’t qualify as major design changes. The STC threshold kicks in when the modification is significant enough that it could affect the aircraft’s structural integrity, performance, weight and balance, or systems operation.
The Certification Process
Getting an STC approved is a multi-stage process that mirrors, on a smaller scale, the original aircraft certification path. It begins when the applicant submits a formal application to the FAA. Early meetings establish what the modification involves and what regulations it needs to comply with. The FAA then develops a certification program plan and establishes the certification basis, which is the specific set of airworthiness standards the modification must meet.
From there, the applicant submits engineering data for FAA review. The agency evaluates the design, performs conformity inspections to verify the modification matches the approved data, and reviews compliance findings. Ground inspections, ground tests, and flight tests follow. The FAA reviews the applicant’s flight test results, may conduct its own certification flight tests, and evaluates the modification from a flight standards perspective.
Before final approval, the applicant must provide a flight manual supplement covering any changes to operating procedures, and the FAA’s airworthiness evaluation group completes a continuing airworthiness determination. Only after all of these steps does the FAA issue the STC. For simple modifications, this process can take months. For complex ones, it can stretch well over a year.
STC Holder Responsibilities
Earning the STC is not the end of the obligation. The STC holder has ongoing legal responsibilities that last as long as the certificate exists. They must report any failures, malfunctions, or defects related to their modification. They’re required to make the type certificate data available to the FAA and the National Transportation Safety Board on request. They must provide Instructions for Continued Airworthiness to every aircraft owner or operator using the modification, and they need to develop and distribute any design changes required by Airworthiness Directives.
Flight manual supplements must accompany each installation of the modification. These documents tell pilots what’s different about operating the aircraft with the STC-approved change, covering anything from new limitations to revised emergency procedures.
What Happens to Abandoned STCs
Some STC holders go out of business, become unreachable, or simply stop supporting their modifications. This creates a problem: aircraft owners with installed modifications can’t get updated documentation or required design changes. Federal law gives the FAA a path to address this. If an STC has been inactive for three or more years and the FAA cannot locate the owner of record (or their heir) using due diligence, the agency can designate the STC as abandoned and release the engineering data to anyone who requests it, provided the release would enhance aviation safety.
In early 2026, the FAA published a notice of intent to designate 30 STCs as abandoned under this process, with a public comment period running through August 2026. This mechanism keeps older modifications supportable even when the original certificate holder is no longer in the picture.
International Recognition
An STC issued by the FAA doesn’t automatically carry legal weight in other countries. International acceptance depends on bilateral agreements between the United States and other nations. Bilateral Airworthiness Agreements (BAAs) and Bilateral Aviation Safety Agreements (BASAs) establish frameworks for reciprocal certification of aircraft and modifications. These agreements include Implementation Procedures for Airworthiness that spell out how design approvals, including STCs, get validated across borders.
The arrangement with the European Union works through a Technical Implementation Procedure, or TIP, which functions like the standard implementation procedures but accounts for the fact that the EU represents multiple sovereign states, some of which retain limited responsibility over certain certification activities. In practice, this means an FAA-issued STC typically goes through a validation process with the foreign authority rather than being accepted outright. The complexity and cost of that validation vary by country and by the nature of the modification.