A supplemental type certificate lets an aircraft owner or manufacturer modify a plane in a way the original design didn't cover

A supplemental type certificate (STC) is a formal approval from the Federal Aviation Administration (FAA) that permits someone to change an aircraft in a significant way — installing new equipment, altering the structure, or changing how the plane operates — without having to get the entire aircraft re-certified from scratch. Think of it as permission to modify something that already has an airworthiness certificate.

The person or company seeking the STC must prove to the FAA that the modification is safe and won't make the aircraft less airworthy than it was before. Once approved, the STC becomes a permanent record attached to that specific aircraft, and any future owner inherits it.

Key Takeaways

  • An STC is issued by the FAA to one specific aircraft (or sometimes a fleet) and allows a modification that wasn't part of the original design approval.
  • The person or company requesting the STC must submit technical data showing the modification is safe and won't reduce the aircraft's airworthiness.
  • Common STCs cover avionics upgrades, engine modifications, interior changes, and structural reinforcements.
  • An STC is permanent — it stays with the aircraft and transfers to new owners, but the aircraft must be maintained according to the STC's requirements.

Why an aircraft needs an STC instead of just making changes

When an aircraft was originally certified by the FAA, that certification covered the design, materials, systems, and performance of that specific model. If you want to change something material — not just routine maintenance or repair — you're stepping outside what was originally approved. The FAA needs to know the change won't create a safety problem or an unintended consequence.

An STC is the formal pathway for that approval. Without it, the modification would be illegal to fly, even if it seems like a straightforward upgrade. For example, installing a new glass cockpit, adding a cargo door, upgrading the engine, or reinforcing the fuselage all require an STC because each one changes how the aircraft performs or operates.

Who requests an STC and what they have to prove

An STC can be requested by the aircraft manufacturer, a supplemental manufacturer (a company that specializes in aircraft modifications), an owner, or a maintenance facility. Whoever requests it must submit a detailed process to the FAA that includes engineering drawings, performance calculations, test data, and a plan for how the modification will be inspected and maintained.

The applicant must show that the modification doesn't reduce the aircraft's safety margin, that it won't interfere with other systems, and that it meets all applicable FAA regulations. For some modifications, this means wind tunnel testing, computer modeling, or actual flight tests. The FAA reviews the submission and either approves it, asks for more information, or denies it.

Common types of modifications that need an STC

Avionics upgrades are among the most frequent STCs — installing a modern glass cockpit, adding a terrain awareness system, or upgrading the autopilot. Engine modifications, such as installing a different engine model or increasing power output, also require an STC because they change the aircraft's performance envelope.

Interior changes like installing new seating, adding a galley, or reconfiguring a cabin layout need an STC if they affect weight distribution, emergency egress, or structural loads. Structural modifications — reinforcing the wing, adding a cargo door, or extending the fuselage — always require an STC. Even some paint schemes or external modifications can require an STC if they affect aerodynamics or weight.

How an STC is documented and transferred

Once the FAA approves an STC, it issues an STC number (for example, SA12345NM). That number is recorded in the aircraft's permanent records, along with the technical data package that describes exactly what was changed and how it must be maintained. The aircraft's airworthiness certificate is not reissued; instead, the STC becomes part of the aircraft's history.

When an aircraft is sold, the STC transfers automatically to the new owner. The new owner inherits both the benefit of the modification and the responsibility to maintain it according to the STC's requirements. Some STCs require recurring inspections or compliance with specific maintenance procedures, and those obligations stay with the aircraft.

The difference between an STC and other types of FAA approvals

An STC is different from a type certificate, which is what the original manufacturer receives when the FAA first approves an aircraft design. A type certificate covers the entire aircraft as designed; an STC covers only the modification. An STC is also different from a field approval, which is a temporary, one-time permission for a minor modification or repair that doesn't meet standard procedures. Field approvals are not permanent and don't transfer to new owners.

A parts manufacturer approval (PMA) is another related but distinct approval — it certifies that a replacement part is equivalent to the original part. An STC, by contrast, certifies that a modification to the aircraft itself is safe and airworthy.

What happens if an aircraft has an STC and needs maintenance or repair

The aircraft must be maintained according to both the original manufacturer's maintenance manual and any additional requirements spelled out in the STC. A maintenance facility working on the aircraft needs to have access to the STC documentation to understand what was changed and how to service it correctly.

If a repair or future modification affects the area covered by an existing STC, the technician may need to refer back to the STC to may support the repair doesn't compromise the modification. Some STCs require that certain inspections be performed by an FAA-designated representative or an authorized inspector, not just any maintenance facility.

Frequently Asked Questions

Can an owner install a modification without an STC if they think it's safe?

No. Flying an aircraft with an unapproved modification is illegal under FAA regulations, regardless of whether the owner believes it's safe. The FAA requires formal approval through an STC, a field approval, or another authorized pathway before the modification can be flown.

Does an STC increase the cost of owning or maintaining an aircraft?

An STC itself doesn't add cost to ownership, but the modification it covers does. Some STCs require additional maintenance procedures or recurring inspections, which can add to maintenance expenses over time. However, many STCs (like avionics upgrades) reduce operating costs or improve capability.

Can an STC be reversed or removed from an aircraft?

Technically, yes, but it's uncommon. Removing a modification would require the aircraft to be returned to its original configuration and re-inspected. In practice, most owners keep the modification and the STC because removing it is expensive and time-consuming. The STC remains part of the aircraft's records permanently.

Who can work on an aircraft that has an STC?

Any FAA-certified maintenance facility can work on an aircraft with an STC, as long as they have access to the STC documentation and follow its requirements. Some STCs specify that certain work must be done by the STC holder or an authorized representative, but most allow any may have access to mechanic to perform maintenance.

How long does it take to get an STC approved?

The timeline varies widely depending on the complexity of the modification. straightforward modifications might be approved in a few months, while complex structural or system changes can take a year or more. The FAA's review time depends on the completeness of the process and how much additional testing or data is needed.