A Supplemental Type Certificate (STC) is a national aviation authority approval issued when an applicant alters an aircraft from its original type design. Understanding when you need an STC is essential for aircraft owners, maintenance directors, and parts brokers undertaking avionics upgrades, engine conversions, or interior reconfigurations.
Modifying an aircraft without approved engineering data violates airworthiness rules and grounds the aircraft. Whether you are installing a modern glass cockpit touchscreen or adding winglets to a business jet, securing and documenting the correct STC ensures your modification remains fully legal and insurable.
Key Takeaways
- An STC is required whenever an aircraft modification alters the original type certificate design in a manner not covered by standard manufacturer data.
- STCs are issued by regulators like the FAA and EASA to approve specific engineering data, installation instructions, and part eligibility.
- Purchasing an STC typically grants a single-installation license or a kit license; unauthorized copying of STC data violates intellectual property and law.
- Installed STCs must be permanently recorded in aircraft logbooks and paired with a completed FAA Form 337 major alteration sign-off.
What Is an STC and When Do You Need One?
When an Original Equipment Manufacturer designs an aircraft, the baseline configuration is approved under a Type Certificate. Any subsequent major change—such as swapping analog instruments for a modern Garmin glass flight deck, installing auxiliary fuel tanks, or changing engine models—exceeds standard maintenance data. Under FAA regulations (14 CFR Part 21) and EASA rules, that modification requires a Supplemental Type Certificate (STC).
An STC approves both the physical product and the engineering data package required to install it. If a mechanic attempts a major structural or system alteration without an approved STC or dedicated field approval, the modification is unapproved, invalidating the aircraft's airworthiness certificate.
| Modification Type | Approval Mechanism | Typical Documentation Required |
|---|---|---|
| Minor Repair / Alteration | Standard logbook sign-off | Standard maintenance manual / AC 43.13 |
| Major Alteration (Standard) | FAA Form 337 Field Approval | Engineering data + Form 337 sign-off |
| Complex System Upgrade | Supplemental Type Certificate | STC certificate, AML, & installation manual |
| PMA Replacement Part | 14 CFR Part 21 Subpart K | PMA stamp & installation eligibility supplement |
STC Data Packages, AMLs, and Licensing Rights
An STC is not just a certificate hanging on a wall; it is a comprehensive engineering package containing master drawing lists, wiring diagrams, weight-and-balance amendments, and instructions for continued airworthiness. Every STC includes an Approved Model List (AML) specifying the exact aircraft makes, models, and serial number ranges eligible for that modification.
Purchasing an STC requires buying a license from the STC holder for each individual aircraft tail number. Buying one STC data package and installing the kit across three different company aircraft without purchasing separate installation licenses is illegal copyright infringement and a regulatory violation. When comparing OEM vs PMA vs surplus parts, note that many STC holders bundle proprietary PMA parts directly into their STC kits, requiring buyers to source replacement components through authorized channels.
How STCs Interact With Logbooks and Form 337s
Once an STC is installed, the modifying repair station must record the installation in the aircraft maintenance logbooks, complete an FAA Form 337 major repair and alteration report, and place a copy of the STC certificate in the permanent aircraft records.
During pre-purchase evaluations following a used aircraft inspection guide, technicians verify that every installed STC matches an entry in the logbooks and that the aircraft serial number appears on the STC's Approved Model List. Unrecorded or mismatched STCs create severe redelivery hurdles during lease returns.
Common STC Compliance Mistakes
Three recurring errors trip up modification projects. First, assuming an STC approved for a twin-engine turboprop automatically applies to a heavier or lighter variant without checking the AML serial number effectivity. Second, losing the physical STC licensing document, which leaves future inspectors unable to verify legal installation rights.
Third, failing to update aircraft weight and balance records immediately after completing an STC installation that alters basic empty weight and center of gravity.
Frequently Asked Questions
Can I copy an STC data package to install a modification on a second aircraft?
No. An STC license is issued for a single specific aircraft tail number or serial number. Installing the modification on additional aircraft requires purchasing separate installation licenses from the STC holder for each airframe.
What is an Approved Model List (AML)?
An Approved Model List is the official annex issued with an STC that explicitly details every aircraft make, model, type certificate number, and serial number range eligible to receive that specific modification.
Is an STC the same thing as an FAA Form 337?
No. An STC is the master design approval issued by the aviation authority for a major modification, whereas FAA Form 337 is the administrative record used by mechanics and repair stations to document that a major repair or alteration was performed in accordance with approved data.