Supplemental Type Certificate (STC)

A Supplemental Type Certificate is the regulatory instrument by which the FAA or EASA approves a design change to an already-certificated aircraft type, where that change is proposed and owned by an entity other than the original type certificate holder. The original TC holder covers the baseline aircraft — the factory-delivered configuration. Every modification that materially alters the type design, and is proposed by a third party, requires either an STC or an Amended Type Certificate (ATC). The STC route is used when the modifier does not own the original TC.

The STC application process under 14 CFR §21.113–§21.119 requires the applicant to demonstrate to the FAA that the proposed change complies with the applicable airworthiness standards — the same certification basis that applied to the original TC, or an updated basis negotiated with the FAA Aircraft Certification Office. For major changes to avionics, the relevant FAA Technical Standard Orders (TSOs) provide the equipment-level standard; RTCA standards (e.g., RTCA DO-178C for software, RTCA DO-254 for hardware) underpin the certification of the modification. FAA Order 8110.4 (Type Certification, current edition) governs the FAA's internal process for STC evaluation. Under EASA, the equivalent pathway is Part-21 Section A Subpart E, with the Design Organization Approval (DOA) framework under Subpart J providing the structural basis for EASA STC applicants.

STCs cover an enormous range of modification types in the general aviation and training environment. Avionics upgrades — Garmin G1000 NXi installations in Cessna 172s and Piper Archers, Garmin GTN 750Xi navigator installations, ADS-B Out transponder installations under the 2020 mandate (14 CFR §91.225) — are almost all STC-based modifications because the avionics manufacturer is not the aircraft manufacturer. Engine modifications — Supplemental engine replacement STCs covering Lycoming-to-Continental or piston-to-turboprop conversions — are common in the military surplus and specialized training fleet contexts. Airframe modifications — extended-range fuel systems, turbine conversions of piston aircraft (e.g., Soloy turbine conversion STCs for Bell 47 and similar helicopters), structural reinforcements for aerobatic certification — are also STC-governed.

The continuing-airworthiness implication of an STC is the most frequently misunderstood operational aspect. When an STC is installed, the aircraft's approved data set changes. Post-STC installation, the aircraft is maintained per: (a) the original AMM for all systems and structures not addressed by the STC; and (b) the STC's Instructions for Continued Airworthiness (STC-ICA) for all systems modified by the STC, where the STC-ICA supersedes the AMM. The boundary between (a) and (b) is defined in the STC-ICA and must be managed explicitly. A common failure mode: a maintenance organization services an avionics STC-installed system using the original AMM procedures rather than the STC-ICA — either because the STC documentation was not available in the maintenance record, or because the technician was not aware the STC existed. This constitutes use of non-approved data under §43.13 and EASA Part-145 145.A.45.

STC documentation requirements are permanent. Under §21.9 and EASA Part-21 Section A Subpart E, the STC holder must make the STC ICA available to any owner of a product modified under the STC. The STC number must be referenced in the aircraft maintenance records whenever STC-related work is performed. The STC documentation — including the STC certificate, the STC-ICA, the installation approval documentation, and any associated FAA-approved data (engineering orders, drawings) — must be retained with the aircraft's permanent maintenance records for the life of the aircraft.

Under the FAA-EASA BASA, STCs approved by the FAA are generally eligible for validation by EASA for aircraft of the same type certified under both systems. The validation process is not automatic — it requires a formal application and EASA review — but the workload is reduced compared to a full STC application. Bilateral validation enables the same STC modification package to be installed on both N-registered and EASA-registered aircraft of the same type without maintaining two completely separate approval documents.

For flight schools, STCs are especially relevant in three contexts. First, ADS-B Out compliance: the FAA §91.225/§91.227 ADS-B Out mandate (effective January 1, 2020) required most aircraft operating in Class B and C airspace, or above 10,000 ft MSL, to be equipped with 1090ES or UAT ADS-B Out transmitters meeting specific performance standards. The vast majority of these installations were completed under STCs issued by GDL 82, Garmin GDL 84, Uavionix tailBeacon, and similar manufacturers. Every school aircraft that underwent ADS-B Out installation has an STC in its records that governs the ADS-B equipment maintenance.

Second, EASA-registered training fleets sourcing used aircraft from the US market may find STCs installed before purchase that require EASA validation. An aircraft imported from the US with a Garmin G500 glass-panel STC installed may not have a corresponding EASA-validated STC — the STC-modified equipment may not be eligible for continued operation on the EASA register until the STC is validated. Import airworthiness assessments for used aircraft routinely surface this issue.

Aviatize's digital data and records module maintains an STC register for each aircraft. Each STC entry stores: the STC number, the issuing authority (FAA or EASA), the STC holder and installer, the installation date, the affected ATA chapters, the STC-ICA document reference, and any associated Airworthiness Directives applicable to the STC modification. When an AMP task is assigned, the system cross-references the task's ATA chapter against the aircraft's STC register — if the chapter is governed by an STC-ICA, the system flags the applicable STC data reference rather than pointing to the AMM, preventing inadvertent use of superseded base-AMM procedures.

For fleets acquiring additional aircraft, the STC register comparison function identifies modifications present on new acquisitions that differ from the existing fleet configuration — ensuring that the maintenance organization is aware of unfamiliar STCs on newly added aircraft before the first maintenance event. For combined ATO/AOC operators maintaining both US- and EASA-registered aircraft, the system separately tracks FAA-approved and EASA-validated STC status for each modification, flagging any STC that has FAA approval but no corresponding EASA validation on an EASA-registered tail.