What Is a Supplemental Type Certificate (STC) in Aviation? A Clear Guide for Operators and MRO Teams

A Supplemental Type Certificate (STC) is one of the most common ways aircraft operators modernize fleets: adding connectivity, updating avionics, modifying interiors, integrating new mission equipment, or making structural changes, without starting from a clean-sheet aircraft certification.

This guide keeps it practical. You will learn:

• • What an STC is (in plain language)
  • When an STC is needed versus other approval routes
  • Who the typical stakeholders are (operator, design organization, and Maintenance, Repair and Overhaul (MRO) team)
  • What regulators mean by an “approved change”
  • What documentation and compliance mapping usually make or break timelines

And a quick reality check: STCs are not niche. A 2024 U.S. Department of Transportation (DOT) Office of Inspector General (OIG) letter notes the Federal Aviation Administration (FAA) has approved over 1,000 STCs for more than 400 organizations since January 2020.

What Is an STC?

The FAA definition is straightforward: an STC is a type certificate issued when an applicant receives FAA approval to modify an aeronautical product from its original design, and it approves both the modification and how it affects the original design.

The regulatory view (Title 14 of the Code of Federal Regulations (14 CFR), Part 21) goes one level deeper:

• • A supplemental type certificate consists of:
    1. FAA approval of a change in the type design of a product, and
    2. the product’s existing Type Certificate (TC).

So, in plain terms: an STC is an “approved change” to an already type-certified aircraft, engine, or propeller, packaged as a controlled design approval.

When Is an STC Needed?

You typically need an STC when the modification is a major change in type design (but not so extensive that it requires a new type certificate).

STC approval can be done by both TC holder and Non-TC holder like MRO/ third party.

Two useful regulatory anchors:

• • If you introduce a major change in type design and you are not the Type Certificate holder (MRO, third party), you must apply to the FAA for an STC (instead of amending the original type certificate).
  • If the change is so extensive that a “substantially complete investigation” is required, the FAA may require a new type certificate.

Practical examples where STCs are common

• • Avionics upgrades (for example, navigation, communication, surveillance integrations)
  • Cabin modifications (layouts, monuments, premium seating changes that affect design compliance)
  • Satellite communication and connectivity installations
  • Structural modifications (doors, reinforcements, conversions)
  • Mission equipment integrations (special mission aircraft, medical, surveillance, utility roles)

The exact “STC or not” decision depends on aircraft category, scope, and how the change is classified under the applicable rules and guidance. The key takeaway for operators and MRO teams: treat STC determination as an engineering + compliance decision early, not a paperwork step late.

What Does “Approved Change” Actually Mean?

“Approved change” does not mean “the parts fit” or “the system works.” It means the applicant has demonstrated—through engineering data, analyses, and any required tests—that the altered product still meets the applicable airworthiness requirements. That expectation is built into Part 21’s STC framework.

Also important: STC approval is tied to the defined configuration: the specific design data set and installation definition that was approved. If the installed configuration drifts from what was approved, the compliance basis can become fragile quickly.

Who Is Involved in an STC Program?

Think of an STC program as a three-lane effort: operations, engineering/compliance, and maintenance/installation.

1) Operator (airline, lessor, or aircraft owner)

• • Defines operational need (cost, capability, dispatch, cabin strategy)
  • Funds and schedules downtime
  • Owns fleet configuration management across tail numbers

2) Applicant / Design organization (often the STC holder)

• • Owns the approved design data and substantiation strategy
  • Interfaces with the regulator for the certification plan and approval path
  • Maintains the STC configuration, eligibility, and continued support

3) Maintenance, Repair and Overhaul (MRO) organization

• • Executes the installation in accordance with approved data
  • Manages embodiment, inspections, and records
  • Ensures maintenance instructions and continued airworthiness tasks are workable in real operations

4) Regulator (commonly FAA; sometimes validation by other authorities)

• • Confirms the certification basis and compliance approach
  • Reviews data and issues the STC approval

If your aircraft operates across jurisdictions, you may also encounter validation pathways. For example, the European Union Aviation Safety Agency (EASA) describes “Basic” classifications of FAA STCs under the FAA–EASA Technical Implementation Procedures (TIP) and notes that streamlined acceptance/validation depends on classification and submission of the complete technical data package.

Installing an Existing STC: One Detail Operators and MRO Teams Must Not Miss

If you find an STC you want to use, the FAA is clear: you must contact the STC holder to seek written permission, because the STC design data (drawings, specifications, and related information) is the property of the STC holder.

That aligns with the rule requirement that an STC holder must provide written permission acceptable to the FAA if someone is allowed to use the STC to alter a product.

A Practical STC Readiness Checklist (Operator + MRO Friendly)

Before you commit schedule and aircraft downtime, validate these early:

Scope clarity

What exactly is changing (systems, structure, interiors, performance, limitations)?

STC existence check

Is there already an STC covering your aircraft make/model/series and use case? (The FAA maintains STC information through its systems, and STC reference information is widely searchable through industry databases; start with the regulator’s sources where possible.)

Eligibility and applicability

Confirm aircraft model/series, serial number effectivity, and any configuration prerequisites.

Documentation and evidence ownership

Confirm you can legally access the approved data (written permission and data package access).

Embodiment plan

Downtime, kit readiness, tooling, manpower, and inspection requirements.

Continued airworthiness plan

Ensure Instructions for Continued Airworthiness (ICA) expectations and maintenance program impacts are addressed (this is often where “easy installs” become long-term burdens).

TAAL Tech: STC Documentation and Compliance Support That Makes Programs Easier to Execute

As an India-based engineering partner supporting aircraft programs in the United States, Europe, and beyond, TAAL Tech supports STC programs by strengthening the work that sits around approvals: engineering documentation, compliance mapping, and submission package preparation support.

TAAL Tech typically supports operators, design organizations, and MRO teams by:

• • Building compliance mapping from the change scope to the applicable requirements and means of compliance (what must be shown, and how)
  • Preparing structured submission-ready documentation packs (traceable, review-friendly, and configuration-controlled)
  • Supporting configuration discipline so the “approved change” remains representative through design, supply chain, and installation realities
  • Aligning with regulator expectations for complex projects (the FAA highlights the role of Advisory Circular (AC) 21-40 as a national reference for how complicated STC projects should be conducted and the importance of technical data package quality)

Frequently Asked Questions

1. Do you provide STC engineering documentation services?

TAAL Tech supports STC programs through engineering documentation development, compliance mapping, and submission package preparation support—helping teams present a clean, traceable “approved change” story.

2. Do you offer STC certification support services in India for global programs?

TAAL Tech is India-based and supports global operators and MRO teams with STC documentation and compliance support—often coordinating with design organizations, applicants, and regulatory pathways tied to FAA and EASA processes.

3. How can an India-based team support STC programs for United States operators?

Many STC programs are executed across regions: design and documentation can be developed globally while approvals and installs are coordinated with United States stakeholders. TAAL Tech supports by improving documentation quality, compliance traceability, and submission readiness so reviews and embodiment run smoother.

4. What does “STC submission package support” typically include?

It typically covers compliance mapping, document structuring, traceability matrices, configuration control support, and preparation of regulator-ready submission sets aligned to the certification plan and evidence expectations.

5. If we are installing an existing STC, do we still need support?

Often yes, because installation still requires correct applicability checks, access to approved data, and a clean record trail. The FAA notes you must contact the STC holder for written permission and access to the STC data.

 The Fastest STC Programs Treat Documentation as Engineering

An STC is not just “approval paperwork.” It is a controlled, regulator-accepted change to type design, supported by evidence that must remain coherent from design intent to MRO embodiment and continued airworthiness.

If your team is planning an STC – new development or installation of an existing STC – TAAL Tech can support with engineering documentation, compliance mapping, and submission package preparation support to help your program move with fewer iterations and clearer traceability.