STCs vs. Field Approvals: When Each Path Makes Sense for Aircraft Modifications

A Supplemental Type Certificate (STC) and a field approval both authorize major aircraft modifications, but they serve different purposes. An STC, issued under 14 CFR §21.113, approves a modified type design and can be installed on every aircraft on its applicability list. A field approval, documented on FAA Form 337 and authorized by an Aviation Safety Inspector, approves a single major alteration on a single aircraft. Choose an STC for fleet-wide or commercial designs; choose a field approval for one-off installations.

For any aircraft modification beyond what the original type design permits, an applicant has two principal paths to FAA approval: the Supplemental Type Certificate and the field approval. They are not interchangeable. Each sits in a different part of the regulations, requires different evidence, and carries different commercial implications. Confusing them or selecting the wrong one for a program can cost months of schedule and significant engineering rework.

This article walks through what each path is, when each applies, who issues them, and how to decide between them. The framing assumes the aircraft is type-certificated under 14 CFR Part 23, 25, 27, or 29. Modifications to UAS approved under §21.17(b) follow a different process, which we cover in our post on Part 21.17(b) special class certification.

What an STC actually is

The STC is established under 14 CFR Part 21, Subpart E. Specifically, §21.113 governs its application and §21.115 sets procedural requirements. An STC is an FAA approval of a change to a type design that is significant enough to require its own approval but not extensive enough to warrant a new type certificate. The STC holder owns the modified design and may install it on any aircraft that matches the applicability listed on the certificate.

Three points define the STC:

1. It is design-level approval. When the FAA issues an STC, it certifies that the modified design including all compliance findings, engineering data, and supporting documentation meets the applicable airworthiness standards. Aircraft on which the STC is installed inherit that approval.

2. It is transferable and licensable. The STC holder may install the modification on multiple aircraft, sell the rights, or license the design to third parties. For modifications intended for commercial deployment avionics suites, sensor installations, cabin reconfigurations, surveillance payload integrations the STC is the only path that supports a scalable business model.

3. It involves engineering data approval. Most STC programs rely on a Designated Engineering Representative to find compliance with the applicable airworthiness standards and sign FAA Form 8110-3, the Statement of Compliance with Airworthiness Standards. We covered the DER’s role in detail in our post on what an FAA DER is and why drone and eVTOL projects need one.

What a field approval is

A field approval is a fundamentally different instrument. It is issued by an FAA Aviation Safety Inspector typically from the Flight Standards District Office and it approves a single major alteration or major repair on one specific aircraft. The authorizing document is FAA Form 337, Major Repair and Alteration. The inspector signs the form to indicate that the submitted data has been examined and found acceptable for the specific aircraft and modification described.

Field approval procedures are governed by FAA Order 8300.16 and supported by Advisory Circular AC 43-210B. The applicable performance rules for the underlying alteration are in 14 CFR §43.13, which requires that work be performed using methods, techniques, and practices acceptable to the Administrator.

Three defining characteristics:

1. It approves one alteration on one aircraft. A field approval does not establish a design that can be reused. If the same modification is installed on a second aircraft, that second installation requires its own approval, though “approval by reference” to a prior Form 337 is sometimes available at inspector discretion.

2. The inspector evaluates the data, not the design. The Aviation Safety Inspector’s role is to determine whether the data submitted is acceptable for the specific aircraft and the specific alteration. This is a lower bar than an STC’s formal compliance finding, but it depends on the inspector’s judgment, the FSDO’s workload, and the complexity of the alteration.

3. Some alterations are off-limits. AC 43-210B identifies categories not eligible for field approval. Complex alterations affecting primary structure, certain avionics integrations, modifications involving flight characteristics, and any alteration requiring a formal engineering finding of compliance generally require an STC.

When each makes sense

The decision is rarely a close call once program scope is clear.

Choose an STC when: - The modification will be installed on more than one aircraft, or program economics depend on amortizing certification cost across a fleet. The modification affects primary structure, flight controls, or performance. The applicant intends to commercialize the design as a product or license it to operators. The modification involves novel technology, integrates with avionics requiring DO-178C or DO-254 compliance, or relies on materials and processes that require formal qualification.

Choose a field approval when:  The alteration is to one specific aircraft operated by the applicant. The modification is straightforward, well-precedented, and within the categories AC 43-210B identifies as eligible. The applicant has no commercial interest in reusing the design. The local FSDO has confirmed, in pre-application discussion, that they will entertain the submission.

Practical considerations

Two further points drive the choice in practice.

Schedule. STC programs run from six months to several years depending on complexity. A field approval, when the inspector is willing and the data is well-prepared, can sometimes be issued in weeks. For one-off operational needs, this schedule difference matters.

Inspector availability. Field approvals depend on the participating ASI’s willingness and the FSDO’s workload. Some FSDOs have largely stopped accepting field approval requests for anything but the simplest alterations, redirecting applicants to the STC path. Confirm availability before assuming the field approval option is open.

Hybrid sequencing. Some programs use a field approval for a proof-of-concept installation on one test aircraft, then transition to an STC once the modification is validated and commercialization is approved. This sequencing is legitimate but should be planned with the certifying ACO from the outset.

How Martin Solutions supports both paths

Martin Solutions provides FAA DER services that span STC programs and the engineering data preparation that supports field approval submissions. For STC programs, our work includes Project Specific Certification Plans, compliance finding on Form 8110-3, and coordination with the certifying Aircraft Certification Office. For field approvals, we prepare the technical data packages, drawings, and substantiation that ASIs need to evaluate Form 337 submissions efficiently.

For details on these workflows, see our airworthiness certificate services and FAA airworthiness certification services pages. For modifications targeting government aviation customers, where commercial derivative aircraft and military mission system installations are common, see our government aviation services page.

Frequently Asked Questions

Is a field approval cheaper than an STC?

Per installation, yes. A single field approval will almost always cost less than a full STC program. But the field approval covers one installation only. If the modification will be installed on more than two or three aircraft, the STC typically becomes more economical over the program life because the design approval is reusable.

Can a DER approve a field approval?

A DER cannot issue a field approval only an FAA Aviation Safety Inspector can. However, a DER can prepare and approve the engineering data that supports a field approval submission. Well-prepared DER data significantly improves the likelihood that an ASI will approve the Form 337.

What’s the difference between a major and a minor alteration?

14 CFR §1.1 defines a major alteration as one not listed in aircraft specifications that might appreciably affect weight, balance, structural strength, performance, powerplant operation, or flight characteristics or that cannot be done by elementary operations. Minor alterations don’t require a Form 337 or formal approval, but they must still be documented and performed by qualified personnel under 14 CFR Part 43.

Can I convert a field approval into an STC?

Not directly, but the engineering data developed for a field approval often forms the foundation of a subsequent STC application. Many programs use a field approval to support an initial prototype installation, then pursue an STC once the modification is proven and commercialization is planned.

Does an STC apply to all aircraft of the same model?

Only to those listed on the STC’s applicability statement. The applicability may be limited to specific make, model, and serial number ranges. Installing the modification on an aircraft outside the listed applicability requires a separate approval either an amended STC, a new STC, or a field approval for that aircraft.

Are field approvals allowed for UAS or eVTOL?

Field approvals exist within the framework that applies to type-certificated aircraft. UAS approved under §21.17(b) and powered-lift aircraft under the March 2026 final rule have their own modification processes that don’t follow the same Form 337 pathway. For UAS modifications, applicants typically work through amendments to the certification basis with the issuing ACO.

What is FAA Order 8300.16?

It is the FAA’s internal procedural guidance for Aviation Safety Inspectors evaluating major repair and major alteration data submissions. It establishes the criteria inspectors apply when deciding whether to grant a field approval and identifies categories of alterations that fall outside their delegated authority.

Who should I contact first when planning a modification?

For an STC, the Aircraft Certification Office with geographic responsibility for the applicant. For a field approval, the local FSDO. An early conversation with the relevant office or with a DER who has working relationships with the office often saves significant rework later in the program.

Bottom line

STCs and field approvals are not different versions of the same thing. They are different regulatory mechanisms with different scopes, different evidence requirements, and different commercial implications. Selecting the right one early and structuring the supporting data accordingly is one of the most consequential decisions a modification program makes.