STC and modification houses

This category covers the companies that change aircraft they did not design: cabin reconfigurations, freighter conversions, special mission installations, avionics retrofits, winglets, auxiliary tanks. The manufacturer of the aircraft belongs in the platform guides, the box being installed belongs in a component guide, and the shop that only performs maintenance belongs in MRO. What sits here is the ownership of an approved change.

The approval, and who owns it

A person who does not hold the type certificate and introduces a major change in type design must apply for a supplemental type certificate.¹ An STC is the FAA's approval of that change together with the type certificate previously issued for the product, and the FAA describes it as approving both the modification and how the modification affects the original design.² The privileges are specific: an STC holder may obtain airworthiness certificates for aircraft, may obtain approval for installation on certificated aircraft, and may obtain a production certificate for the change.¹ The subpart also makes STC holders responsible for providing written permission where an alteration depends on their approval.¹ Two practical consequences follow. If the modification you want already exists as somebody's STC, you are buying access to that certificate as well as the labour, and the FAA's own guidance treats installation under an STC that is not your own as its own subject.² If the modification does not exist, you are funding a certification programme, and it will have a certification basis, a compliance plan and a schedule that belongs to a regulator's queue rather than to your fleet plan.

What the shop must hand you afterwards

The holder of a design approval, including a type certificate or a supplemental type certificate, must furnish instructions for continued airworthiness to the owner of each affected product, and where a rotorcraft maintenance manual carries an airworthiness limitations section, approved changes to replacement times and intervals must be made available on request to any operator of that type.³ This is the part buyers forget. A modification adds inspections, life limits and a supplement your maintenance programme has to absorb. Ask for the instructions for continued airworthiness before you sign, read what they add per year, and check that your maintenance provider can perform them.

Field approval is not a smaller STC

A one-off alteration can in some cases be approved in the field, and the FAA publishes what qualifies, what data is required, and the Form 337 that records a major repair or alteration.⁴ It is a narrower instrument and it is not automatically portable to a second aircraft. Ask which route the shop intends before quoting, and ask what happens to your fleet plan if the route changes mid project.

Where the record can be checked

Supplemental type certificates are public and searchable, so a claimed modification approval can be confirmed, with its holder and the models it covers.² That check proves the certificate exists and what it applies to. It does not prove that the installation on your serial number was done well, that the paperwork travelled with the aircraft, or that the STC holder still supports the design. Ask for the STC number, the approved model list, the installation records and the current status of the holder's support.

Questions that sort shops

Ask whether the approval will be yours, theirs, or licensed. Ask what the downtime estimate is and what the last three comparable jobs actually took. Ask which parts of the work go to subcontractors and under whose certificate they are released. Ask what the instructions for continued airworthiness will add to your annual maintenance.

The companies below are listed on public facts, each with its source linked.