Direct Answer — STCs and Their Records in Four Sentences
A Supplemental Type Certificate is the FAA approval, required by 14 CFR §21.113, for a major change to a product's type design that is not great enough to require a new type certificate — and under §21.117(b) the STC consists of the FAA's approval of the change plus the type certificate previously issued for the product. Installing an STC on a specific aircraft is a major alteration, recorded on FAA Form 337 under §43.9(d); the STC and its FAA-approved installation data are the approved technical data the 337 references. If you are not the STC holder, §91.403(d) prohibits altering the aircraft under that STC unless you have written permission from the holder, which §21.120 requires the holder to furnish in a form acceptable to the FAA. So the per-aircraft STC record set is the STC certificate, the written permission-to-use letter, the approved installation data and any Instructions for Continued Airworthiness, the executed Form 337, and the companion weight-and-balance and flight-manual updates the alteration triggers.
Sources: 14 CFR §21.113, §21.117, §21.120, §91.403(d), §43.9(d).
The STC is one of the most consequential documents in an aircraft's history, because it is the legal basis for a permanent change to the airframe — and one of the most commonly mishandled, because operators tend to file the STC certificate and forget the two records that prove the installation was authorized and performed: the written permission-to-use letter and the Form 337. When the installed configuration on the airframe does not reconcile against those records, the modification reads as undocumented. FAA civil penalties for recordkeeping violations under 49 U.S.C. § 46301 reach $75,000 per violation (with a lower cap of $1,875 for an individual or small-business concern) under 14 CFR §13.301 for violations on or after December 30, 2024 — but the more common cost is commercial: a prebuy that stalls because a winglet kit on the wing has no matching paperwork.
What an STC Actually Is — §21.113, §21.117, §21.119
An STC sits between a minor change and a new type certificate. Three sections of Part 21 define its trigger, its content, and its privileges.
The trigger — §21.113
§21.113 sets out when an STC is required. A person who holds the type certificate and makes a major change in type design — one not great enough to require a new application for a type certificate — must apply for an amended type certificate or an STC. A person who does not hold the TC but alters a product by introducing a major change in type design not great enough to require a new TC must apply to the FAA for an STC. The application must be made in the form and manner the FAA prescribes. That second case is the everyday one: a modification shop or equipment maker who does not own the original TC develops a change and certifies it via STC.
The content — §21.117
Under §21.117(a), an applicant is entitled to an STC if it meets the requirements of §§21.113 and 21.115 — the latter pulling in the change-approval requirements of §21.33 and §21.53 and, where relevant, the noise (Part 36), fuel-venting and emissions (Part 34), and fuel-efficiency (Part 38) standards. §21.117(b) then defines what the certificate is: it consists of the approval by the FAA of a change in the type design of the product, together with the type certificate previously issued for the product. That dual nature — change plus underlying TC — is why it is called supplemental: it does not stand alone, it supplements the existing type design.
The privileges — §21.119
§21.119 lists what the STC holder may do: for aircraft, obtain airworthiness certificates; for other products, obtain approval for installation on certificated aircraft; and obtain a production certificate under subpart G for the change approved by the STC. The middle privilege — approval for installation on certificated aircraft — is the one that created the STC aftermarket. A holder certifies a change once and can then have it installed across every aircraft of the approved make and model, rather than re-proving the design for each tail.
Where the STC sits — the §21.93 change ladder
14 CFR §21.93§21.93 classifies changes in type design. A minor change is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are major changes. The STC universe is the band of major changes that are still not so extensive that the product becomes a fundamentally new design requiring a new type certificate.
Minor change
No appreciable airworthiness effect. No STC. Documented by an ordinary maintenance/alteration entry.
Major change → STC
Major change in type design, not great enough to require a new TC. This is the STC band. Installed as a major alteration on a Form 337.
New design
A change so extensive the product is essentially new — requires a new or amended type certificate, not an STC.
The distinction between major and minor is the same hinge that decides whether a job needs a Form 337 at all — which is why the STC story and the major-versus-minor story are two halves of the same compliance question. If you want the determination side in depth, see major vs minor repair and alteration determination and the mechanics of the form itself in FAA Form 337 major repair and alteration records.
The Permission-to-Use Rule — §91.403(d) and §21.120
This is the STC obligation operators miss most often. You generally cannot install someone else's STC without their written permission — and that permission is a record.
The prohibition — 14 CFR §91.403(d)
§91.403(d) provides that a person must not alter an aircraft based on a supplemental type certificate unless the owner or operator of the aircraft is the holder of the STC, or has written permission from the holder. The duty lands on the side performing the alteration: no permission, no authorized installation — even if the work is otherwise correct.
This is a Part 91 maintenance-responsibility rule, so it reaches every US-registered civil aircraft, not just air carriers.
The holder's duty — 14 CFR §21.120
§21.120 is the mirror image: a supplemental type certificate holder who allows a person to use the STC to alter an aircraft, aircraft engine, or propeller must provide that person with written permission acceptable to the FAA. So the permission letter is not a courtesy — furnishing it is the holder's regulatory obligation, and obtaining it is yours.
In practice this is the STC license, authorization, or permission-to-use letter that shops obtain from the certificate holder for each installation.
If you hold the STC yourself, you do not need a permission letter — but you still need the 337
§91.403(d) carves out the owner or operator who is the STC holder. Some operators and shops own STCs they developed; for those, the permission-to-use letter is not in play. What never goes away is the rest of the package — the installation is still a major alteration that needs an executed Form 337, approved data, a return to service by an appropriately authorized person, and the companion record updates. Holding the STC removes one record; it does not remove the alteration.
The STC is approved data — but it does not return the aircraft to service
An STC, with its FAA-approved installation instructions, is the approved technical data that makes the alteration eligible for a Form 337 and supports the return to service. But the data approval and the return to service are different acts. The return to service after a major alteration is performed by an authorized person — typically an inspection-authorization holder approving the work against the approved data, a certificated repair station, or an air carrier under its own rules. The STC supplies the why it is airworthy; the 337 and the authorized signature supply the it was installed and returned to service on this tail. For the deeper return-to-service question, see major vs minor determination and IA renewal requirements.
Could you produce the permission letter and 337 for every STC on your fleet today?
FileFlo classifies STC certificates, permission-to-use letters, installation data, and Form 337s the day they enter your records, indexes them to the right tail, and flags any installed configuration whose authorizing paperwork is missing — so the gap surfaces now, not at prebuy. See the full Aviation compliance coverage.
The Per-Aircraft STC Record Set
An STC installation generates a small constellation of records — not one document. Here is what belongs in the aircraft file for each installed STC.
The STC certificate
§21.117(b)The FAA-issued certificate identifying the approved change and the make/model(s) it applies to. It consists of the FAA approval of the type-design change plus the underlying type certificate.
Written permission to use the STC
§91.403(d) / §21.120When you are not the STC holder, the holder must furnish written permission acceptable to the FAA before the STC may be used to alter your aircraft. This letter is a record you keep per affected tail.
Approved installation instructions + ICA
STC data packageThe FAA-approved installation data the 337 references, plus any Instructions for Continued Airworthiness the holder furnishes — new recurring inspections, life limits, or maintenance steps fold into your program.
The executed FAA Form 337
§43.9(d) / §91.417(a)(2)(vi)The record that the alteration was performed on this aircraft and approved for return to service. A major-alteration 337 is a permanent record that transfers with the aircraft at sale.
Revised weight and balance + equipment list
companion recordsSTC installs routinely change empty weight, CG, and the equipment list. The revised W&B and equipment list are the classic companion records an inspector expects to find next to a fresh alteration.
AFM supplement / limitation changes
§43.5(c) / §91.9Where an STC changes operating limitations or flight data, the approved flight manual must be revised and the AFM supplement carried as required — another per-aircraft record the modification generates.
Two records the STC quietly creates downstream
- New recurring inspections or life limits. When an STC alters the maintenance picture, the holder commonly furnishes Instructions for Continued Airworthiness. Those can introduce recurring inspections, new life-limited parts, or supplemental maintenance steps that did not exist on the unmodified aircraft — and the resulting inspection and parts records then live in your aircraft records, not in the installation folder. Fold the ICA into your aircraft records and inspection program.
- The reconfigured airframe baseline. An STC permanently changes what the aircraft is. The revised weight and balance, the equipment list, and — for many STCs — an Airplane Flight Manual supplement become the new operating baseline. A current installed-configuration list that does not include every active STC is itself a records defect, because it no longer describes the aircraft an inspector is standing next to.
For a Part 135 operator, the STC set is part of the airworthiness surface an inspector reconciles during surveillance: installed configuration against the alteration record, against the ARROW documents, against the AD status — and STCs interact with ADs in a way worth watching: an AD can be written against an STC's installed equipment, so an aircraft can inherit AD applicability the moment the STC is installed. The records have to reflect the aircraft as it actually flies, not as it left the factory. See what records a Part 135 operator must keep for the broader set.
How Long STC Records Live — the §91.417 Permanent Tier
Because an STC installation is a major alteration, its Form 337 lands in the permanent-record tier — and the rest of the STC package is most usefully treated the same way.
The alteration 337 — permanent
§91.417(a)(2)(vi): transfers with the aircraft at sale
§91.417(a)(2)(vi) lists copies of the forms prescribed by §43.9(d) for each major alteration to the airframe and currently installed engines, rotors, propellers, and appliances among the records retained and transferred with the aircraft when it is sold — effectively the life of the aircraft. An STC installation is a major alteration, so its 337 sits in that permanent tier alongside life-limited parts status and AD status.
The rest of the package — keep it as long
Permission letter, STC certificate, installation data, ICA
The STC certificate, the permission-to-use letter, and the installation data are the proof that the 337 was authorized and that the configuration is approved. There is little reason to keep the 337 forever but discard the documents that make it meaningful. The provable practice is to retain the entire STC package for as long as the modification is on the aircraft — and to transfer it with the aircraft, so the next owner inherits a configuration they can prove rather than one they have to reconstruct.
Two retention nuances worth knowing
- Removing an STC is also an alteration. Taking a modification back out — returning the aircraft to its prior configuration — is itself a change in type design that must be performed and documented under approved data, with its own Form 337 where the change is major. The records should show both the install and any later removal, so the configuration history is continuous rather than implied.
- CAMP operators keep equivalent records under their own rules. §91.401(b) provides that §91.417 (among others) does not apply to an aircraft maintained under a continuous airworthiness maintenance program as provided in Part 121, 129, or §§91.1411 or 135.411(a)(2). Those operators keep equivalent maintenance and configuration records under their air-carrier rules — for Part 135 CAMP operators, the parallel requirements covered in Part 135 CAMP maintenance recordkeeping — so the STC trail lives there rather than in §91.417.
Want a structured look at where your aircraft records stand against the FAA's expectations? The free FAA Readiness Score walks your document set in about three minutes.
Get Your Readiness ScoreThe Five Ways STC Records Go Wrong
Every one of these is an administrative failure, not an engineering failure — which is exactly why disciplined document handling prevents all five.
The STC certificate on file, the permission letter missing
Operators routinely keep a copy of the STC certificate and assume that proves authorization. It does not. §91.403(d) requires written permission from the holder when you are not the holder, and §21.120 makes furnishing it the holder’s duty. A configuration installed under someone else’s STC with no permission-to-use letter is an authorization gap that a thorough prebuy or FAA records review will surface.
No Form 337 for the installation
An STC install is a major alteration, and §43.9(d) requires major alterations to be recorded on a Form 337. When the 337 was never executed — or was executed but never made it into the aircraft records — the installation is undocumented under §91.417(a)(2)(vi). The STC certificate alone does not record that the change was actually performed on this tail and returned to service.
Configuration does not match the records
A modification is on the airframe — winglets, an avionics suite, an auxiliary fuel tank — but the records do not list it, or list a different revision than what is installed. Reconciling installed configuration against the alteration record is a standard inspection and prebuy step; a mismatch reads as undocumented until an appropriately rated person evaluates and re-documents it against the approved data.
STC ICA never folded into the maintenance program
Where an STC came with Instructions for Continued Airworthiness — new recurring inspections, life limits, or maintenance steps — those obligations have to live in the aircraft’s maintenance and inspection program, not in the installation folder. An ICA item that was never added to the program is a recurring requirement nobody is tracking, and the missed inspection becomes the finding.
Companion records left stale after the install
STC installs change empty weight, CG, equipment lists, and often the flight manual under §43.5(c) and §91.9. The 337 gets executed; the weight and balance, equipment list, and AFM supplement lag. An inspector who finds a fresh alteration next to an outdated W&B sheet and an unrevised equipment list has found multiple findings, not one.
Where FileFlo fits — the document layer, honestly scoped
FileFlo does not develop your STC, approve your installation data, perform the alteration, or return your aircraft to service — those are acts of certificate holders, repair stations, and certificated mechanics. What FileFlo does is keep the paper trail those people create provable: it classifies STC certificates, permission-to-use letters, installation data, ICA, and Form 337s as they enter your records, indexes them to the right tail, separates the permanent §91.417(a)(2) set from shorter-lived records, and flags installed modifications whose authorizing documents are missing — before an inspector or a buyer does.
When the FSDO letter or the prebuy request arrives, the answer is an organized, audit-ready export instead of a week of folder archaeology.
Frequently Asked Questions
What is a Supplemental Type Certificate (STC)?
An STC is the FAA approval that authorizes a major change to a product's type design that is not great enough to require a brand-new type certificate. The requirement comes from 14 CFR §21.113: if you make a major change in type design and you do not hold the original type certificate, you must apply to the FAA for an STC. Under §21.117(b), the supplemental type certificate itself consists of two things — the FAA's approval of the change in the type design of the product, and the type certificate previously issued for the product. In plain terms, an STC lets a third party design and certify a modification (an avionics suite, a winglet kit, an interior, an engine conversion, an extended-range fuel system) and then make it available for installation on other aircraft of the same approved type.
Does installing an STC count as a major alteration that needs a Form 337?
In nearly all cases, yes. An STC exists because the underlying change is a major change in type design (§21.113), and installing it on a specific aircraft is a major alteration as defined in 14 CFR §1.1 — an alteration not listed in the aircraft, engine, or propeller specifications that might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness. Major alterations are documented on FAA Form 337 under §43.9(d) and Appendix B to Part 43. The STC and its FAA-approved installation instructions are the approved technical data that the Form 337 references; the 337 is the record that the alteration was actually performed on this tail and approved for return to service. The two documents travel together for the life of the aircraft.
Do I need permission to install an STC I did not develop?
Yes — and this is the single most overlooked STC record. Under 14 CFR §91.403(d), a person must not alter an aircraft based on a supplemental type certificate unless the owner or operator of the aircraft is the holder of the STC or has written permission from the holder. The flip side is §21.120, which places the duty on the STC holder: a supplemental type certificate holder who allows a person to use the STC to alter an aircraft, aircraft engine, or propeller must provide that person with written permission acceptable to the FAA. So when you install someone else's STC, you need a permission-to-use letter (often called an STC license, authorization, or permission letter) from the certificate holder, and that letter is a record you keep. Without it, the installation is not properly authorized even if the work and the data are otherwise correct.
What STC records should an operator actually keep on file?
Plan to retain, per affected aircraft: a copy of the STC certificate itself (the FAA-issued document listing the make/model the STC applies to and the approved configuration); the written permission-to-use letter from the STC holder when you are not the holder, required by §91.403(d) and furnished under §21.120; the FAA-approved installation instructions and any Instructions for Continued Airworthiness the STC holder furnishes; the executed FAA Form 337 documenting the installation as a major alteration; and the resulting updates the alteration triggers — revised weight and balance, equipment list, and any Airplane Flight Manual supplement or operating-limitation changes. The Form 337 for the alteration is a permanent record under §91.417(a)(2)(vi) and transfers with the aircraft at sale; treat the whole STC package the same way.
What is the difference between an STC and a type certificate?
A type certificate (TC) approves the original type design of a product as a whole. A supplemental type certificate approves a change to that already-approved type design — supplemental meaning it rides on top of the existing TC rather than replacing it. The line is drawn by the size of the change. 14 CFR §21.93 classifies changes in type design: a minor change has no appreciable effect on weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting airworthiness, and all other changes are major. A minor change does not need an STC. A major change that is still not great enough to require an entirely new TC is the STC's domain under §21.113. A change so extensive that the product is essentially a new design would instead require a new or amended type certificate.
Can the same STC be installed on more than one aircraft?
Yes — that is the central feature of the STC system. Under §21.119, the holder of an STC may obtain airworthiness certificates for aircraft built under it, obtain approval for installation of the change on certificated aircraft, and obtain a production certificate for the change. A single STC can be applied to every aircraft of the make and model listed on the certificate's approved model list, which is why STCs created an aftermarket: one party develops and certifies a modification once, then licenses its installation across a fleet. Each individual installation is still its own major alteration with its own Form 337 and, where the installer is not the STC holder, its own §91.403(d) written permission. The STC is the reusable approval; the 337 and permission letter are per-aircraft records.
Who provides the Instructions for Continued Airworthiness for an STC modification?
The STC holder is responsible for furnishing the data needed to keep the modified product airworthy. When an STC alters the maintenance, inspection, or life-limit picture, the holder typically provides Instructions for Continued Airworthiness (ICA) — supplemental maintenance and inspection instructions specific to the modification — which the operator must then fold into the aircraft's maintenance and inspection program. This matters for records because an STC can introduce new recurring inspections, new life-limited components, or new recordkeeping obligations that did not exist on the unmodified aircraft. If an installed STC came with ICA, those instructions belong in your maintenance program and the resulting inspection and parts records belong in your aircraft records, not in a drawer with the installation paperwork.
What happens if the STC paperwork is incomplete when the aircraft is sold or inspected?
It becomes a configuration-versus-records gap, which is one of the most common and most expensive findings at a prebuy inspection or an FAA records review. The pattern is recognizable: an inspector or buyer walks the airframe, sees an installed modification — winglets, an avionics suite, an auxiliary fuel system — and asks for the STC, the permission-to-use letter, and the Form 337 that authorized it. If the installed configuration does not reconcile against the alteration records, the modification is treated as undocumented until proven otherwise, which can stall a sale, ground a configuration question, or require an appropriately rated person to evaluate and re-document the installation against the approved data. The cheaper path is structural: capture the STC, the permission letter, and the 337 the day the work is done, indexed to the tail, and reconcile your installed configuration against those records before a transaction rather than during one.
Related Aviation Compliance Reading
FAA Form 337 Major Repair & Alteration Records
The form that records an STC installation as a major alteration
Major vs Minor Repair & Alteration Determination
The §1.1 / §21.93 line that decides when a 337 and STC apply
Part 91 Aircraft Records Requirements
The full §91.417 record set, including the (a)(2) permanent tier
Airworthiness Directive Compliance Records
ADs can be written against STC-installed equipment
Aircraft Weight & Balance Records
The companion record every STC installation changes
ARROW Documents Explained
AFM supplements and what rides aboard the modified aircraft
Every STC, permission letter, and 337 — indexed before anyone asks
FileFlo is a compliance document intelligence platform: it classifies aviation maintenance documents against the governing CFR — STC certificates, §91.403(d) permission-to-use letters, installation data, Form 337s under §43.9(d), and the §91.417(a)(2) permanent record set — tracks what must transfer at sale, and generates inspector-format audit binders on demand. It proves your records; your IA, repair station, STC holder, and FSDO still run the aviation. Starter $89/month · Professional $299/month · 5-day free trial, no credit card required.