14 CFR Part 21 — Certification Procedures for Products and Articles

Why Part 21 Matters to a Mechanic

14 CFR Part 21 establishes the certification procedures for all civil aeronautical products and articles sold or installed in the United States. As a mechanic, you are personally responsible for knowing whether a part is legally approved before you install it. Installing an unapproved part on a certificated aircraft can ground the aircraft, expose you to certificate action, and — most critically — create an unsafe condition that may not reveal itself until flight. Part 21 is the regulatory foundation that answers the question: Is this part legal to install?

The Type Certificate (TC)

A Type Certificate is issued by the FAA to the designer of an aircraft, aircraft engine, or propeller. It confirms that the design meets all applicable airworthiness standards. The TC data sheet lists the approved configuration: approved engine models, operating limitations, required equipment, and more. When you install a replacement part, that part must conform to the type design documented in the TC. The TC holder is typically the original equipment manufacturer (OEM). Parts manufactured under the TC holder's quality system are called OEM parts and carry the TC holder's part number.

Production Approvals: PC and PMA

A Production Certificate (PC) authorizes a TC holder to manufacture duplicate products that conform to the approved type design. It demonstrates that the manufacturer has a quality system capable of producing consistent, airworthy products.

A Parts Manufacturer Approval (PMA) is a combined design and production approval issued by the FAA to a company other than the TC holder. A PMA part has been independently approved and may legally replace the corresponding OEM part. PMA parts must be marked with the manufacturer's name, part number, and the letters "FAA-PMA." When you receive a PMA part, verify that marking is present and that the part number matches the aircraft's parts catalog or maintenance manual. Missing or altered markings are a red flag.

Technical Standard Orders (TSO)

A TSO authorization allows a manufacturer to produce articles — typically appliances and components such as altimeters, life vests, or transponders — that meet a specific FAA-accepted minimum performance standard. A TSO article is marked with the TSO number (for example, TSO-C119 for TCAS equipment). TSO authorization alone does not automatically approve an article for installation on a specific aircraft; you must also confirm that the aircraft's TC or an STC permits that article in that position. Think of the TSO as a quality floor — it proves the article meets a standard, but the mechanic still must verify it is the correct article for the aircraft.

Supplemental Type Certificates (STC)

An STC approves a modification or alteration to an existing type-certificated product. If an operator wants to install a new avionics system or a different engine model, the STC documents the approved design change. As a mechanic performing STC work, you must have the STC data sheet and the associated maintenance or installation instructions in hand before starting work. Parts called out in the STC are approved only in conjunction with that STC.

Field Approvals

A field approval is granted by an FAA Aviation Safety Inspector and allows a specific major repair or alteration on a specific aircraft when no existing DER-approved data covers the work. The approval is recorded on an FAA Form 337. Field approvals are aircraft-specific; the data does not transfer to another aircraft without a new approval.

Unapproved Parts — A Safety and Legal Hazard

An unapproved part is any part that has not been produced under an FAA production approval (TC/PC, PMA, or TSO) or that cannot be shown to conform to an approved design through acceptable documentation. Unapproved parts enter the supply chain through counterfeit manufacturing, improper salvage operations, or paperwork fraud. Advisory Circular AC 21-29 provides detailed guidance on detecting and reporting suspected unapproved parts (SUPs).

Key warning signs include:

• Missing, illegible, or altered part markings
• Absence of an FAA Form 8130-3 (Airworthiness Approval Tag) or equivalent release document from a certificated repair station
• Packaging inconsistent with the part's claimed origin
• Price significantly below market value

If you suspect a part is unapproved, do not install it. Quarantine the part, document your findings, and report to your supervisor and, if appropriate, to the FAA's Suspected Unapproved Parts Program. Attempting to install a known unapproved part is a violation of 14 CFR Part 21 and may also violate Part 43 and Part 65, risking your A&P certificate.

What to Verify Before Installation

Before installing any replacement part, confirm all of the following:

1. The part has a valid production approval — OEM (TC/PC), PMA, or TSO as appropriate.

2. Part markings are present, legible, and match the paperwork.

3. An airworthiness release document (FAA Form 8130-3 or equivalent) accompanies the part.

4. The part number matches what is called out in the aircraft maintenance manual, illustrated parts catalog, or applicable STC/field approval data.

5. The part has not been altered or shows no signs of damage or improper repair.

This verification process is not optional. It is a direct regulatory duty under Part 43 and Part 65, and it begins with understanding the Part 21 approvals described in this lesson.