Supplemental Type Certificates

FAR 21.115 — Supplemental Type Certificates

FAR 21.115 explains the applicable requirements for a Supplemental Type Certificate (STC), including noise, emissions, and fuel efficiency compliance.

In Plain English

FAR 21.115 spells out what an applicant must show to earn a Supplemental Type Certificate (STC) — the approval needed when someone modifies an already type-certificated product (like adding a new avionics suite, engine, or interior).

The altered product must meet the applicable airworthiness requirements in § 21.101, plus any of these that apply to the change:

Acoustical changes (§ 21.93(b)) — must comply with the noise standards in Part 36.
Emissions changes (§ 21.93(c)) — must comply with the fuel venting and exhaust emissions standards in Part 34.
Fuel efficiency changes (§ 21.93(d)) — must comply with the fuel efficiency standards in Part 38.

The applicant must also meet § 21.33 (inspection and tests) and § 21.53 (statement of conformity) for each change in the type design.

Why it matters: STCs are how legal modifications get approved. As a pilot, knowing an aircraft has an STC tells you the alteration was reviewed against these standards — not a backyard fix.

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Regulation Text

14 CFR § 21.115

§ 21.115 Applicable requirements. (a) Each applicant for a supplemental type certificate must show that the altered product meets applicable requirements specified in § 21.101 and— (1) In the case of an acoustical change described in § 21.93(b), show compliance with the applicable noise requirements of part 36 of this chapter; (2) In the case of an emissions change described in § 21.93(c), show compliance with the applicable fuel venting and exhaust emissions requirements of part 34 of this chapter; and (3) In the case of a fuel efficiency change described in § 21.93(d), show compliance with the applicable fuel efficiency requirements of part 38 of this chapter. (b) Each applicant for a supplemental type certificate must meet §§ 21.33 and 21.53 with respect to each change in the type design. [Amdt. 21-17, 32 FR 14927, Oct. 28, 1967, as amended by Amdt. 21-42, 40 FR 1033, Jan. 6, 1975; Amdt. 21-52A, 45 FR 79009, Nov. 28, 1980; Amdt. 21-61, 53 FR 3540, Feb. 5, 1988; Amdt. 21-68, 55 FR 32860, Aug. 10, 1990; Amdt. 21-71, 57 FR 42854, Sept. 16, 1992; Amdt. 21-77, 65 FR 36266, June 7, 2000; Amdt. No. 21-107, 89 FR 12653, Feb. 16, 2024]

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Oral Exam Questions a DPE Might Ask

Q1What is a Supplemental Type Certificate, and what must an applicant show to obtain one?

Per FAR 21.115, an STC approves a major change to an already type-certificated product. The applicant must show the altered product meets the applicable airworthiness requirements of § 21.101, plus § 21.33 and § 21.53 for each design change.

Q2If a modification creates a noise or emissions change, what additional standards apply under FAR 21.115?

FAR 21.115 requires acoustical changes to comply with Part 36 noise rules, emissions changes to comply with Part 34 fuel venting and exhaust emissions rules, and fuel efficiency changes to comply with Part 38.

Q3Besides § 21.101, which other regulations does an STC applicant have to meet for each design change?

FAR 21.115(b) requires the applicant to meet § 21.33 (inspection and tests) and § 21.53 (statement of conformity) for each change in the type design.

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§21.15 Type Certificate Application

FAA application form, aircraft drawings, and engine data

§21.17 Certification Basis

Applicable airworthiness, noise, and emission rules

§21.20 Compliance Showing

How applicants show compliance to the FAA

§21.31 Type Design