Interface component

(5) Interface component means an article that serves as a functional interface between an aircraft and an aircraft engine, an aircraft engine and a propeller, or an aircraft and a propeller. An interface component is designated by the holder of the type certificate or the supplemental type certificate who controls the approved design data for that article;

Source

14 CFR § 21.1

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Scoping language

This part prescribes
(1) Procedural requirements for issuing and changing—
(i) Design approvals;
(ii) Production approvals;
(iii) Airworthiness certificates; and
(iv) Airworthiness approvals;
(2) Rules governing applicants for, and holders of, any approval or certificate specified in paragraph (a)(1) of this section; and
(3) Procedural requirements for the approval of articles.

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