Airworthy

Airworthiness is the measure of an aircraft's suitability for safe flight. Certification of airworthiness is initially conferred by a certificate of airworthiness from a national aviation authority, and is maintained by performing the required maintenance actions.

The application of airworthiness defines the condition of an aircraft and supplies the basis for judgment of the suitability for flight of that aircraft, in that it has been designed with engineering rigor, constructed, maintained and is expected to be operated to approved standards and limitations, by competent and approved individuals, who are acting as members of an approved organization and whose work is both certified as correct and accepted on behalf of the State.

An example of an aircraft that was not legally airworthy is Larry Walters' "Lawn chair flight" in 1982.

In the U.S., Title 14, Code of Federal Regulations, Subchapter F, Part 91.7 states: "a) No person may operate an aircraft unless it is in an airworthy condition. b) The pilot in command of a civil aircraft is responsible for determining whether that aircraft is in condition for safe flight. The pilot in command shall discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur."

The USA Federal Aviation Regulations, Part 21, §21.183(d) has a procedural definition of airworthy:
"other aircraft An applicant for a standard airworthiness certificate for aircraft not covered by paragraphs (a) through (c) (a:"New aircraft manufactured under a production certificate." b:"New aircraft manufactured under type certificate only." c:"Import aircraft.") of this section is entitled to a standard airworthiness certificate if -- (1) He presents evidence to the Administrator that the aircraft conforms to a type design approved under a type certificate or a supplemental type certificate and to applicable Airworthiness Directives; (2) The aircraft (except an experimentally certificated aircraft that previously had been issued a different airworthiness certificate under this section) has been inspected in accordance with the performance rules for 100-hour inspections set forth in part 43.15 of this chapter and found airworthy by-- (i)The manufacturer; (ii)The holder of a repair station certificate as provided in part 145 of this chapter; (iii)The holder of a mechanic certificate as authorized in part 65 of this chapter; (v)The holder of a certificate issued under part 121 of this chapter, and having a maintenance and inspection organization appropriate to the aircraft type; and (3)The Administrator finds after inspection, that the aircraft conforms to the type design, and is in condition for safe operation."

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Wikipedia