Applicability

This regulation establishes which special airworthiness requirements apply to different types of aircraft operated under Part 121 (scheduled air carrier operations).

In practical terms:

Very old aircraft certified before November 1, 1946 must meet extensive special requirements (§§121.215-121.283), though the FAA can grant relief if literal compliance would be extremely difficult for cargo operations without improving safety.

All Part 121 operators must comply with §§121.285-121.291 regardless of aircraft type.

Nontransport category airplanes certified after December 31, 1964 must meet additional requirements in §121.293.

This matters because it determines which airworthiness standards your aircraft must meet beyond basic certification requirements. Older or non-standard aircraft face more stringent special requirements to operate in scheduled service. Most modern transport category aircraft won't be affected by paragraphs (b) or (e), but all operators must address the universal requirements in paragraph (c).

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*