US-FAA14 CFR 121.493
Flight time limitations: Flight engineers and flight navigators
Read the official ruleOfficial FAA regulation text (public domain)
(a) In any operation in which one flight engineer or flight navigator is required, the flight time limitations in § 121.483 apply to that flight engineer or flight navigator. (b) In any operation in which more than one flight engineer or flight navigator is required, the flight time limitations in § 121.485 apply to those flight engineers or flight navigators.
Verbatim text of a US Federal Aviation Regulation (public domain), reproduced for reference — not legal advice and not for navigation. A plain-English explainer may be added later; always rely on the current official rule linked above.