While most US carriers operate under 14 CFR Part 121 (a classification for regularly-scheduled air carriers), JSX (formerly known as Jet Suite X), operates under 14 CFR Part 135 (a classification primarily for commuter and on-demand operations).
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§ 121.159 Single-engine airplanes prohibited. No certificate holder may operate a single-engine airplane under this part.
§ 121.481 Flight time limitations: One or two pilot crews.(a) A certificate holder conducting flag operations may schedule a pilot to fly in an airplane that has a crew of one or two pilots for eight hours or less during any 24 consecutive hours without a rest period during these eight hours.
Tier 1 airlines board more than 25 million passengers per year. Tier 2 airlines board between 10 and 25 million passengers per year. Tier 3 airlines board between 3 and 10 million passengers per year. Tier 4 airlines board fewer than 3 million passengers per year.