Crew duty limits
Crew duty limits are the regulatory caps on how long pilots may be on duty and how much rest they need between duties. Under FAA Part 135 a two-pilot crew is generally limited to 14 hours of duty and 10 hours of flight time, with at least 10 hours' rest; EASA rules are broadly similar but vary with start time and number of sectors. The clock starts when the crew report, not when you board, so a late departure or a long lunch stop can push a crew out of limits and require a second crew or an overnight. This is why operators ask for realistic timings and why a 4 a.m. departure after a late-night arrival is often impossible without a crew change.