Section 121.509 - Flight time limitations: Four pilot crews: airplanes(a) No certificate holder conducting supplemental operations may schedule a pilot- (1) For flight deck duty in an airplane that has a crew of four pilots for more than eight hours in any 24 consecutive hours; or(2) To be aloft in an airplane that has a crew of four pilots for more than 16 hours in any 24 consecutive hours.(b) No pilot of an airplane that has a crew of four pilots may be on duty for more than 20 hours in any 24 consecutive hours.Doc. No. 6258, 29 FR 19218, Dec. 31, 1964; 30 FR 3639, Mar. 19, 1965, as amended by Amdt. 121-253, 61 FR 2613, Jan. 26, 1996