14 C.F.R. § 121.509 Flight time limitations: Four pilot crews: airplanes.
Title 14 - Aeronautics and Space
(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]
Title 14: Aeronautics and Space
PART 121—OPERATING REQUIREMENTS: DOMESTIC, FLAG, AND SUPPLEMENTAL OPERATIONS
Subpart S—Flight Time Limitations: Supplemental Operations
§ 121.509 Flight time limitations: Four pilot crews: airplanes.