121.509—Flight time limitations: Four pilot crews: airplanes.
(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.