Block time 8-12 hours: '3-pilot' crew consisting of 2C + 1F or 1C + 2F (one F/O type-rated), scheduled for one sector in duty period
Block time over 12 hours: 'two-set' crew consisting of 2C + 2F, scheduled for one or two sectors in duty period
One sector in duty period: crew A does takeoff + 1/2 flight time, crew B does 1/2 flight time + landing
Two sectors in duty period: (here is the 'other' part lol)
Crew A does takeoff and 1/2 flight time of first sector
Crew B does 1/2 flight time + first sector landing followed by takeoff and 1/2 flight time of second sector
Crew A returns to duty and does last 1/2 second sector flight time + landing
UNLESS
One of the two sectors has block time less than two hours, in which case one crew does the short flight and the other takes a greater share of the longer flight so as to divide block time equally between the crews
If you are confused by this lat bit, I understand
