Good question and the answer (a factor of crew availability and loaded cargo) lies in the USAF Instructions Series 11 (Flying Operations). As an example, a C-5 can accommodate 73 folks in the passenger compartment but some flights may depart with only 19 passengers. Other factors could include type (security) of cargo or availability of adequate safety equipment but the main reasons for seat limitations are listed below for the more common aircraft:
- Patriot Express Missions: These are commercial contracted missions and the contract only provides a fixed number of seats even though the aircraft size might accommodate more passengers.
- C-5: Cargo (not crew) limitation. 20 passengers and above requires an open pallet position for passenger baggage. Typically 73 Seats iempty but if there is a full cargo load thenlimited to 19 seats.
- C-17: Crew limitation. Above 40 passengers requires an additional crew member. Typically, 53 Seats empty.
- KC-10: Crew limitation. Above 40 passengers requires an additional crew member.
- KC-10: Above 10 passengers requires requires an open pallet position for passenger baggage.
- KC-135: Crew limitation. Above 10 passengers requires an additional crew member.