It is not unusual to call an airline many weeks in advance of travel to book a cheap seat or a coveted frequent flier free award seat, only to be told it is already booked. Airlines post schedules well in advance, and follow a complex system of refinement, and understanding it can avoid disappointment.
According to major airline website booking engines, schedules are posted approximately 330 days prior to departure. Seats cannot be booked prior to the posted schedule.
Airlines want to garner as much traffic as possible, and post early schedules to serve those customers desiring confirmations for planned vacations. Initial allocations of free frequent flier seats are also available, particularly in economy class.
Often these early schedules are simply a continuation of the schedule currently in effect, and as the departure date approaches, they are often subject to change. If the aircraft type changes, so does the seating configuration and any advance seat assignments.
According to a research study titled "Stability of Airline Schedules," airlines first determine what aircraft to assign to a flight. This is based on passenger demand models, aircraft “flow” from station to station and optimum use of aircraft for profitability.
Analysts continually review flight bookings, and may change aircraft type should bookings be greater or less than expected. They may also adjust inventories of cheap and free seats. As noted in Fare Compare, an Internet forum for frequent travelers, cheap fares can be published and then disappear quickly. In rare cases, a flight may be dropped from the system. Passengers are always protected if they have an early reservation, either on a different flight, or even a different airline.