R2D
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Not the flight plan, the load plan. The flight plan isn’t available until about 1 hour prior for a domestic and 1 1/2 hours prior for an international flight.
The flight plan is sent 1.5 to 3.5 hours before departure, but not available to the crew until the time you mentioned, unless requested.
The dispatcher, when they push the button on the flight plan, determines if the flight will be PO based on estimated weights and structural or performance limits. If PO is not noted on the flight plan when it is sent and it becomes necessary later, the load planner will call and confirm it is okay.