14 CFR § 121.667

No person may take off an aircraft unless the pilot in command has filed a flight plan, containing the appropriate information required by part 91, with the nearest FAA communication station or appropriate military station or, when operating outside the United States, with other appropriate authority. However, if communications facilities are not readily available, the pilot in command shall file the flight plan as soon as practicable after the aircraft is airborne. A flight plan must continue in effect for all parts of the flight.

When flights are operated into military airports, the arrival or completion notice required by §§ 91.153 and 91.169 may be filed with the appropriate airport control tower or aeronautical communication facility used for that airport.