49 U.S.C. § 44101: Operation of aircraft

(a) Registration Requirement.—Except as provided in subsection (b) of this section, a person may operate an aircraft only when the aircraft is registered under section 44103 of this title. (b) Exceptions.—A person may operate an aircraft in the United States that is not registered—(1) when authorized under section 40103(d) or 41703 of this title; (2) when it is an aircraft of the national defense forces of the United States and is identified in a way satisfactory to the Administrator of the Federal Aviation Administration; and (3) for a reasonable period of time after a transfer of ownership, under regulations prescribed by the Administrator.