14 CFR § 47.43

The registration of an aircraft is invalid if, at the time it is made—

The aircraft is registered in a foreign country;

The applicant is not the owner;

The applicant is not qualified to submit an application under this part; or

The interest of the applicant in the aircraft was created by a transaction that was not entered into in good faith, but rather was made to avoid (with or without the owner's knowledge) compliance with 49 U.S.C. 44101-44104.

If the registration of an aircraft is invalid under paragraph (a) of this section, the holder of the invalid Certificate of Aircraft Registration, if issued in paper form, must return it as soon as possible to the Registry.