49 U.S.C. § 46109

A person interested in or affected by a matter under consideration in a proceeding before the Secretary of Transportation (or the Administrator of the Transportation Security Administration with respect to security duties and powers designated to be carried out by the Administrator of the Transportation Security Administration or the Administrator of the Federal Aviation Administration with respect to aviation safety duties and powers designated to be carried out by the Administrator) or civil action to enforce this part or a requirement or regulation prescribed, or an order or any term of a certificate or permit issued, under this part may be joined as a party or permitted to intervene in the proceeding or civil action.