620 ILCS 5/56

At the time fixed for any hearing, pursuant to the laws of this State pertaining to aeronautics, or the rules, rulings, regulations, orders or decisions issued by the Department thereunder, whether upon a complaint or otherwise, persons, municipalities or other political subdivisions having a substantial interest in the subject matter thereof, and in case of a complaint, the complainant and the person, municipality or other political subdivision complained of, and, in every case, such persons, municipalities or other political subdivisions as the Department may allow to intervene, shall be entitled to be heard and introduce evidence. The Department shall issue process to enforce the attendance of all necessary witnesses.