Mich. Comp. Laws § 259.801

Sec. 1. Within 90 days of the effective date of this act, counties, any portion of whose boundaries are within 10 miles of any state-owned airport and any city located within the boundaries of any such counties and having a population of over 100,000, by a resolution passed by each of their legislative bodies by a majority of the entire membership of each board or council voting separately shall join to form an airport authority, hereafter referred to as the "authority". The authority shall be deemed to be a charter authority within the meaning of section 6 of article 9 of the state constitution.