Nothing in this chapter shall apply to, or be construed or held to apply to, any common carrier by aircraft which the commission shall by order determine to be engaged mainly and principally in interstate commerce and whose intrastate business is incidental to its interstate business, provided the commission shall find that its operations are conducted pursuant to a certificate or certificates of public convenience and necessity issued by the Civil Aeronautics Board of the United States of America or by any other governmental agency successor thereto.