Pilots, copilots, stewardesses, and other regular flying employees of a regularly established airline operating under a certificate of convenience and necessity granted by the competent authorities of the United States of America and operating regularly scheduled flights in interstate or foreign commerce shall be exempt from the compulsory provisions of this title while engaged in work, the duties of which primarily involve interstate or foreign flying operations. Employees not regularly engaged in interstate or foreign flying operations, and the flying employees of any such airline as has its principal operating base in North Dakota, shall not be included in this exemption.