First-class medical is required for pilots who exercise airline transport pilot (ATP) privileges; that is, those flying scheduled airliners and other pilots whose employers require this level of certification. A first-class medical is valid for ATP privileges for twelve months for pilots under age 40. Pilots who are age 40 and older must renew First class medical certificates every six months.
A Second class is required for pilots who fly commercially—in operations such as crop dusting or carrying passengers or cargo for hire. The Second class medical is valid for commercial privileges for 12 months.
A Third class medical certificate is appropriate for student pilots or private pilots who fly for fun business, but not for hire. A third-class medical is valid for 60 months for pilots under age 40 and 24 months for applicants who are age 40 or older.