Pilots use medical certificates to show that they meet the health requirements to perform the duties and privileges of their pilot certificate.
The requirement for a pilot to possess a medical certificate to perform the privileges granted by their pilot certificate is found under 14 CFR part 61.23
All pilots, except glider and free air balloon pilots, must possess an FAA Medical Certificate to exercise the privileges of their airman certificates. Pilots must also have their medical certificates in their physical possession while exercising their flight privileges.
FAA medical certificates come in three "classes" or types. These medical classes are:
The type of medical you require depends on the pilot-in-command privileges you wish to exercise.
Generally speaking, the first-class medical is for pilots operating at the airline level with Airline Transport Pilot (ATP) certificates.
Second-class medicals are required for pilots with commercial pilot certificates, and third-class medicals are for pilots with student, recreational, and private pilot certificates. Sport pilots may receive a third-class medical if they wish. However, a driver's license is sufficient to prove medical competence, and a third-class medical is not required.
Pilot medicals work under the idea of a hierarchy with the health standards required by the pilot to receive a medical getting progressively more strict, with the third class medical being the least restrictive to the first-class medical being most restrictive.
Therefore, a pilot that possesses a first-class medical also meets the requirements of the second and third-class medical.
For example, a pilot who flies professionally at the ATP level and then leisurely on the side only requires a first-class medical. Not a first and third-class medical.
All medicals granted by the FAA expire based on the pilot's age when they received the medical.
Medicals received by a pilot when they were under 40 years old expire 60 calendar months from the date of issuance. Medicals received by a pilot older than 40 years old expire 24 calendar months from the date of issuance.
However, this does not mean a 30-year-old pilot working at an airline only needs to renew their first-class medical once every 60 calendar months. Instead, the pilot must renew their medical certificate every 12 calendar months.
The reason for this is because of medical privilege. Medical privileges operate in the exact same manner as medical classes.
First-class privileges are required for any pilot wishing to exercise the pilot-in-command privileges of an ATP certificate.
Second-class privileges are required for any pilot wishing to exercise the pilot-in-command privileges of a commercial certificate.
Third-class privileges are required for any pilots wishing to exercise the pilot-in-command privileges of a private or recreational pilot certificate. Third-class privileges are also required for sport pilot certificates if that pilot does not have a driver's license.
An ATP certificated pilot requires a first-class medical because it grants that pilot first-class privileges for either 12 or 6 calendar months after the date of issuance, depending on the pilot's age.
This first-class privilege grants the pilot the ability to exercise the pilot-in-command privileges of an airline transport pilot certificate. Similar to how a pilot with a first-class medical does not also need a second and third-class medical, a pilot that possesses first-class privileges also possesses second and third-class privileges for the time being.
Therefore, a 25-year-old pilot with a first-class medical received eight calendar months ago possesses a first-class medical with first-class privileges. 48 calendar months after being issued the medical, that first-class medical would become a first-class medical with third-class privileges.