Pilot certificates do not expire, although they may be suspended or revoked by the FAA. However, a pilot must maintain "currency" — recent flight experience that is relevant to the flight being undertaken. To remain current, every pilot has to undergo a flight review with an instructor every 24 calendar months unless she or he gains a new pilot certificate or rating in that time or satisfies the flight review requirement using an alternate approved means. For most types of certificate, she or he must also undergo a medical examination at intervals ranging from six months to five years, depending on the pilot's age and desired flight privileges. Other currency requirements apply to the carriage of passengers or to flight under instrument flight rules (IFR).