Aeronautics and Space. Part135:Operating requirements: Commuter and on demand operations and rules governing persons on board such aircraft. Section135.297:Pilot in command: Instrument proficiency check requirements.
(a) No certificate holder may use a pilot, nor may any person serve, as a pilot in command of an aircraft under IFR unless, since the beginning of the 6th calendar month before that service, that pilot has passed an instrument proficiency check under this section administered by the Administrator or an authorized check pilot.