14 C.F.R. § 135.245
(c) No certificate holder may use any person, nor may any person serve, as second in command under IFR unless that person meets the following instrument experience requirements:
(1) Use of an airplane, powered-lift, or helicopter for maintaining instrument experience. Within the 6 calendar months preceding the month of the flight, that person performed and logged at least the following tasks and iterations in-flight in an airplane, powered-lift, or helicopter, as appropriate, in actual weather conditions, or under simulated instrument conditions using a view-limiting device:
(2) Use of an FSTD for maintaining instrument experience. A person may accomplish the requirements in paragraph (c)(1) of this section in an approved FSTD, or a combination of aircraft and FSTD, provided:
(d) A second in command who has failed to meet the instrument experience requirements of paragraph (c) of this section for more than six calendar months must reestablish instrument recency under the supervision of a flight instructor qualified under § 135.338 or a check pilot qualified under § 135.337. To reestablish instrument recency, a second in command must complete at least the following areas of operation required for the instrument rating practical test in an aircraft or FSTD that represents the category of aircraft for the instrument experience requirements to be reestablished:
[44 FR 26738, May 7, 1979, as amended by Docket FAA-2016-6142, 83 FR 30283, June 27, 2018; Docket FAA-2023-1275, Amdt. 135-147, 89 FR 92487, Nov. 21, 2024]