Section 121.432 - General(a) Except in the case of operating experience under § 121.434 and ground training for mentoring required by §§ 121.419 , 121.420 , 121.427 , and 121.429 , as applicable, a pilot who serves as second in command of an operation that requires three or more pilots must be fully qualified to act as pilot in command of that operation.(b) No certificate holder may conduct a check or any training in operations under this part, except for the following checks and training required by this part or the certificate holder: (1) Line checks for pilots.(2) Flight engineer checks (except for emergency procedures), if the person being checked is qualified and current in accordance with § 121.453(a) .(3) Flight attendant training and competence checks.(c) Except for pilot line checks and flight engineer flight checks, the person being trained or checked may not be used as a required crewmember.Doc. No. 9509, 35 FR 95, Jan. 3, 1970, as amended by Amdt. 121-130, 41 FR 47229, Oct. 28, 1976; Amdt. 121-366, 78 FR 67840 , Nov. 12, 2013; Amdt. 121-382, 85 FR 10925 , Feb. 25, 2020