14 CFR §91.529

1. (a)No person may operate the following airplanes without a flight crewmember holding a current flight engineer certificate:
   1. (1)An airplane for which a type certificate was issued before January 2, 1964, having a maximum certificated takeoff weight of more than 80,000 pounds.
   2. (2)An airplane type certificated after January 1, 1964, for which a flight engineer is required by the type certification requirements.
2. (b)No person may serve as a required flight engineer on an airplane unless, within the preceding 6 calendar months, that person has had at least 50 hours of flight time as a flight engineer on that type airplane or has been checked by the Administrator on that type airplane and is found to be familiar and competent with all essential current information and operating procedures.