Which required document includes descriptions of the necessary crewmember functions during an emergency according to 14 CFR Part 121?

The certificate holder's manual is the required document that includes descriptions of the necessary crewmember functions during an emergency according to 14 CFR Part 121. This manual provides detailed procedures, responsibilities, and guidelines that ensure all crewmembers understand their roles in emergency situations. It outlines the standard operating procedures for emergencies, ensuring that each crew member is trained and ready to respond appropriately, thereby enhancing overall safety and effective crisis management.

The other documents mentioned serve different purposes. The Airplane Flight Manual primarily focuses on the specific performance characteristics and operational limits of the aircraft, while the Pilot's Emergency Procedures Handbook typically contains emergency procedures relevant to pilots but may not encompass the full range of required crewmember functions as mandated by the regulations. The certificate holder's manual, therefore, is specifically designed to meet regulatory requirements for crew coordination and response during emergencies.