Final answer:
Life preservers are required on board an aircraft to ensure passenger safety during emergencies over water. Regulations vary by aircraft type and route, with checklists used to confirm equipment availability.
Step-by-step explanation:
Life preservers are required to be carried on board an aircraft under various regulations to ensure the safety of all passengers and crew in the event of an emergency. The specific regulations can depend on the type of aircraft, its size, and whether the flight is over water or near water bodies where an emergency landing might necessitate their use. For example, some regulations dictate that life preservers must be accessible for each passenger when the aircraft operates beyond a certain distance from shore. Compliance with safety checklists, as recommended by organizations such as the World Health Organization (WHO), is crucial in the aviation industry to ensure all essential equipment, including life preservers, are in place and in working order before takeoff.