According to FAR 23, what must be included in the empty weight of the aircraft?

The correct answer is that the engine oil capacity must be included in the empty weight of the aircraft. Empty weight refers to the weight of the aircraft including all mandatory items, which encompasses the airframe, engines, and any other permanently installed equipment. According to Federal Aviation Regulations (FAR) 23, this also specifically includes the weight of full engine oil.

Engine oil is considered an essential part of the aircraft's operation; it contributes to proper lubrication and functioning of the engine systems. Therefore, its weight is necessary to accurately represent the operational weight of the aircraft when it is not carrying any passengers, cargo, or fuel.

The other options pertain to items that are not included in the calculation of the empty weight. Fuel capacity, for instance, is not counted in the empty weight but is considered when calculating the maximum takeoff weight or gross weight of the aircraft. Passenger weight and cargo weight also fall under discretionary loading and are not part of the base empty weight as defined by the regulations. These distinctions are important for accurate weight and balance calculations and overall safety of flight operations.