Which of the following is not included in the empty weight?

Empty weight refers to the total mass of an aircraft without any payload, passengers, or usable fuel. It includes everything that is permanently installed in the aircraft but does not account for the items that may be loaded onto the plane for a specific flight.

Fuel is considered a consumable and variable part of an aircraft’s operation; it is not counted in the empty weight because it can change significantly between flights. The weights of fluids such as hydraulic fluid, oil, and fixed ballast are included in the empty weight since they are essential components necessary for the aircraft's normal operation and are inherently part of the aircraft structure when it is not loaded with passengers or cargo. Thus, the distinction lies in the fact that fuel is a variable element that changes based on the flight requirements, while the other fluids and materials mentioned are constants that do not fluctuate.