In aircraft weight definitions, what does "unusable fuel" refer to?

Unusable fuel refers to the portion of fuel in the aircraft's tanks that cannot be accessed or used by the engines due to various system limitations. These limitations can include the design of the fuel system, where certain angles of operation or fuel slosh may prevent the fuel from reaching the fuel lines or being drawn into the engine. This is important to consider in weight and balance calculations, as it contributes to the total weight of the aircraft without being available for flight operations. Understanding unusable fuel helps pilots accurately assess the total amount of usable fuel and ensure proper weight management for safe flight operations.