Which component is NOT included in the empty weight calculation of an aircraft?

The correct answer pertains to the weights and balances considered in calculating the empty weight of an aircraft. Empty weight is defined as the weight of the aircraft with all its installed components and equipment, but it does not include the weight of the pilot and passengers.

In the context of the choices presented, unusable fuel, hydraulic fluid, and all installed operating equipment are all considered as part of the empty weight calculation. Unusable fuel refers to the fuel that cannot be burned during flight but still adds weight to the aircraft. Hydraulic fluid is essential for the operation of various systems and is always included in the empty weight calculation as it is part of the aircraft's operational requirements. All installed operating equipment is equally important as it includes anything permanently installed in the aircraft that contributes to its functionality.

In contrast, pilot and passenger weights are categorized as part of the loading weight and not the empty weight, hence their exclusion from the empty weight calculation. This distinction is crucial for maintaining accurate weight and balance calculations for safe flight operations.