Which of the following correctly describes usable fuel?

Usable fuel refers specifically to the amount of fuel that the aircraft can utilize during flight operations. This includes the fuel that is accessible to the engines for the purpose of generating thrust. Properly understanding usable fuel is crucial for flight planning and ensuring that an aircraft has sufficient fuel for its intended journey along with safety margins.

In aviation, it’s important to distinguish between total fuel capacity and usable fuel, as not all fuel in the tanks may be available for immediate use. For example, certain tank configurations may prevent the complete drainage of fuel, thus making some of it unusable during flight. Hence, knowing the quantity of usable fuel helps pilots execute weight and balance calculations effectively, ensuring the aircraft operates within its limitations.

The other options do not accurately represent the concept of usable fuel. Fuel that is not stored in the aircraft would not be part of any calculations pertaining to flight operations. Fuel measured without considering temperature does not take into account the density changes that affect fuel volume and weight, which is critical for accurate calculations. Lastly, fuel kept for emergency use only implies that it is not intended for regular operational use, which again does not align with the definition of usable fuel during standard flight.