What is defined as any fuel remaining in the tanks after the system has been drained?

Residual fuel refers to the quantity of fuel that remains in the tanks after the fuel system has been drained for maintenance or inspection purposes. This fuel is not accessible for use by the aircraft's engines during flight operations, but it is still present in the fuel tanks. Understanding the concept of residual fuel is crucial for maintenance and safety, as it can affect the total fuel weight and balance of the aircraft as well as must be accounted for when planning fuel use and endurance during flights.

Usable fuel, on the other hand, is the amount of fuel that can be drawn from the tanks and effectively used to power the engines under operating conditions. Unusable fuel is fuel that remains in the system but cannot be utilized due to various technical or design constraints, often accounting for the last fragments of fuel that are difficult to extract. Flight fuel typically refers to the amount of fuel that is necessary for the aircraft to complete its intended journey.

The distinction between these terms is important for aviation mechanics to ensure precision in fuel management and maintain operational efficiency and safety.