The endurance time entered on an IFR flight plan should be based on which fuel quantity?

The endurance time entered on an IFR flight plan is most appropriately based on the total useable fuel on board. This is because the endurance calculation uses the amount of fuel that is available for consumption during flight, reflecting what the aircraft can actually utilize under operational conditions.

The total useable fuel takes into account both the fuel quantity and any limitations that may exist due to fuel system design, which can affect how much fuel is genuinely available to the engines. This ensures accurate calculations for flight planning, as it directly impacts the expected performance and safety margins during the flight.

While other options might address various aspects of fuel requirements for the flight, such as fuel needed to reach the destination and alternate with reserves, or total fuel required for the flight, these do not specifically represent the fuel that is available for that particular flight once the aircraft is airborne. The focus on useable fuel allows pilots to make informed decisions about endurance and helps maintain safety during the operation of the aircraft.