At what point should reserve fuel be calculated?

Calculating reserve fuel during the flight planning stage is crucial for ensuring a safe and compliant flight. During flight planning, pilots assess the total fuel requirements based on various factors, including the distance to the destination, expected weather conditions, and specific performance data of the aircraft. It is essential to incorporate reserve fuel at this time to account for unexpected contingencies that could arise, such as deviations from the planned route due to weather or other circumstances.

By determining the reserve fuel during flight planning, pilots can ensure that they have an adequate safety margin and comply with regulatory requirements, which often specify minimum reserve fuel levels. This proactive approach minimizes the risk of encountering fuel shortages later in the flight. Calculating it only before departure, upon reaching cruising altitude, or after fuel burn calculations would not provide a comprehensive overview needed for safe flight operations, as it would not consider variables that can change during the flight.