What temperature condition is needed to estimate endurance flights at 18,000 ft for VFR day reserves?

Estimating endurance for flights at 18,000 feet requires understanding how temperature affects aircraft performance, particularly fuel consumption. At standard temperature, which at 18,000 feet is typically around -4.7 °C (or 255.4 Kelvin), the aircraft's systems and the atmospheric conditions are stable and predictable, allowing for more accurate calculations of fuel reserves needed for VFR (Visual Flight Rules) day operations.

Using standard temperature provides a reliable baseline since the performance calculations, including endurance and fuel consumption, are often derived from conditions that reflect average sea level standard atmosphere. When conditions deviate significantly from standard temperature, adjustments need to be made to account for variations in aircraft performance, which could complicate the estimation of endurance.

This is essential for planning purposes, as pilots are required to calculate the minimum fuel reserves to satisfy VFR requirements, which include ensuring enough fuel to reach the destination and cope with unforeseen circumstances. As a result, estimating endurance under standard temperature conditions ensures a more straightforward and safe approach to flight planning and adherence to regulatory requirements for minimum fuel reserves in VFR day conditions.