Which factor is most significant when determining optimum cruise altitude?

The most significant factor in determining the optimum cruise altitude is the gross weight at the beginning of the cruise. As an aircraft climbs, its gross weight affects its performance, including engine thrust, fuel efficiency, and aerodynamic drag. Heavier aircraft benefit from cruising at higher altitudes where the thinner air reduces drag and enhances fuel economy.

When a plane is at maximum gross weight, it may not be able to achieve efficient cruise at extremely high altitudes due to performance limitations. Therefore, the weight directly influences the altitude at which the aircraft can fly most efficiently, balancing fuel usage and flight safety. The goal is to find an altitude where fuel burn is minimized while still maintaining a safe margin above stall speeds and operational ceilings.

While aircraft wing design plays a role in overall aerodynamics, it is less influential on the choice of cruise altitude than the specific weight of the aircraft at the beginning of cruise. Weather conditions can also impact altitude selection, but they typically do not outweigh the influence of the aircraft's weight. Similarly, the flight route length is a consideration for overall operational planning but does not directly dictate the most efficient altitude for a given weight scenario. Thus, the gross weight at the start of cruise is the primary determinant of optimum altitude for fuel efficiency and performance