When determining optimum cruise altitude, which factor is most significant at the beginning of the cruise?

The gross weight of the airplane is the most significant factor to consider at the beginning of the cruise when determining the optimum cruise altitude. This is because the aircraft's weight affects its climb performance and maximum altitude capabilities. A heavier airplane requires a higher indicated airspeed to maintain lift and may have a limited capability to climb, thus determining how high it can go.

At higher altitudes, the air density decreases, which can affect engine performance and lift as well. Therefore, if the aircraft is still relatively heavy at the start of the cruise, it may need to operate at a lower altitude where the air is denser, ensuring efficient aerodynamic performance and engine efficiency. As fuel is burned and the airplane becomes lighter, it may then be able to ascend to a more optimum altitude for cruise, which generally offers fuel efficiency and better performance.

While altitude, speed, and temperature are also important elements in flight planning, the gross weight directly influences how these other factors interact with the aircraft's performance early in the cruise phase.