What factor most directly affects an airplane's capability at different altitudes?

Air density is the key factor that most directly affects an airplane's capability at different altitudes. As an aircraft ascends, the density of the air decreases. This reduction in air density affects several critical performance metrics of an airplane, including lift generation, engine performance, and aerodynamic efficiency.

Lift is primarily dependent on air density; with lower density at higher altitudes, an aircraft requires a higher speed or a greater wing area to produce the necessary lift to maintain flight. Additionally, engine performance is often tied to the amount of oxygen available in the air. Less dense air means there is less oxygen for combustion, which can reduce the power output of jet engines.

Consequently, as air density decreases, pilots must be aware of these changes in aircraft performance, making air density a primary consideration for safe and efficient flight operations at altitude.