What factor directly affects aircraft performance the most?

Density altitude is a critical factor that directly affects aircraft performance because it combines the effects of temperature, humidity, and atmospheric pressure into a single measurement of air density at a given altitude. As density altitude increases, aircraft performance generally decreases. This is due to the reduction in air density, which diminishes lift generated by the wings, reduces engine power output, and ultimately leads to longer takeoff and landing distances.

When density altitude is high, the aircraft will require a longer distance to take off, climb more slowly, and may have reduced control effectiveness, impacting overall safety and performance. A lower density altitude corresponds to more efficient engine performance, greater lift, and improved aircraft handling characteristics, which is why it is considered the most significant factor in assessing aircraft performance.

While temperature, humidity, and wind speed do play a role in performance, they are often components of the overall density altitude calculation, making density altitude the most comprehensive and direct influence on how an aircraft performs in various conditions.