What ultimately determines an airplane's performance?

Density altitude is a critical factor that ultimately determines an airplane's performance because it directly affects the air density, which influences engine performance, aerodynamic lift, and propeller efficiency. As density altitude increases, air becomes less dense, which means the aircraft's engines do not produce as much power, propellers generate less thrust, and wings produce less lift. This can result in longer takeoff distances, decreased climb rates, and increased landing distances.

For example, at higher elevations or on hot days, the density altitude can significantly exceed the actual elevation, creating a scenario where the aircraft's performance can be limited. Pilots must understand how density altitude impacts aircraft performance to make informed decisions during flight operations, especially in terms of weight and balance, fuel loads, and runway length requirements.

While factors such as altitude above sea level, engine horsepower, and weight of the aircraft do play roles in performance, they are all influenced by density altitude. Thus, understanding density altitude provides a comprehensive insight into how these other factors interact and affect overall aircraft performance in varying conditions.