Which of the following is NOT a factor of in-flight stability?

In-flight stability of an aircraft is heavily influenced by several key factors that contribute to how well an aircraft can maintain its intended flight path without excessive input from the pilot. Proper weight, speed, and drag surfaces all play critical roles in stability during flight.

When examining the factor of "Environment," it encompasses external conditions, such as wind, turbulence, and weather conditions, which can indeed impact flight stability but do not directly contribute to the fundamental aerodynamic stability of an aircraft's design. The other factors—proper weight, speed, and drag surface—are intrinsic to the aircraft's characteristics and directly affect its stability during flight.

Proper weight allows the aircraft to have a suitable center of gravity, which directly influences balance and maneuverability. Proper speed ensures that the aircraft remains within its performance envelope, which is crucial for maintaining control. Proper drag surfaces, such as flaps or ailerons, also help manage the airflow around the aircraft, promoting stability through various flight modes. Therefore, while environmental conditions can affect the flight experience, they are not considered fundamental factors of in-flight stability in the same way that the other elements are.