Which of the following is NOT a factor affecting UAS stability during flight?

Altitude is not a direct factor affecting the stability of an Unmanned Aircraft System (UAS) during flight when compared to the other listed factors.

Rotor efficiency plays a crucial role in how well a UAS can maintain stability, as it determines the effectiveness of lift generation. If the rotors are not operating efficiently, it can lead to instability. Similarly, UAS weight and balance significantly influence how the aircraft behaves in the air. An imbalanced UAS can lead to unwanted pitching or rolling motions, making stability harder to maintain.

Wind conditions also greatly affect flight stability, as changes in wind speed and direction can lead to turbulence that the UAS must compensate for. Therefore, while altitude may influence performance characteristics such as air density and lift, it is not a primary factor in immediate flight stability when compared to rotor efficiency, weight and balance, and wind conditions.