Which combination of factors most significantly reduces helicopter performance?

High altitude, high humidity, and high temperature significantly reduce helicopter performance due to several interconnected factors. At high altitudes, the air density decreases, which impacts the lift generated by the helicopter rotor blades. This diminished lift force necessitates a longer distance for takeoff and landing, thereby affecting overall performance and maneuverability.

High temperatures contribute to further decreases in air density, which exacerbates the challenges posed by altitude. As temperature rises, the air becomes less dense, effectively resulting in reduced lift and engine performance. This combination means that helicopters operating in these conditions may experience increased fuel consumption and reduced power output.

High humidity can also influence helicopter performance, as moist air is less dense than dry air at the same temperature. As humidity increases, the overall density of the air decreases, complicating the lift and performance challenges already posed by high temperature and altitude.

Together, these factors create a scenario where the helicopter's ability to generate sufficient lift and maintain effective performance is severely compromised, which is why this combination is particularly detrimental for helicopter operations.