What is the impact of humidity on turboshaft engine performance?

Humidity generally decreases turboshaft engine performance. While seemingly counter-intuitive, this is because water vapor displaces oxygen in the air. Turboshaft engines rely on a specific mixture of air and fuel for efficient combustion. When the air is humid, it contains a higher percentage of water vapor and a lower percentage of oxygen. The lower oxygen content reduces the efficiency of the combustion process, resulting in less power output from the engine. High humidity can also affect the density of the air, further reducing engine performance. This is why helicopters operating in hot and humid environments often experience a reduction in their maximum payload and hover ceiling. While the effect of humidity is usually less pronounced than that of temperature and altitude, it is still a factor that pilots must consider during pre-flight planning.