The performance data of some helicopter types is referenced to the Density Altitude. Most of the performance regulations have to do with the take-off phase and contain the following phrase, regarding compliance with the requirements: ‘’the take-off mass shall be such that ……”, which means that, if a take-off weight at a specific altitude in specific ambient conditions does not permit any compliance with the requirements, then the take-off weight must be reduced.

Density Altitude is the pressure altitude corrected for non-standard temperature, the ISA deviation. In other words, it is the altitude in ISA, which gives the same air density as the prevailing non-ISA combination of temperature - pressure altitude and is used to establish performance, as it is a figure that expresses where your engine thinks it is, as opposed to where it actually is.

Density varies with:

• Temperature
• Altitude
• Humidity

Thus, since Airfield elevation and Humidity are factors that determine the Density Altitude, then the determined Density Altitude has a greater effect on the helicopter's performance than each factor separately.

The effect of a light precipitation on helicopter's performance is negligible.

The change in Density Altitude due to humidity is very low, at only 400 feet from between 0 to 100%, so the effect on the rotors is not much. In other words, the range of humidity between 0-100% makes a difference of only 400 feet in Density Altitude, so when not limited by engine output, that would be a difference of about 20-25 kg off your Maximum All-Up-Mass.