Calculate the density altitude of a helicopter being at an airfield with a Pressure Altitude of 3 500 ft and OAT +27°C. (Assume 120 ft per degree deviation from ISA.)
The performance data of some helicopter types is referenced to the Density Altitude.
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 and pressure altitude.
Apart from the Flight/Nav computer and CAP 758 Figure 1.1, an alternative method to calculate the density altitude is the following:
- Density Altitude = Pressure altitude + (120 ft x ISA deviation)
- The ISA deviation is: OAT - ISA temperature at 3 500 ft = +27°C - (15°C - 2°C x 3 500 ft/1 000) = +19°C.
- Thus, the Density Altitude is: 3 500 ft + (120 ft x 19°C) = 5 780 ft.
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