You are preparing for take-off from an airfield where the pressure altitude is 2000 ft. The outside air temperature is +15°C, and the QNH is 1013 hPa. Using this information, what is the approximate density altitude at the aerodrome?
1. Given data:
- Pressure altitude = 2000 ft
- Temperature = +15°C
- QNH = 1013 hPa (so pressure altitude is already correct, no correction needed).
2. Standard temperature at 2000 ft:
ISA at sea level = 15°C.
Standard lapse rate = 2°C per 1000 ft.
At 2000 ft: 15 − (2 × 2) = 11°C.
3. Temperature difference (OAT – ISA):
15 − 11 = +4°C warmer than standard.
4. Density altitude correction:
Rule of thumb: For every 1°C that the environment temperature differs from ISA, the density altitude will differ from the pressure altitude by 120 ft.
= 4 × 120 = 480 ft.
5. Density altitude:
Pressure altitude + correction = 2000 + 480 ≈ 2480 ft.
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