FAA Private Pilot Helicopter (PRH)WeatherMedium
A pilot departs an airport with a pressure altitude of 3,000 feet and an outside air temperature of 25°C. Using the standard lapse rate of 2°C per 1,000 feet, what is the approximate density altitude?
- A4,900 feet
- B3,000 feet
- C6,000 feet
- D1,900 feet
Show answer & explanationAnswer & explanation
Correct answer: A. 4,900 feet
Standard temperature at 3,000 feet is 15°C − (2°C × 3) = 9°C. The temperature deviation from standard is 25°C − 9°C = 16°C above standard. Density altitude increases approximately 120 feet for each degree Celsius above standard, so 3,000 + (120 × 16) = 3,000 + 1,920 ≈ 4,900 feet.
Why the other options are wrong
- B. This ignores the temperature deviation from standard entirely.
- C. This overestimates the temperature correction beyond the actual 16°C deviation.
- D. This would result from being below standard temperature, not above it.
Density Altitude Calculation
Density altitude is pressure altitude corrected for nonstandard temperature; it increases when temperature is above standard for that altitude.
- Standard temperature decreases 2°C per 1,000 feet from 15°C at sea level
- Density altitude changes about 120 ft per °C deviation from standard
- High density altitude reduces aircraft/helicopter performance
- Formula: DA = PA + (120 × (OAT − Standard Temp))
Memory trick: Find standard temp, find the difference, multiply by 120