FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
A helicopter departs a 5,000-foot elevation airport where the altimeter is set to 29.92 in. Hg (pressure altitude = field elevation) and the outside air temperature is 30°C. Using the rule that standard temperature decreases 2°C per 1,000 feet from 15°C at sea level, and that density altitude increases approximately 120 feet for each degree Celsius above standard temperature, what is the approximate density altitude?
- A5,000 feet
- B8,000 feet
- C9,500 feet
- D6,500 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 8,000 feet
Standard temperature at 5,000 ft is 15°C − (2°C × 5) = 5°C. The actual OAT of 30°C is 25°C above standard. Density altitude = pressure altitude + (120 × temperature deviation) = 5,000 + (120 × 25) = 5,000 + 3,000 = 8,000 feet.
Why the other options are wrong
- A. This is just the pressure altitude, ignoring the temperature deviation entirely.
- C. This overstates the deviation, as if using 37.5°C above standard instead of 25°C.
- D. This would result from using only a 12.5°C deviation, an arithmetic error.
Density Altitude Calculation
Density altitude is pressure altitude corrected for nonstandard temperature; it determines actual helicopter and rotor performance.
- Standard temperature decreases 2°C per 1,000 ft from 15°C at sea level
- Rule of thumb: DA = PA + 120 x (OAT - standard temp)
- High density altitude reduces rotor efficiency and available power
Memory trick: Hot and high robs your rotor of bite — density altitude eats performance.