FAA Private Pilot Helicopter (PRH)Performance and LimitationsEasy
An airport has a pressure altitude of 2,000 feet and an outside air temperature of 30°C. Using the standard rule of thumb (add 120 feet for each degree Celsius above the ISA temperature for that altitude), what is the approximate density altitude?
- A3,800 feet
- B4,280 feet
- C5,120 feet
- D2,280 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 4,280 feet
ISA temperature at 2,000 ft = 15°C − (2 × 2) = 11°C. The temperature deviation is 30 − 11 = 19°C. Density altitude ≈ 2,000 + (120 × 19) = 2,000 + 2,280 = 4,280 feet.
Why the other options are wrong
- A. This underestimates the temperature deviation used in the calculation.
- C. This overstates the deviation; the correct correction is 19°C, not larger.
- D. This is only the correction added, not the total density altitude.
Density Altitude
Pressure altitude corrected for nonstandard temperature; it directly affects helicopter performance such as lift, power available, and hover capability.
- Rises with higher temperature and higher elevation
- High density altitude reduces engine and rotor performance
- Rule of thumb: DA = PA + 120 × (OAT − ISA temp at that altitude)
Memory trick: Hot and high, helicopters sigh (less lift, less power).