FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesHard
An airport is at a field elevation of 5,000 feet MSL with the altimeter set to 29.92 inches Hg, giving a pressure altitude of 5,000 feet. The outside air temperature is 30°C. Using the rule of thumb that density altitude increases approximately 120 feet for every 1°C the actual temperature exceeds the standard temperature for that altitude, what is the approximate density altitude?
- A6,500 feet
- B8,000 feet
- C9,500 feet
- D5,000 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 8,000 feet
Standard temperature at 5,000 feet is 15°C − (2°C × 5) = 5°C. The actual temperature of 30°C is 25°C above standard. Using 120 ft per °C deviation: 25 × 120 = 3,000 ft added to pressure altitude, so density altitude = 5,000 + 3,000 = 8,000 feet.
Why the other options are wrong
- A. This underestimates the temperature deviation correction.
- C. This overestimates the correction beyond the actual 25°C deviation.
- D. This ignores the temperature deviation entirely.
Density Altitude Calculation
Density altitude is pressure altitude corrected for nonstandard temperature; higher density altitude reduces aircraft and engine performance.
- Standard temp decreases 2°C per 1,000 ft from 15°C at sea level
- Rule of thumb: add ~120 ft density altitude per °C above standard
- High density altitude reduces lift, engine power, and propeller efficiency
Memory trick: Hot and high, performance says goodbye.