FAA Private Pilot Airplane (PAR)Performance, Weight and BalanceHard
An airport has a field elevation of 5,000 ft MSL. The current altimeter setting is 30.42 inHg, and the outside air temperature is 30°C. Using the rule of thumb that density altitude increases approximately 120 ft for each degree Celsius above the standard temperature for that pressure altitude, what is the approximate density altitude?
- A4,500 ft
- B7,380 ft
- C9,500 ft
- D5,000 ft
Show answer & explanationAnswer & explanation
Correct answer: B. 7,380 ft
Pressure altitude = field elevation − (altimeter setting − 29.92) × 1,000 = 5,000 − (30.42−29.92)×1,000 = 5,000 − 500 = 4,500 ft. Standard temperature at 4,500 ft ≈ 15°C − (4,500/1,000 × 2°C) = 15 − 9 = 6°C. Temperature deviation = 30 − 6 = 24°C above standard. Density altitude ≈ 4,500 + (120 × 24) = 4,500 + 2,880 = 7,380 ft.
Why the other options are wrong
- A. This is only the pressure altitude, before applying the temperature correction.
- C. This overstates the correction beyond what the temperature deviation supports.
- D. This is only the field elevation, not corrected for pressure or temperature.
Density Altitude Estimation
Density altitude is calculated by first finding pressure altitude (correcting for altimeter setting) and then adjusting for temperature deviation from the standard lapse rate.
- Pressure altitude = elevation − (altimeter setting−29.92)×1,000
- Standard temperature decreases 2°C per 1,000 ft from 15°C at sea level
- Rule of thumb: DA ≈ PA + 120 ft per °C above standard temperature
Memory trick: Pressure first, then heat, gives you the density altitude treat.