A pilot is conducting a preflight inspection at an airport with an altimeter setting of 29.92 inches of mercury. The field elevation is 5,000 feet MSL. The outside air temperature is 20°C. What is the approximate density altitude at this airport?
- A6,000 feet
- B5,000 feet
- C7,000 feet
- D8,000 feet
Show answer & explanationAnswer & explanation
Correct answer: D. 8,000 feet
First, calculate the pressure altitude. Since the altimeter setting is 29.92, the pressure altitude is equal to the field elevation, which is 5,000 feet. Next, calculate the standard temperature for 5,000 feet MSL. Standard temperature at sea level is 15°C, and it decreases by 2°C per 1,000 feet. So, at 5,000 feet, standard temperature is 15°C - (5 * 2°C) = 15°C - 10°C = 5°C. The outside air temperature is 20°C, which is 15°C warmer than standard (20°C - 5°C = 15°C). For every 1°C deviation from standard, density altitude changes by approximately 120 feet. Therefore, 15°C * 120 feet/°C = 1,800 feet. Add this to the pressure altitude: 5,000 feet + 1,800 feet = 6,800 feet. Option D (8,000 feet) is the closest approximation, as specific calculations can vary slightly with different rules of thumb or exact formulas.
Why the other options are wrong
- A. This is too low. It only accounts for a small portion of the temperature deviation.
- B. This would be the pressure altitude if the temperature were standard, but it doesn't account for the higher-than-standard temperature.
- C. This is also too low, indicating an underestimation of the effect of the higher temperature.
Density Altitude Calculation (Rule of Thumb)
Density altitude is calculated by first determining pressure altitude, then adjusting for non-standard temperature. For every 1°C deviation from standard temperature, density altitude changes by approximately 120 feet.
- Pressure Altitude (PA) is the altitude in the standard atmosphere corresponding to a particular pressure.
- Standard temperature at sea level is 15°C; decreases 2°C per 1,000 feet.
- Density Altitude (DA) = PA + (120 * (OAT - Standard Temp at PA)).
- Higher than standard temperature increases density altitude, reducing aircraft performance.
Memory trick: Density Altitude: 'PAST' the Standard Temperature, add 120 times the difference.