A pilot is planning a departure from an airport at a pressure altitude of 5,000 feet with an outside air temperature of 30°C. Using the standard lapse rate assumption, what is the approximate density altitude, and what performance concern should this raise for the CFII to brief?
- AApproximately 8,000 feet; significantly reduced takeoff and climb performance
- BApproximately 5,000 feet; no significant performance degradation expected
- CApproximately 10,000 feet; oxygen requirements become a factor
- DApproximately 6,500 feet; slightly reduced climb performance
Show answer & explanationAnswer & explanation
Correct answer: A. Approximately 8,000 feet; significantly reduced takeoff and climb performance
Standard temperature at 5,000 feet pressure altitude is 5°C (15°C minus 2°C per 1,000 feet). The temperature deviation from standard is 30°C − 5°C = 25°C above standard. Using the approximation of 120 feet of density altitude added per degree Celsius above standard, density altitude = 5,000 + (120 × 25) = 5,000 + 3,000 = 8,000 feet, which significantly reduces engine performance, propeller efficiency, and lift, requiring longer takeoff rolls and reduced climb rates.
Why the other options are wrong
- B. This ignores the significant temperature deviation above standard, which substantially raises density altitude.
- C. 10,000 feet is too high based on the calculation, and oxygen requirements aren't the primary CFII briefing concern here.
- D. This underestimates the density altitude increase; the math shows a much higher value.
Density Altitude Calculation
Density altitude is pressure altitude corrected for temperature deviation from standard; higher density altitude significantly degrades aircraft performance.
- Standard temp decreases 2°C per 1,000 ft (ISA lapse rate)
- Approximate rule: add 120 ft density altitude per °C above standard
- High density altitude reduces lift, engine power, and propeller efficiency
Memory trick: Hot and high robs you of sky—density altitude steals performance.