FAA Part 107 Remote Pilot (UAG)WeatherMedium
A technician preparing to launch an sUAS at a field elevation with a pressure altitude of 6,000 feet notes the outside air temperature is 25°C. The standard temperature at 6,000 feet is 3°C. What is the approximate density altitude, and how will it affect the aircraft's performance?
- AApproximately 3,000 feet, resulting in improved climb performance
- BApproximately 8,640 feet, resulting in decreased lift, reduced thrust, and longer takeoff distance
- CApproximately 6,000 feet, with no measurable effect on performance
- DApproximately 11,000 feet, resulting in increased propeller efficiency
Show answer & explanationAnswer & explanation
Correct answer: B. Approximately 8,640 feet, resulting in decreased lift, reduced thrust, and longer takeoff distance
Temperature deviation from standard = 25°C − 3°C = 22°C. Density altitude ≈ pressure altitude + (120 × temperature deviation) = 6,000 + (120 × 22) = 6,000 + 2,640 = 8,640 feet. Higher density altitude means thinner air, which reduces propeller efficiency, lift, and thrust, degrading takeoff and climb performance.
Why the other options are wrong
- A. Uses the wrong direction of temperature deviation; density altitude increases, not decreases.
- C. Ignores the significant temperature deviation above standard, which raises density altitude substantially.
- D. Overestimates density altitude and incorrectly claims improved efficiency, which is opposite of reality.
Density Altitude
Pressure altitude corrected for nonstandard temperature; represents the altitude the aircraft 'feels' it is flying at aerodynamically.
- Rule of thumb: DA ≈ pressure altitude + (120 × temp deviation from standard)
- Higher DA = thinner air = reduced lift, thrust, and climb performance
- Hot, high, and humid conditions increase density altitude
Memory trick: 'High, hot, humid' all push density altitude up and performance down.