FAA Private Pilot Airplane (PAR)Performance, Weight and BalanceMedium
A pilot is calculating takeoff performance on a hot, humid day at a high-elevation airport. In addition to the effects of temperature and altitude, what effect does high humidity have on aircraft performance?
- AHigh humidity only affects landing performance, not takeoff performance
- BHigh humidity increases air density, improving performance
- CHigh humidity further decreases air density, slightly reducing engine power and requiring a longer takeoff roll
- DHumidity has no measurable effect on performance calculations
Show answer & explanationAnswer & explanation
Correct answer: C. High humidity further decreases air density, slightly reducing engine power and requiring a longer takeoff roll
Water vapor is less dense than dry air, so high humidity slightly decreases overall air density, which can further reduce engine power output and lift, compounding the effects of high density altitude and requiring a longer takeoff roll.
Why the other options are wrong
- A. Humidity affects both takeoff and landing performance similarly, not just landing.
- B. Humid air is actually less dense than dry air, not more.
- D. Although POH charts rarely quantify it, humidity does have a real, if small, adverse effect.
Humidity and Performance
High humidity slightly reduces air density (water vapor is lighter than dry air molecules), compounding the performance-robbing effects of high density altitude.
- Moist air is less dense than dry air at the same temperature/pressure
- Humidity effects are usually not on performance charts but are real
- Combines with heat and altitude to reduce power and lift further
Memory trick: Moist air makes muscle weaker.