FAA Private Pilot Helicopter (PRH)Performance and LimitationsEasy
A pilot is preparing for a departure on a hot, humid day at a high-elevation airport. Compared to the same temperature and pressure altitude with low humidity, the high humidity will:
- AIncrease available rotor thrust because moist air is denser
- BHave no measurable effect on helicopter performance
- CDecrease performance similar to an increase in density altitude
- DOnly affect engine cooling, not aerodynamic performance
Show answer & explanationAnswer & explanation
Correct answer: C. Decrease performance similar to an increase in density altitude
Water vapor is less dense than dry air, so high humidity effectively lowers air density, reducing engine power output and rotor lift capability, much like an increase in density altitude. Although not included in the standard density altitude formula, pilots should account for it as a performance-reducing factor.
Why the other options are wrong
- A. Moist air is actually less dense than dry air, so thrust and lift decrease, not increase.
- B. Humidity has a real, measurable negative effect on both engine and rotor performance.
- D. Humidity affects aerodynamic lift and engine power output, not just cooling.
Humidity Effect on Performance
High humidity displaces heavier oxygen and nitrogen molecules with lighter water vapor, decreasing air density and degrading engine and rotor performance.
- Water vapor is lighter than dry air molecules
- High humidity effectively increases density altitude conditions
- Not included in the standard DA computation but should be considered by pilots
Memory trick: Humid air is 'thin' air in disguise—less punch for engine and rotor