FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceHard
A pilot notices that on a hot, humid day, density altitude computed from temperature and pressure altitude alone may understate the actual reduction in aircraft performance. What is the primary reason for this?
- AWater vapor molecules are lighter than the nitrogen and oxygen molecules they displace, further reducing air density
- BHumidity has no measurable effect on performance and is disregarded in all POH data
- CHumidity increases propeller efficiency, which offsets any loss of engine power
- DHumid air is denser than dry air at the same temperature and pressure, improving performance
Show answer & explanationAnswer & explanation
Correct answer: A. Water vapor molecules are lighter than the nitrogen and oxygen molecules they displace, further reducing air density
Water vapor molecules are lighter than the nitrogen and oxygen they displace, so humid air is actually less dense than dry air at the same temperature and pressure, further degrading engine power and aerodynamic performance beyond standard density altitude calculations.
Why the other options are wrong
- B. Incorrect; while charts may not always account for it, humidity does affect performance.
- C. Fabricates an effect; humidity does not improve propeller efficiency.
- D. Reverses the actual physics; humid air is less dense, not more.
Humidity Effect on Performance
High humidity slightly reduces air density beyond that predicted by temperature and pressure alone because water vapor molecules are lighter than the oxygen/nitrogen molecules they replace, further reducing engine power and lift.
- Water vapor is lighter than dry air's main components
- Humid air is less dense than dry air at same temp/pressure
- Effect is smaller than temperature/pressure but still real
- POH performance charts often assume dry air, understating hot/humid degradation
Memory trick: 'Wet air is light air' — humidity thins the air further.