Humidity Vs. Horsepower

By the time student pilots near the practical exam, they’ve usually got a pretty good idea of how and why to calculate density altitude (DA). If they’re lucky, they’ve even done some high-altitude takeoffs with an instructor, or at least simulated DA’s effects by using much-less-than-full power settings on a few takeoffs. Those tables and graphs overlook an important characteristic of the air in which we’re trying to fly: its humidity. Two classic concerns with mountain flying are density altitude and pressure altitude. Actual altitude doesn’t affect aerodynamic performance. Most of us plan our high-elevation arrivals and departures as early in the day as practical, and are extra attentive on warmer days when seated behind a normally aspirated engine. While reduced horsepower is certainly one reason to be wary of high-DA situations, the thinner air also means higher true airspeeds—and lower indicated ones—resulting in the airplane “thinking” it’s higher than it really is. The impact is felt through mushier controls, since there are fewer air molecules flowing over them. There’s also an impact on propeller efficiency, since its blades are airfoils. The net effect, of course, translates into longer, faster takeoff rolls.

Key Takeaways:

• Standard density altitude (DA) calculations and aircraft performance charts frequently overlook the critical impact of humidity on normally aspirated engine performance.
• Humidity reduces the available oxygen in the air, directly diminishing engine horsepower and leading to significantly degraded takeoff and climb capabilities, particularly at high-altitude airports.
• Pilots should account for humidity by using the dew point to determine a higher "oxygen equivalent" pressure altitude for more accurate density altitude and performance calculations during pre-flight planning.

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