Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.")

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

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Aero-Tips 08.11.06

"It's not the heat, it's the humidity." A tired saw, but it holds a truth. Moisture content does affect the way we perceive a given temperature... but does humidity have an impact on aircraft performance?

Density Altitude

Density altitude is the "working" altitude for an aircraft. Lift and thrust vary with the amount of available air; the hotter the air the less dense it becomes, and the less is available to flow over wings and propellers, and to burn in engines.

Moisture displaces atmospheric gases in a given volume of air, so it should affect lift and power development also. The Pilot's Handbook of Aeronautical Knowledge tells us:

The small amount of water vapor suspended in the atmosphere may be almost negligible under certain conditions, but in other conditions humidity may become an important factor in the performance of an airplane. Water vapor is lighter than air; consequently, moist air is lighter than dry air. It is lightest or least dense when, in a given set of conditions, it contains the maximum amount of water vapor. The higher the temperature, the greater amount of water vapor the air can hold. When comparing two separate air masses, the first warm and moist (both qualities tending to lighten the air) and the second cold and dry (both qualities making it heavier), the first necessarily must be less dense than the second. Pressure, temperature and humidity have a great influence on airplane performance, because of their effect on density.

High humidity, then, has a negative effect on aircraft performance. Yet I know of no Pilots Operating Handbook or other source that provides Performance Chart adjustments for high humidity. We have no data to determine what humidity does to performance for most aircraft.

Aero-tip to the day: If the humidity is high -- say, above 50% -- it may be wise to add another 10% or more to runway requirements over what you calculate using the more traditional density altitude inputs of temperature and pressure altitude.

FMI: Aero-Tips