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.")
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-- 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.