HEAT + HUMIDITY = TROUBLE

It’s time to recall some of the things you learned in ground school but haven’t thought about for a while—at least not since last summer. You remember the old memory shortcut about “high, hot, and heavy” degrading performance. “Heavy” is obvious. And while we all know that our aircraft don’t perform as well at high altitudes and hot temperatures, it’s important to understand why not.

If you’ve walked across a hot parking lot, you know it’s even hotter on the pavement.  Paved surfaces, especially black asphalt, absorb the sun’s rays and make the air above them even hotter.  Of course, this happens on runways too. 

Runway temperatures can be as much as 40°F hotter than what the AWOS or ATIS says on a sunny summer day.  Plan for that and adjust your takeoff distance calculations appropriately.  You might want to get the temperature from your airplane’s Outside Air Temperature gauge in addition to the AWOS or ATIS to use on your takeoff performance chart.

When we’re talking about “high altitude,” it’s density altitude that we’re talking about. At high altitudes, the air is less dense because heat expands things. The hotter the air, the less dense it becomes. As you know, wings generate lift by interacting with air, and engines develop power by combining fuel with air.  Then propellers turn power into thrust by reacting with the air.  Anything that reduces the density of air will reduce the airplane’s performance.  Hence, “high” and “hot.”

But what about humidity? Though it might seem counterintuitive, air loaded up with water vapor is lighter than dry air. According to the FAA’s Pilot’s Handbook of Aeronautical Knowledge¹:

“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 aircraft. Water vapor is lighter than air; consequently, moist air is lighter than dry air. Therefore, as the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance.  It is lightest or least dense when, in a given set of conditions, it contains the maximum amount of water vapor…” Then, we add heat to the equation. “…As temperature increases, the air can hold greater amounts of water vapor.   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 upon density.

Take a look at the performance section of your POH. If the humidity is high — say, above 50 or 60% — it may be wise to add additional length to the runway requirements over what the takeoff performance chart says to account for moist air.  You might also want to plan for a decreased rate of climb, especially if you have to clear an obstacle.

The NTSB files are full of accidents caused by pilots who didn’t take heat and humidity, let alone altitude, into account before blasting down a runway

To make sure you don’t become one of them, please spend a few minutes in the books accounting for takeoff performance loss next time you fly.

¹ The Pilot’s Handbook of Aeronautical Knowledge 2016. Published by the FAA and currently located at https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/

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