The Heat is On

By Michael Adams, Senior Vice President, Avemco Insurance Company

July 2017

Summers’ heat is now upon us. One of the perks of being a pilot is that when temperatures climb, so can you, right up to altitude where it’s nice and cool. But to get there, you first need to get safely off the ground.

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. And the hotter the air, the less dense it becomes.

That’s why it’s even more important to make a takeoff distance calculation before taking off on a hot day. You need to know you have enough runway to get off the ground.

If you’ve walked across a hot parking lot lately, you know it’s even hotter on 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.

“It’s not the heat, it’s the humidity.” How many times have you heard that old summer cliché? But does humidity do anything to airplane performance? 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… 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.

This tells us that high humidity has a negative effect on aircraft performance. Yet I’m not aware of any Pilots Operating Handbook or other source that provides Performance Chart adjustments for high humidity. 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.

To make sure you reach that cool air on hot summer days please spend a few minutes in the books accounting for takeoff performance loss when the heat is on!

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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/

Mike Adams, Senior Vice President of Underwriting, is an instrument-rated pilot, and a former President of the Oregon Pilots Association. Mike holds a property/casualty insurance license in all 50 states. His more than 30 years of combined experience with general aviation and the aviation insurance industry helps pilots to understand why many of Avemco’s coverages and underwriting decisions are designed to help keep them safe.