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As I write this, the mercury here in New Jersey is inching into the 90s. It seems like just a few weeks ago the trees were still budding. But summer has arrived, and with it, all the new flight planning considerations of the season. Most often, we think of summer as the time for thunderstorms and, maybe, those murky low-visibility days brought on by lingering high pressure, especially here in the Northeast. But the effect of high density altitude on aircraft performance can be sneaky. And if you're not keeping a watchful eye out, it can bite you. Especially flatland pilots venturing into areas of higher elevation on summer vacation trips.
Most pilots have probably seen the YouTube video of an A36 Bonanza trying to take off from a short strip in the heat of the day and wallowing into the terrain off the end of the runway. Any of us can imagine the sickening feeling the pilot must have experienced as his airplane just didn't lift off the way he expected.
To hark back to primary flight instruction days: density altitude is a combination of air temperature, barometric pressure and humidity as calculated by dew point. Hot, humid weather with low pressure reduces the density of the air beneath our wings — as well as the air pushed by our propellers and sucked into our cylinders for combustion. Manufacturers' takeoff performance numbers are calculated by density altitude, and most of the time we have plenty of margin. But sometimes, when we're up high, closer to maximum gross weight (or even above it?) and we're using a runway that's shorter than we're accustomed to, density altitude causes the required runway length number to creep up and over the amount of actual pavement in front of us. And don't forget the specter of rising terrain off the end.
The humidity/dew point factor is a surprising wild card. Standard ICAO conditions are calculated with zero humidity — no moisture in the air at all. On one of those gooey summer days, humidity can climb up to more than 80 percent, represented by dew points edging closer and closer to the actual ambient temperature. On a 95-degree day with a dew point of 80 degrees and ICAO-standard pressure of 29.92 inches, an airport at sea level has a density altitude of 2,718 feet. An airport at 5,000-feet elevation, at the same temperature with a baro pressure of 29.45 inches treats your airplane as though it were at 9,375 feet.
Gulp.
There is a simple density altitude calculator available at the pilotfriend.com website. Click here to have a look. It's an eye-opening exercise to punch in some not-so-unlikely scenarios like the one in the last paragraph. Coupled with the takeoff performance numbers in your aircraft's pilot's operating handbook (POH), that should give you an idea of when you'll need to start paying attention.
Call to action: If you have any tips of your own you'd like to share, or have any questions about flying technique you'd like answered, send me a note at enewsletter@flyingmagazine.com. We'd love to hear from you.
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