Plane & Pilot
Tuesday, June 19, 2012

The Joys Of Summer

Depending on where you live, summer can be the time to fly

It was late summer, and I nursed the old Bellanca Cruisemaster higher as we passed over Blue Mesa Reservoir near Gunnison, Colo. The engine was leaned for our 9,500-foot altitude, but looking straight ahead at the crest of the Rockies, it was obvious I still had higher to climb.

The Continental Divide cut directly across our path 30 miles ahead. Monarch Pass reared up as I followed U.S. Highway 50 east into the tall rocks, leaning the mixture a little more and hoping the 190 hp Lycoming could find extra power to somehow lift us above the jagged rocks of the pass at 11,300 feet.

I had passed this way probably a dozen times before in a variety of airplanes, from Super Cubs and Decathlons to Scouts and Skyhawks, and I was grateful that it was a clear day with little wind and hardly any turbulence. I knew it was unusual to see an OAT of 25 degrees C, warm for the high Rockies in late July.

I had already crunched the numbers through my trusty E6B, and the density altitude at 9,500 feet pressure altitude worked out to a discouraging 12,700 feet. The crest of the pass was almost another 2,000 feet above our current height, so we'd probably need to be at a theoretical 14,500-foot density to squeak across the summit with a few hundred feet to spare.

Perhaps in anticlimax, we picked up a little lift as we lofted uphill toward the high mountains. We soared over the final high-way switchback to the viewpoint on the Continental Divide, peppered with cars and tourists, many probably startled by our 500-foot pass above them. Then, we dropped over the eastern rim and headed toward Montrose and the Midwest.

Understanding Density Altitude
High density altitude isn't much of a concern over most of the U.S., but it becomes a problem each summer in the Southwest. The simple increase in temperature by 15 or 20 degrees C in the middle of the year thins the air and makes life more difficult for pilots of general aviation airplanes when the days grow long.

One of the reasons pilots get into trouble in summer is that they don't fully understand the effect of the temperature change. Reading the OAT and translating that through the performance charts often doesn't even come close to predicting airplane performance.


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