Plane & Pilot
Tuesday, June 19, 2012

The Joys Of Summer

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

High density altitude becomes a problem each summer in the southwestern U.S. Grand Canyon National Park Airport (KGCN) sits at an elevation of 6,609 feet, but density altitude reaches much higher during the hot summer months.
flight to the U.S. after delivering a new airplane in late January. As it happened, I knew the captain of the flight, and he asked if I wanted to ride jump seat for part of the trip. (This was obviously well before 9/11/01.)

I boarded with the crew, stowed my gear and climbed up to the cockpit. The captain was out attending to last-minute details, so the first officer and I discussed the flight ahead. Elevation at Johannesburg is just over a mile above the sea, and Jan Smuts Airport has one runway that stretches nearly 14,500 feet. Our departure was scheduled for midnight, but the temperature was still almost 30 C as we prepared for pushback.

I asked the FO about our takeoff roll, he shuffled through some papers and finally found the number. "It'll be about 13,800 feet," he said casually.

I was stunned. It had never occurred to me that anyone could predict performance that accurately on a 400-ton airliner. Sure enough, a half-hour later when we rolled down the runway, I was amazed at the accuracy of the copilot's calculation. When he called "Rotate" and the captain finally lifted off, the REILs were directly ahead, probably less than 700 feet away. We roared above the lights at 50 feet AGL and climbed away toward London.

Fortunately, we don't have the same problems in personal aircraft, but figuring accurate density altitude can still be a challenge.

Part of the problem is that pilots sometimes use the wrong temperature to calculate DA. Most of the time, we fly from smooth, dry runways, typically constructed of concrete, asphalt or a combination of the two. When the outside air temperature soars above 38 degrees C (100 degrees F), runways tend to absorb heat and bake in the sun, and the actual runway temperature may be far warmer than the reported OAT.

On a very hot day, consider doing part of your checklist before engine start to save time and avoid unnecessary engine temp increase.
In the true dog days of summer, airports at places such as Phoenix, Tucson, Las Vegas, El Paso, Dallas and other hot spots may announce the runway temperature on the ATIS ("Temperature 39, runway temperature 48 degrees), making density altitude calculation easier. Sometimes, ATIS will even include a computer-generated density-altitude computation. If they don't, the higher runway temp is the one to use for density calculation.

But, wait. There's another factor that should be considered. Tightly cowled engines operate on induction air that may be considerably hotter than even the runway temperature. Induction tubes and intake manifolds heat up disproportionately as the engine warms, heating air trapped in the engine compartment and contributing to a further escalation of temperature inside the cowling. If you're operating from a major terminal such as those mentioned above, you may be forced to wait in the run-up area for several minutes. Meanwhile, the induction temperatures may be climbing well above any reasonable limit.

There are ways to shortstop this problem if you're operating from an uncontrolled strip or an airport that will allow you to depart on pretty much your own schedule. If you use the standard C-I-G-A-R-S-R-LCA checklist or some variation thereof, you can shorten the run-up time without compromising safety.


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