Flying Smart

Density Altitude

"Front Range Airport automated weather observation - 1812 Zulu weather. Winds calm, visibility more than 10, sky: few clouds at 9,500. Temperature 20 Celsius, dew point minus 1, altimeter 29.87. Remarks: Density altitude, 7,400." The field elevation at this Colorado airport is 5,512 feet.

As hot weather settles in for the duration of summer, the words density altitude should be an integral part of your flight planning. You may have learned in ground school that density altitude is pressure altitude corrected for nonstandard temperature and humidity, and you can probably repeat that sentence in your sleep. But you also need to know how high temperatures, high elevation, and aircraft weight work together to spell trouble for the unsuspecting pilot. Certificated flight instructors frequently call these "the three Hs" - hot, high, and heavy.

The factors that cause dramatic reduction in air mass density - high altitude, temperature, and humidity, combined with low barometric pressure - deprive wings and propellers of lift. Density altitude hampers takeoff and climb performance and, hence, your ability to safely clear obstacles that lie beyond the runway. This is why you need to include density altitude in flight planning and why you should be intimately familiar with the performance charts in the pilot's operating handbook for the aircraft you fly. Even then, remember that those calculations were made when the aircraft was brand-new. Your trainer aircraft may no longer perform to those standards.

You may fly out of at an airport 50 feet above sea level with a runway that's more than a mile long. But that won't always be the case. If you land at a mountain airstrip with a planeload of passengers and bags, can you get back out safely with full fuel tanks? If the figures look dicey, you'll want to be flexible. Reschedule your departure until later in the evening or early in the morning, when cooler temperatures can work to your favor. Better yet, don't load your airplane to its maximum takeoff weight. Aviation experts advise operating your aircraft as though it had fewer seats. You can find on AOPA Online a detailed discussion of density altitude's effects ( www.aopa.org/members/files/pilot/1997/dens9706.html ). And don't forget to read AOPA's A Pilot's Guide to Mountain Flying, which reviews density altitude and high-elevation airport operations (www.aopa.org/members/files/guides/mntfly.htm ).