Density altitude high? Know before you fly

July 18, 2007

Density altitude high? Know before you fly

Throughout a pilot's flight training, there are constant reminders about the detrimental effect high density altitude has on aircraft performance. But when the sky is blue and the summer sunshine is hot, pilots need to be reminded again why it's important to carefully calculate takeoff, climb, cruise, and landing performance during preflight planning.

In addition to your pilot's operating handbook, read AOPA's subject report on density altitude, featuring instructor tips and pilot stories. For more learning tools, articles, and other online resources to increase your knowledge about summer weather, check out the AOPA Air Safety Foundation's Safety Hot Spot: Summer Weather. If your flight path takes you over areas of high terrain, enhance your knowledge with the foundation's Mountain Flying online course, which introduces pilots to the challenges of mountain flying and offers ways to minimize the risks.

Remember that density altitude is pressure altitude corrected for nonstandard temperature: In a sense, it's the altitude at which the airplane "feels" it's flying. Refresh your memory by taking the foundation's Safety Quiz " Aircraft Performance," featuring questions about high density altitude and how it can reduce lift, the efficiency of the propeller, and the power output of the engine.

On a hot and humid day, the aircraft will accelerate more slowly down the runway, will need to move faster to attain the same lift, and will climb more slowly - all of which can cause an accident if the poor performance has not been anticipated.

"Accident reports in the AOPA Air Safety Foundation database indicate that the biggest density altitude-related problems occur when flatlander pilots operate at a higher elevation airport but fill their tanks, load their airplanes in excess of maximum gross weight, leave the mixture full rich for takeoff (as they were taught at lower elevation airports), and operate in what they think are pleasant 70- or 80-degree F mountain temperatures. But those can be killer temperatures," wrote Alton K. Marsh in " Density Altitude: It isn't just for mountains," in the July 2007 AOPA Pilot.

Read details from the foundation's accident database to learn about one pilot's experience while trying to maintain clearance above a forest while at a density altitude of over 10,000 feet.

Bottom line? When in doubt, be flexible on weight, fuel, and departure time.

(July 18, 2007)