May 6, 2011
It’s a cool spring morning when you take off on your cross-country flight. The weather forecast is good, but the day is expected to become “a scorcher.”
Your smooth ride will become bumpy as the sun heats the ground and creates thermal turbulence. What conditions should you expect in a few hours at your destination, an airport at 2,000 feet msl with a 3,000-foot-long runway?
You studied the nearest terminal forecast, but it won’t be until you hear the automated surface observing system (ASOS) report that you will learn details including the destination’s surface temperature. Fortunately, ASOS and many preceding-generation automated weather observation system (AWOS) units provide another valuable piece of information for hot weather operations: the field’s current density altitude.
If the surface temperature is 90 degrees Fahrenheit—well above standard temperature of about 52 degrees Fahrenheit for 2,000 feet msl—density altitude will be about 4,550 feet. When you depart for home, on takeoff and climbout your low-powered trainer will perform as if departing an airport at 4,550 feet msl under standard conditions. Plan accordingly.
“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,” explains this AOPA subject report. “All of these factors can lead to an accident if the poor performance has not been anticipated.”
Question: If humidity is so important, why isn’t it part of density-altitude calculations? Most consider only nonstandard temperature and pressure.
“Humidity should not be ignored but almost always is because aircraft performance charts do not include it as a factor, but should,” wrote AOPA Pilot columnist Barry Schiff in his July 2007 Proficient Pilot column. “Although even the most humid air is not that much lighter than dry air, it causes reciprocating engines to lose considerable power.”
He offered an alternative way to estimate humidity’s effect: “Because the precise effect of humidity involves complex calculations and is difficult to determine, pilots can compensate by raising the calculated DA by 1,000 feet on hot, humid days and decreasing calculated performance by a conservative fudge factor of 10 percent.”
Remember, no airport is “too low” for a density altitude check and aircraft performance calculations in hot weather!
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Note: Products listed have not been evaluated by ePilot editors unless otherwise noted. AOPA assumes no responsibility for products or services listed or for claims or actions by manufacturers or vendors.
Question: When flying in the traffic pattern, my flight instructor recommends that I fly 1,000 feet agl, but I've heard other instructors teaching their students to fly 800 feet agl. I'm a bit confused on this issue.
Answer: Traffic pattern altitudes for propeller-driven aircraft can range from 600 through 1,500 feet agl, according to Chapter 4-3-3 of the Aeronautical Information Manual (AIM). The AIM also recommends using a 1,000-foot-agl pattern altitude unless a different altitude has been established for the airport. If a traffic pattern altitude (TPA) is not listed for a particular airport, the 1,000-foot recommendation would apply. Sometimes airport management decides to set or change a TPA (within the parameters of 600 to 1,500 feet agl), and the new TPA is not listed in the airport/facility directory. AOPA Airports updates this information through airport surveys and questionnaires. If in doubt, though, AOPA recommends you call the airport. For more information, download the Air Safety Institute’s Safety Advisor, Operations at Nontowered Airports .
Got a question for our technical services staff? E-mail email@example.com or call the Pilot Information Center, 800/872-2672. Don’t forget the online archive of “Final Exam” questions and answers, searchable by keyword or topic.
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