Flight Training - July 2011 - 39

than floods, lightning, tornadoes, and hurricanes combined. At 2:15 p.m. in Newton, Iowa, the apparent temperature was 133 degrees, in the NWS’ “extreme danger” category. The NWS website has more information about the dangers of heat and a chart you can use to find apparent temperatures (www. weather.gov/om/heat/index.shtml).

WHY HUMID AIR IS LIGHTER THAN DRY AIR
Molecules of water vapor in the air replace some of the molecules of nitrogen and oxygen that account for approximately 98 percent of the air. The water molecules are lighter than the molecules they displace. You can learn about this online (www.weatherjackwilliams.com/?p=654).

You often hear density described in pounds per cubic foot, which works well enough for ordinary uses. But pounds are a HEAT AND YOUR AIRPLANE measure of force, not mass. Scientists and As air becomes hotter and more humid, it engineers working in American units use becomes less dense, because air—like most the slug for mass. Near the Earth’s surface, other things—expands as it heats up. a slug is approximately 32.2 pounds. Less dense air degrades aircraft perforThe table shows us that on a “normal” mance, because lift depends on the mass day, the air pressure at 1,000 feet is 28.86 of the gases in the air that flow over and inches of mercury, the temperature 55.4 under the wings each second. This, in turn, degrees F, and the density is 0.002309 depends both on the number of molecules slugs per cubic foot. But, if on a parof the various gases in each cubic foot of air ticular day a higher temperature at an as well as the mass of the molecules, and airport with an elevation of 1,000 feet has how fast the air is flowing over the wing. made the density only 0.002112 slugs per An airplane’s engine produces power cubic foot, we would say that the density by burning the proper mixture of fuel UNDERSTANDING DENSITY altitude is 4,000 feet, no matter what the and oxygen from the air. As air density ALTITUDE true altitude, atmospheric pressure, and decreases, each cubic foot of air that the Density altitude is defined as the “prestemperature at the time. engine sucks in has less oxygen (along sure altitude corrected for nonstandard Today, of course, no one looks up values with less of air’s other gases); thus, temperature variations.” What does that for density altitude in a table; he or she power decreases as air density decreases. mean? The best way to answer is to fly uses a computer to calculate them. An The amount of thrust depends on a little off course and examine the staneasy-to-use online computer is on the the mass of air that a propeller or jet dard atmosphere. website of the El Paso, Texas, NWS office engine can push toward the aircraft’s This term refers to tables with the (www.srh.noaa.gov/epz/?n=wxcalc). rear. Decreasing air density reduces values of air pressure, temperature, and To find density altitude, scroll down thrust. Drag also decreases as air density air density for each altitude listed. The to the Pressure Conversions box. First, decreases, which helps a little, but the table below shows standard atmosphere decreases in power, thrust, and lift more figures for sea level up to 4,000 feet above select Station Pressure—you’ll need this to calculate the density altitude. Station than offset the drag reduction. sea level using the units commonly used pressure is the actual barometric readin the United States, including density in ing at a location. It’s used to calculate the CALCULATING PERFORMANCE slugs per cubic foot. altimeter setting, which you use While the term “density along with the field elevation to altitude” is used to describe STANDARD ATMOSPHERE calculate station pressure at the the effects of temperature bottom of the box. and air density on aircraft ALTITUDE PRESSURE TEMPERATURE DENSITY (FEET) (IN. HG) (DEGREES (SLUGS PER performance, many airplane FAHRENHEIT) CUBIC FOOT) BACK TO NEWTON, IOWA operating handbooks include Using the El Paso weather performance charts that pilots 0 29.92 59.0 0.002378 calculator and data from the use to find figures such as take1,000 28.86 55.4 0.002309 METAR (aviation weather off distance and rate of climb 2,000 27.82 51.9 0.002242 observations), including the without ever using a figure for 3,000 26.82 48.3 0.002176 temperature and dew point the density altitude. observations for Newton at 2:15 For example, the takeoff 4,000 25.84 44.7 0.002112 p.m. on July 24, 2010, we found distance chart in the Cessna 5,000 24.89 41.2 0.002049 the density altitude. 172M pilot’s operating handbook uses the airplane’s takeoff weight, temperature, and pressure altitude to calculate takeoff distance. The pressure altitude is the figure your altimeter reads when it’s set at 29.92 inches of mercury. The chart shows that if you were at sea level with the standard atmospheric pressure in a Cessna 172M weighing 2,300 pounds, you would need 775 feet of runway to lift off if the temperature were 32 degrees Fahrenheit. If the temperature were 104 degrees at the same air pressure, you’d need 1,030 feet of runway before the wheels leave the pavement.

JULY 2011 FLIGHT TRAINING

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http://www.weatherjackwilliams.com/?p=654 http://www.srh.noaa.gov/epz/?n=wxcalc

Flight Training - July 2011

Table of Contents for the Digital Edition of Flight Training - July 2011

Flight Training - July 2011
Contents
President’s Perspective
Right Seat
Letters
The Wind in Your Hair
This Weekend
Success Story What It Looks Like
ASI News Member Products
Tech Tip
Since You Asked
AOPA Action
Final Exam
AOPA Action
News
Insights
Flying Carpet
Emergency Practice
I Love Low and Slow
Technique
Weather
Checkride
Flight Lesson
Rotorworld
Career Advisor
Tech Talk
Get Inside the Checklist
Nothing to See Here
Ultimate Communicators
Advertiser Index
Debrief
Flight Training - July 2011 - Flight Training - July 2011
Flight Training - July 2011 - Cover2
Flight Training - July 2011 - Contents
Flight Training - July 2011 - 2
Flight Training - July 2011 - 3
Flight Training - July 2011 - President’s Perspective
Flight Training - July 2011 - 5
Flight Training - July 2011 - Right Seat
Flight Training - July 2011 - 7
Flight Training - July 2011 - Letters
Flight Training - July 2011 - 9
Flight Training - July 2011 - The Wind in Your Hair
Flight Training - July 2011 - 11
Flight Training - July 2011 - This Weekend
Flight Training - July 2011 - Success Story What It Looks Like
Flight Training - July 2011 - ASI News Member Products
Flight Training - July 2011 - Tech Tip
Flight Training - July 2011 - Since You Asked
Flight Training - July 2011 - 17
Flight Training - July 2011 - AOPA Action
Flight Training - July 2011 - Final Exam
Flight Training - July 2011 - AOPA Action
Flight Training - July 2011 - 21
Flight Training - July 2011 - News
Flight Training - July 2011 - Insights
Flight Training - July 2011 - Flying Carpet
Flight Training - July 2011 - 25
Flight Training - July 2011 - Emergency Practice
Flight Training - July 2011 - 27
Flight Training - July 2011 - 28
Flight Training - July 2011 - 29
Flight Training - July 2011 - 30
Flight Training - July 2011 - 31
Flight Training - July 2011 - I Love Low and Slow
Flight Training - July 2011 - 33
Flight Training - July 2011 - 34
Flight Training - July 2011 - 35
Flight Training - July 2011 - Technique
Flight Training - July 2011 - 37
Flight Training - July 2011 - Weather
Flight Training - July 2011 - 39
Flight Training - July 2011 - 40
Flight Training - July 2011 - Checkride
Flight Training - July 2011 - 42
Flight Training - July 2011 - 43
Flight Training - July 2011 - Flight Lesson
Flight Training - July 2011 - Rotorworld
Flight Training - July 2011 - Career Advisor
Flight Training - July 2011 - 47
Flight Training - July 2011 - Tech Talk
Flight Training - July 2011 - Get Inside the Checklist
Flight Training - July 2011 - Nothing to See Here
Flight Training - July 2011 - Ultimate Communicators
Flight Training - July 2011 - Advertiser Index
Flight Training - July 2011 - 53
Flight Training - July 2011 - 54
Flight Training - July 2011 - 55
Flight Training - July 2011 - Debrief
Flight Training - July 2011 - Cover3
Flight Training - July 2011 - Cover4
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