For your selected airfield, include the following: ICAO identifier (the ICAO identifier is the 4-letter airport code, starting with a ‘K’ for the continental US) Field elevation [ft MSL] (field elevation can be found in the location information section at skyvector.com or the overview section at AOPA’s flight planning tool) Current weather report at the time of work on this assignment to include: Date and time Current altimeter setting [in Hg]: Current temperature [°F or °C, but stay consistent]: Note: When operating at an airfield within the US, the altimeter is adjusted according to current conditions (i.e. the reported altimeter setting that you found) in order to always indicate the correct field elevation when on the ground. Therefore, your indicated altitude will remain equal to your field elevation when being on the surface of that airfield, but your pressure altitude will be subject to change depending on changes in the altimeter setting. Using your researched data, find the Pressure Altitude of your airfield [ft]. Use the found altimeter setting and the rule of thumb lapse rate of 1 in Hg = 1000 ft, i.e. 00.01 in Hg = 10 ft change from the field elevation, with standard atmospheric altimeter setting being 29.92 in Hg (see also tutorial and example problems). Keep in mind that an increase in altimeter setting above standard will lead to a positive shift of Indicated Altitude above Pressure Altitude (or in other words, a lower pressure altitude than what is indicated) and vice versa. Note: In some cases (low field elevation, coupled with high altimeter setting) it may lead to negative pressure altitudes, which is completely correct. However, to allow further work in the atmospheric table excerpt in your textbook (Table 2.1, which does not include the negative values), you may change your altimeter setting in 4.b) to a lower value (please include a note) for all further work or select a different airfield (preferably above 1000’ MSL). Based on your determined pressure altitude, find the Pressure Ratio, d (delta), in the Standard Atmosphere Table (“Flight Theory and Aerodynamics”, Table 2.1). Interpolate as necessary. Using your researched current temperature and the known standard sea-level temperature, determine the Temperature Ratio, ? (theta). (Remember to convert °F or °C into an absolute temperature, i.e. °R or °K, and stay consistent within one system of measurement.) From your #5 and #6 results, find the Density Ratio, s (sigma). With your result, from #7, e-enter the Standard Atmosphere Table (“Flight Theory and Aerodynamics”, Table 2.1) to find the corresponding Density Altitude. Interpolate as necessary. Notice that the above relationships between Indicated, Pressure, Temperature, and Density Altitudes hold for any point in the atmosphere, not just for aircraft on the surface of an airfield. If atmospheric conditions such as altimeter setting and temperature or temperature offset are known, any Indicated Altitude can be converted in a similar way.
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