The formula provides an accurate answer as it is directly traceable to the 1976ISA, and is not an approximation. The formula below is from the 1976 International Standard Atmosphere, and reduced by a friend (physicist) who has developed a set of formulas for navigation. Therefore, the resultant figure may be close, but is not the true altitude for the aircraft. In this instance, we don't have that, rather we have a Regional Pressure Setting. So for survey work, or precision remote sensing, there would need to be an observation at the point below an aircraft, or at the very least an interpolation (modeled) using known values. Such QNH data is accurately called a Regional Pressure Setting, and is the lowest forecast pressure for a defined region within an hour. A QNH is for a station, and in ATC that location is arbitrary, and in places such as the UK is one of 20 or so regions. Remember, this is all an exercise in estimation which is helpful in reinforcing the relationships between temperature, pressure, and altitude.įirst, True Altitude is applicable to a column of air above a point on the sphere. In such cases, substitute pressure altitude for indicated altitude in the above formula. In some cases, a question might ask for true altitude based on a pressure altitude, instead of an indicated altitude. $A_T$ is our estimated true altitude, in feet Given that the temperature is ISA+8☌, and knowing 6 above, we can calculate our true altitude relative to pressure altitude: Follow that step carefully, because the signs can be confusing. Given an ISA of -23☌, we see that our OAT of -15☌ shows a difference of +8☌ above ISA. Since ISA at the surface is 15☌, using the standard lapse rate of -2☌ per thousand feet, we know that ISA at 19,000 ft will be -23☌. Let's assume a flight at FL190, with an OAT of -15☌, on a standard pressure day. This all might sound complicated, so lets run through an example problem. Knowing all this, we can calculate the following: To find true altitude, the difference from indicated altitude is 4 ft per 1☌ deviation from ISA for every 1,000 ft.Standard lapse rate is -2☌ per thousand feet.Now, to solve for true altitude, we need to know a few additional items. FL170 represents a pressure altitude of 17,000 ft.See this page for other examples of this type of question. Note that all these calculations are an estimate based on standard ISA temperature and altitude relationships. This can be calculated by knowing a few formulas, or temperature relationships related to altitude.
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