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How to Calculate a QFE

During flight, pilots need to keep track of many different measurements from the cockpit. Some important measurements include the plane's speed, outside temperature and wind direction. Pilots also keep track of pressure at their plane's height by keeping tabs on QNH (Question Nil Height), the sea level pressure of a standard atmosphere, and QNE (Question Nil Elevation), the barometric pressure as reported from a specific station. They also ensure their QFE (Question Field Elevation), is set properly. QFE can be determined most accurately using special pressure instruments. It can be approximated, however, using the hypsometric equation.

Instructions

- 1
  Determine the altitude of the airplane. This measurement must be in meters. If your instruments record in feet, you must convert feet to meters using this equation: M = F * 0.3048. In this equation, "M" is the altitude in meters and "F" is the altitude in feet.
- 2
  Determine the pressure of the sea level, or QFF, in millibars. Without equipment, finding the QFF is extremely difficult because the atmosphere is constantly changing. If you do not have equipment, you may have to find sea level pressure using a weather report. If you do not have access to a weather report, use the QNH instead of QFF. The QNH is the sea level pressure of a standard atmosphere. QNH is equivalent to exactly 1000 millibars. 1000 millibars is exactly equal to one atmosphere, 760 millimeters of mercury, 29.92 inches of mercury, 14.7 pounds per square inch and 101.3 kilopascals.
- 3
  Find the average temperature of the atmosphere. This can be done by using the following equation: AT = (Tb -- Ta) / 2. In this equation, "AT" is the average temperature, "Tb" is the temperature at the airplanes altitude and "Ta" is the temperature at sea level. Using the equation only yields an approximation. Weather reports may have more accurate atmospheric average temperatures. Temperature in this equation, like most atmospheric science equations, must be in Kelvin. Celsius can be converted to Kelvin by using this equation: Tk = Tc + 273.15. Here, "Tk" is the temperature in Kelvin, while "Tc" is the temperature in Celsius. Fahrenheit can be converted to Celsius using this equation: Tc = (Tf -- 32) * (5 / 9). Here, "Tf" is the temperature in Fahrenheit.
- 4
  Calculate the value for QFE using a modified version of the hypsometric equation. The hypsometric equation is as follows: Za -- Zb = (R / g) * AT * ln(pb / pa). In this equation, you can find the QFE by isolating "pa." This is the altered hypsometric equation with "pa" isolated: pa = pb * exp((R * AT) / ((za -- zb) * g)). In this modified equation, "pa" is the QFE, "pb" is either the QFF or QNH depending on which you decided to use and "R" is gas constant for dry air which is equivalent to 287 J/(kg*K) . Furthermore, "za" is the altitude of the QFE in meters, "zb" is the altitude of the QFF or QNH and "g" is a constant for gravity. Gravity in this instance is equivalent to 9.81 meters per second squared.

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