From CAS to EAS – Calculating and Plotting the Compressibility Correction Chart

Purpose – The conversion between calibrated airspeed (CAS) and equivalent airspeed (EAS) is relatively cumbersome, because it involves the calculation of incompressible flow, for which the equations are quite long. If calculations on the computer are required, conversions with equations are necessary. In contrast, this project calculates a CAS to EAS Compressibility Correction Chart, which allows to convert CAS to EAS very quickly by reading the correction from a graph. --- Methodology – In Excel, compressibility correction is achieved through flight mechanics formulas. The correction is calculated with two distinct functions, one based on Mach Number and the other on pressure altitude. These functions are graphed individually and then integrated to produce the Compressibility Correction Chart. --- Findings – The Compressibility Correction Chart was successfully recreated as a 2-D graph. Upon comparison with other correction charts, the EAS-CAS-results demonstrate a mere 0% deviation, proving the accuracy of the findings and validating their near-perfect alignment. --- Research Limitations – Due to a limitation in Excel, which allows for 255 series for plotting, the range of input parameters had to be adjusted accordingly. The iterations of altitude span 1000 ft intervals, while those for Mach Number span 0.05 intervals. --- Practical Implications – Pilots can easily use the Compressibility Correction Chart for quick and highly accurate calculations when needed. --- Originality – CAS-EAS Compressibility Correction Charts are available in other sources. This paper represents a recreation of the 2-D Correction Chart by the combination of plots: one as function of Mach Number and the other of pressure altitude, using the Excel Software.