This is the original problem:

A turboprop aircraft with a weight of 162 kN is equipped with a power plant with a power derating model described by $\frac{P_A}{P_{A,0}} = 0.9\sigma^{0.75}$, where $P_{A,0}$=3.5 MW is the sea level available power. The aircraft has the following aerodynamic parameters: $C_{D,0}$=0.018, wing area S=52 m2, wingspan b=24 m and efficiency factor e = 0.82. Determine the maximum true airspeed at an altitude of 6 km.

I evaluated the available power to be 1.981 MW in an earlier part of the question. By solving $\sqrt{\frac{2W^3(C_{D,0}+KC_L^2)^2}{\rho{S}C_L^3}}=1.981\times10^6$ for $C_L$, I found that $C_L$ could either be .3057 or 12.82. I chose to use $C_L = .3057$ in solving $\frac{2W}{\rho{S}V^2} = C_L$ for $V$ to find the maximum TAS.

Aside from being ridiculously large, would there be a better explanation for why I should disregard $C_L = 12.82$?

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