A long$-$range commercial jet is performing a cruise flight at $20000ft$ on a day with ISA conditions. If the weight of the aircraft, $W=2,500,000N,$ wing area, $S=450m^2,$ wing drag polar equation, $CD=0\mathrm{.}017+0\mathrm{.}042C{L}^2,$ specific fuel consumption, $c=2\mathrm{.}5{10}^{-4}\frac{N}{s}\frac{?}{N}$ and fuel weight, $z=0\mathrm{.}15$

What is the maximum endurance at constant altitude and constant lift coefficient?