This paper continues researches of optimal control in active systems started before. In application to an aircraft maximal duration horizontal flight control problem solution, it is proposed to use the individual's subjective preferences entropy extremization principle as a helpful tool. Basing oneself upon that principle alone, one can reveal the optimal mode of operational control even without knowing the extremal one. Since the subjective entropy extremization principle allows; independently on the conditions of transversality, Weierstrass-Erdmann, principle of maximum by L.S. Pontryagin (USSR), as well as principle of optimality by R. Bellman (USA); finding the extremals, their optimal conjunctions of all kinds: either breaks with shifts, or both at smooth and corner points, for closed and restricted areas; stipulated by obeying the only a priory condition of the Euler-Lagrange equations; it is suggested to call this principle by the name of its author, professor Vladimir Aleksandrovich Kasianov, National aviation university (Kyiv, Ukraine). The necessary calculations are fulfilled. Plotted corresponding diagrams.
