Vertical profile flight trajectory of heavy military airlift aircraft, which is often time-critical was optimized. Flight time was chosen as the cost function and divided into climb, cruise and descent parts. Flight point-mass motion equation was transferred into equivalent-energy form using energy-state method. Climb and descent segment flight trajectory optimization was carried out by applying Pontryagin's minimum principle after optimizing cruise altitude and speed. Numerical result was obtained that trajectory of such aircraft was made up of three parts: a climb segment along a maximum feasible speed path, a maximum level speed cruise segment at lower altitude and a descent and approach segment along a maximum feasible speed path. This result will be significant for improving airlift efficiency and safety of military transport aircraft which is operated in hazardous frontline environment.