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I am confused a lot about the efficiency and thrust relation with altitude and velocity of an aircraft. If thrust is related to air mass flow and velocity diffrence of bypassed air - exhaust gases and intake air velocity, then thrust will decrease as altitude increases which is true as CFM56 stationary thrust is 140kN while at cruising altitude it is 24kN. Everyone points out that the efficiency is higher at higher altitudes, but how? Yes fuel flow decreases too to maintain O/F ratio but what about the reduction of thrust? Does propulsive efficiency goes higher too? And how the aircraft can still maintain its speed with 1/5 of the thrust at TO, someone also pointed out drag actually increases with higher altitude and speed, is it true?