Flying at a high altitude has numerous advantages over flying at lower levels. At higher flight levels there will be a great reduction in Total Drag and therefore a reduction in required thrust. From sea level to 40,000 ft a reduction of approximately 75% is achieved. Also the higher you go the higher your TAS and GS. So by increasing your speed you get extra benefits too-
(1) you get an increase in propulsive efficiency resulting from the higher velocity
(2) a greater "ram" effect at the entrance to the compressor, reducing slightly the work done to drive this component. In summary it can be said that the reason we fly jet a/c at high Flt. Levels is due to SFC being lower due to-(i)lower inlet temp(ii)improved propulsive efficiency(iii)"Ram" effect at higher velocity
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