Slide 1 [3 graphs] illustrates near linearity and non linearity in a particular example of long range cruise under ISA conditions.
http://i28.photobucket.com/albums/c2...alyth/FUEL.jpg Slide 2 collates these into a fuel burnout plot.
http://i28.photobucket.com/albums/c2...h/FUELFLOW.jpg The tsfc graph should be read right to left. At FL310 tsfc is high but the characteristic is flat and has actually dipped to minimum and started to rise before step up. At FL350 the pilot sees only the rising portion of the tsfc curve in this case which has a much steeper gradient. Then a further step to FL390 offers a chance of further improvement including a slight dip before tsfc begins to climb relentlessly again.