Trend in overall efficiency of propulsion h (solid circles), computed from
aircraft specific fuel consumption (SFC) data (open circles; data as in Chapter 7, adapted
from Aylesworth, 1997), according to h = V(Q SFC)-1, with V as the aircraft speed (about
240 m s-1) and Q as the specific heat of combustion aviation fuels (43 MJ kg-1). The solid
circles also denote the critical altitude z (right axis) above which contrails form (for 100%
relative humidity in the mid-latitude standard atmosphere) for the years 1960 to 2010.