Experimental test of the influence of propulsion efficiency on contrail formation

According to a previously established thermodynamic theory, contrails are expected to form at a threshold temperature that increases with the overall efficiency of the aircraft propulsion. As a consequence, aircraft with modern engines, with higher overall efficiency, cause contrails over a larger range of cruise altitudes. To validate this theory, an experiment was performed in which contrail formation was observed behind two different four-engine jet aircraft with different engines flying wing by wing. Photographs document the existence of an altitude range in which the aircraft with high engine efficiency causes contrails whereas the other aircraft with lower engine efficiency causes none. For overall efficiencies of 0.23 and 0.31 and an ambient temperature lapse rate of 12 K km(-1), the observed altitude difference is 80 m. This value would be larger (200 m) in a standard atmosphere with smaller temperature lapse rate (6.5 K km(-1)). In a standard atmosphere, an increase of overall efficiency from 0.3 to 0.5, which may be reached for future aircraft, would cause contrails at about 700 m lower altitude.