Influence of Propulsion Efficiency on Contrail Formation

Abstract

Aircraft cause contrails when flying in an atmosphere colder than a threshold temperature which depends on the overall efficiency η of propulsion of the aircraft/engine combination. Higher η causes contrails at higher ambient temperatures and over a larger range of flight altitudes. The ratio of temperature increase relative to moisture increase in engine plumes is lower for engines with higher η. Thermodynamic arguments are given for this fact and measurements and observations are reported which support the validity of the given criterion. The measurements include contrail observations for identified aircraft flying at ambient temperature and humidity conditions measured with high precision in-situ instruments, measurements of the temperature and humidity increases in an aircraft exhaust plume, and an observation of contrail formation behind two different four-engine jet aircraft with different engines flying wing by wing. The observations show that an altitude range exists in which the aircraft with high efficiency causes contrails while the other aircraft with lower efficiency causes none. Aircraft with more efficient propulsion cause contrails more frequently. The climatic impact depends on the relative importance of increased contrail frequency and reduced carbon dioxide emissions for increased efficiency, and on other parameters, and has not yet been quantified.