Abstract <p>The problem of determining the conditions of formation of aircraft condensation trails (contrails) during the interaction of a turbofan engine’s (turbojet) exhaust jet with the environment is considered. The gas dynamics equations (Reynolds equations) are solved numerically with regard for the nozzle shape of the turbojets under consideration. The calculations take into account the geometric and gas-dynamic parameters, including the bypass ratio, the internal and external flow characteristics, the water vapor emission, and others, that define the key processes for various types of turbojets. The obtained results are necessary for developing a criterion for the formation of stable contrails, based on the degree of water vapor supersaturation in the engine jet. Examples of contrail formation are analyzed, based on full-scale flight test data.</p>