The saturation temperature Tsat for the formation of polar stratospheric clouds (PSC) strongly depends on the local partial pressures of nitric acid and water vapor, and thus is sensitive to NOx and H2O-injection due to exhaust from aircraft in the stratosphere. The present paper investigates this effect, using daily stratospheric temperature data from the northern hemisphere compiled over the last 25 years by the Free University of Berlin. For Type-I PSC the data were examined for temperatures T<Tsat-3 K, which would allow for a supercooling of about the 3 K measured in a previous arctic winter balloon mission, in effect corresponding to a large HNO3-supersaturation. We show that this is required to overcome the heterogeneous nucleation energy barrier. We compare the analyses of both a background atmosphere and one perturbed by a fleet of 600 stratospheric aircraft flying at ~22 km altitude. The result is that between December and March in the polar cap region there might be more than a doubling in the occurrence of Type-I PSCs and an even stronger increase of Type-II PSCs, and accordingly a substantial enhancement in the potential ozone destruction by chlorine radicals.