The direct deposition of emissions from the 1992 aircraft fleet is studied using different aircraft emissions inventories, meteorological data from 1983--1996, and definitions of the tropopause. Globally, between 18 and 44% of current emissions of water vapor and nitrogen oxides are deposited in the stratosphere, depending on the exact definition of the tropopause and on season, in agreement with previous work. Largest emissions in the stratosphere are found to occur in January. The sensitivity of this result is tested with respect to the definition of the tropopause, the data used to analyze the tropopause, and the interseasonal and interannual variability of tropopause height. On a global basis, the stratospheric deposition is most sensitive to the definition of the tropopause. Approximately 20% of emissions are within ¿1 km of the tropopause, and 35% are within ¿2 km of the tropopause. Deposition into the stratosphere is not significantly affected by the choice of meteorological data set or by the interannual variability of the tropopause. These results have important implications for chemical model studies of aircraft emissions impact because they highlight the sensitivity to the exact flight altitudes, choice of tropopause height, and resolution in the tropopause region. ¿ 1999 American Geophysical Union