To evaluate the impact of air traffic on the upper and lower troposphere, one must find an effective method to measure the actual gas emissions of aircraft engines at defined thrust levels and at all altitudes. FTIR-emission-spectroscopy detects the thermal radiation of hot exhaust gases, yielding all information about its compounds during one measurement. This remote technique can be used under ground- and flight-conditions. The theoretical line-by-line retrieval of the measured spectra simulates the radiative transfer through several plume- and foreground-layers and is based on the HITRAN 92 database. After the spectroscopic determination of the plume temperature and its profile from the CO2-band around 2400 cm-1, one obtains the toal mass of the single gas species in the field of view of the spectrometer. Comparing the measured data with the theoretical emission index of CO2 from ideal stoichiometric combustion, one obtains the emission indices for the other measured species. Knowing the fuel consumption of the engine, one may get the emission rates of the compounds in g/a. Several engine types, old fashioned engines (no bypass) and modern JT8 and CFM56 bypass at different thrust levels have been analyzed. H2O, CO2 CO, and NO concentrations can be derived immediately from the measurements right behind the nozzle exits, where the temperature profile is known to be homogeneous. The retrieval of the measured data far behind the nozzle exit uses a computer time consuming multilayer model. Formaldehyde and other hydrocarbon species are seen in some spectra and shall be implemented in the computer code. Apart form future applications for the turbine development and the engine-status control after a certain flight time, this remote sensing system can deliver emission data of aircraft engines and the temperature decay of the exhaust plumes at all altitudes.