Comparison of the measurement of the weft insertion tie with yarn tensions determined at a test rig show that the forces transmitted in the mixing tube of a main jet have a decisive effect on the weft insertion behaviour of a yarn on air-jet looms, tests having been carried out on viscose rayon, cotton., cotton/polyester, polyester, plied wool and filament yarns. Measurements on an air-jet loom and simulation results show that the weft insertion behaviour and the loads on weft yarns in air-jet weaving can be predicted. An attempt is made to develop a measuring method into an industrial testing instrument. The test results make it possible to determine the suitability of yarns as weft on air-jet looms without weaving tests. Simulation of the weft insertion using the measuring results permits accurate weft insertion behaviour. Results of the simulation calculation can be used for improving and speeding up the setting of air-jet looms. Weft insertion can also be automatically optimized using microprocessor control of the loom as a function of the weft parameters and regression equations. These results are important for the German and the European textile industry, as a rapid response to market requirements and the saving of labour and energy costs to ensure competitivity is indispensable.