This paper describes the econometric system approach developed by MKmetric to perform short and long-term air transport demand forecasts while considering various determinants such as socio-economy, policy, infrastructure and land use. The necessities for modelling air transport evoking from a transport system point of view and the changes of the aviation world occurred during the last decade are investigated. Based on these findings the mathematical framework is outlined considering shortfalls of traditional models used in aviation forecasting and restrictions caused by classical functional forms. The increasing gap of information for modelling is described and alternative data sources used for the development of the system approach are listed. As all models are imperfect describing just a part of real life, it sheds a light on the necessity to validate models and the prerequisite of complexity needed to cope with multi-sector scenario simulations for strategic, tactical and operational developments as well as political decisions. Finally some analysis examples demonstrate the power of the approach used focusing on the choice modelling reflecting consumers’ behaviour.