In this communication, the problem of long-term forecasting of the traffic growth in a large air transportation network is considered. This problem is crucial when planning the necessary investments in airports, fleets and air traffic control equipments. The resulting optimization problem is a multilevel one. The proposed approach makes use of two different optimization models: One model is devoted to demand forecasting, while the other defines the air transport supply according to a profit maximization behaviour of the airlines sector. The proposed solution scheme is composed of an iterative process between the current solutions of the demand and the supply optimization problems. Convergence conditions are discussed for this iterative process between these two problems which can be seen as inverse of each other. The proposed optimization approach is briefly illustrated in the case of the long-term forecasting of air traffic in the West Africa region.