The subject of this research is a new concept of modular aircraft designed to cope with the seasonal variation in passenger demand by opening the fuselage and increasing its length with extra bits of fuselages. The goal is to find out if this new aircraft concept is more profitable than the current alternatives. Previous work have looked at increasing the size of existing aircraft only once in their lifetime. Or offered opening mechanisms for jet fighters or studied modularity for products or Unmanned Aerial Vehicles (UAV). The idea is to design a new aircraft from scratch able to change its size twice a year in order to improve the offer to the passenger demand. The research was performed in four phases by making two tools. The first phase is making the first tool which allows the design of a family of aircraft according to an input network and passenger demand. This is going to serve as basis of comparison for the modular aircraft. The second phase is making the second tool, based on The Initiator, able to design a modular aircraft. The third phase is performing a structural analysis to compute the mass penalty caused by the connection mechanism. The final phase is studying the profitability of the modular aircraft compared to the optimal family form the first phase. This economical study is performed at two levels: the aircraft level and the airline level. The best concept is starting with the long version. Then the short version uses the same wing and tail but smaller engines and landing gear. When using a safety factor of 8 for the connection mechanism, the mass penalty is relatively small ranging from 1 to 6% of the fuselage mass depending on the aircraft configuration. The principal factor driving the performance of the modular aircraft is not the mass penalty but the non-optimal wing used for the short version. To generate the same profitability as the optimal design, the modular aircraft should reach a load factor of 85.6% instead of 80%. Even in a network, the increased fit between the offer and demand cannot outweigh the design penalty. As a result, the potential for a modular aircraft seems low when compared to the alternatives able to increase the aircraft utilization such as real-time-health monitoring of aircraft to improve the maintenance and price-setting algorithms able to improve both the load factor and the profitability.