| چکیده انگلیسی مقاله |
Air transportation has an important position among the other modes of transportation due to its significant impact in the economy and welfare of a society. Within the several components of air transportation market, flight network plays a fundamental role and considerably affects the airlines revenue. Improvement of the network system requires an accurate plan and programming. Hub-and-spokes are of further interest; as such networks reduce the operational costs, create proper ground for flight network development and extension, and help in competition. However, several models have been introduced for hub-and-spokes design purposes based on the diversity of the effective factors, decision-making variables and different forms of the network. Generally speaking, hub-and-spokes are categorized into two principal sectors: single allocation and dual or multiple allocations. Within a single allocation, traffic is accumulated in a single hub and then distributed to the destinations, while within a dual-allocation network, the gathered traffic at the first hub is again distributed to another hub before directing it to the final destination. This research presents a linear model for hub-and-spokes evaluation and planning, determining optimum flight routes and fleet assignment. The model considers both direct flights and hub connections, and outputs an optimum network based on the mixture of these two options. Sets of airport connections are so designed to well cover all the necessary inter-airport trips. This particular is done by utilizing hub-and-spoke system as the airport networks. To fulfill the requirements of the study location (Iran), in this paper, single allocation was selected to develop the models, meaning that just one hub has been considered in the modeling process. Inter-airport demands of the passengers were inputted in the network and the model works only for passenger transportation. The objective was to design the hubs so as to obtain an optimum network. In other words, the model is to suggest the best option with which the demand is handled cost-effectively. Trips are planned to be either direct or meeting a one-hub maximum. As the model is to minimize the cost, such variables as demand for variety of routes and type and quantity of the available aircrafts were included. The model was developed in two stages to ease the process. The first stage dealt with the target function and the fleet was assigned to the Abstracts 136 outputs of the stage one. Iran's internal flight network was chosen as the case of study to develop the model based on the country's geographical situation and available data. 59 airports were chosen as the total set of airports and trip origins. The number of hub candidates and destination airports were eight including Isfahan, Ahwaz, Bandar Abbas, Tabriz, Tehran, Shiraz, Kerman and Mashhad airports. Based on the availability, the data of five types of aircrafts were used in the model development. Lingo Version 8 has run the model using branchand- bound method to obtain accurate and reliable outputs. Up to eight hub networks were considered by the model and the model confirmed that with increase in the number of hubs, operational cost decreased. However, cost reduction had lower rate for the systems with six hubs and over. The results also suggested that the probability of stop in a hub rises for longer trips. Flights longer than 1.5 hours had to stop at one hub in three-, four-, five-, six- and eighthub networks. Four-hub network was found to be the optimum one due to having the shortest stop slot where fixed cost has been allocated for selecting an airport as the hub. The hubs of the optimum fourhub network are Tehran, Shiraz, Kerman and Isfahan airports. The results showed that demand is not the only effective factor in the selection of the hub; it means that another key factor, geographical positions, has effect and the airport with higher demand is not necessarily selected as the hub. Therefore, hub-and-spokes can enhance the efficiency of the airports with lower demands. As an example out of the results, passengers intended to use Isfahan and Kerman airport as their hubs in a four-hub network were more than the ones targeting Isfahan and Kerman airports as destinations. Analysis of the four-hub network cleared that, according to the current demand and operational costs of different aircrafts, large planes (e.g. Airbus 300) and small lpanes (e.g. Foker 50) will perform more flights in comparison the with other types. Using the model developed in this research, airlines will be able to forecast and plan their required fleet combination based on the demands. |