In this research we propose an integrated model to optimize both aircra ft's fight assignment asnd "A" check schedules at the same time. We formulate the problem into a two-tier multicommodity flow problem based on a time-space network. The first tier models the movement of aircraft in the time-space network, and the second models the flow of available flight hours between "A" checks. The two tiers are bundled gy a set of side constraints. The objective is to minimize the sum of the aircraft's operating cost and the opprotunity cost of the unused flight hours. We tested our model with the data from a small fleet, and solved the mixed interger prograpmming problem with the CPLEX package. Preliminary results show that the model is able to generate promising reults. We also observe that overlapping a portion of consecutive planning periods can eliminate some potential problems that might occur otherwise, at the cost of a slight decrease in the objective function value.