This paper formulates the problem of aircraft conflict avoidance as a multiphase mixed-integer optimal control problem. In order to find optimal maneuvers, accurate models of aircraft nonlinear dynamics and flight envelop constraints are used. Wind forecast and obstacles in airspace due to hazardous weather are included. The objective is to design aircraft maneuvers that ensure safety while minimizing fuel consumption. The solution approach is based on conversion of the multiphase mixed-integer optimal control problem into a mixed-integer nonlinear programming problem. Two case studies for the Airbus 320 aircraft illustrate the approach. © 2012 IEEE.