We propose a novel method for conflict resolution for aircraft flying in a self-separation airspace, with the possibility of some ground assistance. Our method is based on navigation functions, that can guarantee decentralized conflict avoidance for the short term horizon, supported by a model predictive controller on the ground, responsible for the optimization of the aircraft flight plans in the mid term horizon. As a consequence, our method combines the short term safety guarantees provided by navigation functions with the long term optimality and constraint satisfaction guarantees (in terms of airspeed, turning radius etc.) provided by model predictive controllers. The efficiency of the approach is demonstrated on simulations involving a number of aircraft converging in planar configurations.