A method and a system for assisting air traffic controllers that automatically detects conflicts between aircraft trajectories and selects the conflicts that can be solved by minor modification(s) of aircraft speed, climbing rates or descending rates and lateral shifts of route. Minor modifications are selected so as to not interfere with current controllers' decision making process thereby circumventing the basic rule of uniqueness of control in a given piece of airspace. The minor modifications are automatically transmitted to aircraft for execution without requiring controllers' prior agreement. Thus, the method solves most conflicts, such that the air traffic delivered to the controllers is free of most of the pre-existing conflicts.