The RTA (Required Time Arrival) capabilities of aircraft FMS (Flight Management Systems) offer new opportunities to solve mid-term horizon conflicts (20 minutes in advance) with small speed adjustments. The ERASMUS project has shown promising results of up to $80\%$ conflict resolution using small speed adjustments in the $[-6\%;+3\%]$ range with 20 minutes advance notice. The hypotheses were based on very accurate trajectory predictions (TPs). In this article we show how the quality of these results decreases as the uncertainties on the trajectory prediction increase. Therefore we used the CATS (Complete Air Traffic Simulator) developed in the late 90s at CENA (Centre d'Etudes de la Navigation A\'erienne) and constrained the solver to use only speed maneuvers for leveled or descending aircraft with different hypotheses on speed ranges and speed uncertainties. Results show that Traffic Control using Speed Adjustments (TCSA) can solve most of the conflicts even when we consider uncertainty on the TP. However, the number of maneuvers that need to be given to aircraft is highly influenced by the uncertainties used in the TP.