Current European Air Traffic Control (ATC) system is far exceeded by the demand and the resulting delays are a financial and psychological burden for airlines and passengers. The Central Flow Management Unit (CFMU), which is in charge of regulating the flights to respect operational en-route capacity constraints of Air Traffic Control Centres (ATCC), fails to allocate efficiently departure slots and merely satisfy a weak and unrealistic modelisation of the fuzzy-stated workload constraints. Disregarding the minimization of the sum of the delays, this paper presents new models, based on the Constraint Programming paradigm, of the slot allocation problem focused on the controllers workload: an extension of the current model with a standard formulation, and a novel approach involving the sort constraint, both able to maintain workload constantly below a given capacity and the latter also providing efficient failure proof on over-constrained instances. The behaviours of the different models are discussed with partial and full instances from real French air traffic data set and we show the potential operational improvement supplied by these continuous models.