The recirculating flow in the vaned passage in the case surrounding the tips of the fan blades of a turbo fan engine or ducted fan propulsor is modified or blocked off during certain engine or aircraft operating modes. In one embodiment an aneroid valve closes off the vaned passage upon attaining a given altitude and fails open to assure that the safe stall margin is retained during take off. The active VPCT serves to increase fan efficiency and engine thrust while maintaining stall margin at the required safety level. The vaned passage in another embodiment is utilized to abate or preclude the stall precursor waves manifested by the rotor blade by modulating the recirculating flow as a function of time.