Tip leakage flow has a significant impact on fan blade performance. It is therefore critical to understand the effect of both the uniform and non-uniform tip clearance variations that occur in engines. Firstly, a detailed understanding of the flow physics present in the tip region of a modern axial flow fan blade is developed using RANS CFD. Analysis is under-taken of the tip leakage flow structure, tip vortex characteristics and tip leakage massflow distribution. It is shown how flow acceleration upstream of the pressure surface passage shock causes an unusual chordwise tip leakage distribution with'reverse'tip leakage near the leading edge. The importance of the leakage distribution on the tip flow behaviour is explained. The sensitivities of blade efficiency to both uniform and non-uniform tip clear-ances are studied, and it is shown how increasing tip clearances towards the leading edge has the greatest impact on efficiency.