The constant amplitude and flight simulation (TURBISTAN) fatigue crack growth behaviour of Ti-6A1-4V fan disc forgings tested in the AGARD engine disc cooperative test programme was investigated by fractographic and microstructural analysis. The crack growth curve shapes and fractographic characteristics were similar. Transitions in the fatigue crack growth curves correlated with a change from structure-sensitive to continuum-mode crack growth, primarily in the transformed and aged β grains, and decreased fracture surface roughness. The transitions were most probably caused by the maximum plane strain cyclic plastic zone sizes becoming equal to and exceeding the average platelet a packet sizes. The significance of such transitions for prediction of fatigue crack growth and service failure analysis is discussed.