Using fracture mechanics principles, crack growth rates can be predicted accurately for some simple crack configurations subjected to constant amplitude loading., However, for more complex loading sequences such as flight simulation loadings, the results are conservative by a factor 3 to 10 or more. The main objective of this study is to interpret the fatigue behaviour of two aluminum alloys under programmed block load-ings. The effects of cycle ratio R (61 min/6" max) and material thickness on crack propagation rate are analysed. Fatigue crack growth under programmed block loading is presented. Linear damage accumulation is established for some simple flight simulation tests. Aspects covered include microscopic. and fractographic observations. The incidence of crack closuro is examined and the agreement between predictions and test results is very promising.