Full-scale structural tests enable an in-depth understanding of how composite blades respond to specific applied loads. Blade strength can be validated, and necessary modifications can be made to improve structural performance and/or reduce blade weight. This study revisits the structural collapse of a 52.3m composite blade with new research content. Specifically, the present work examines the chain of events captured in the video record of the blade collapse and provides direct phenomenological evidence of how the blade collapsed in its ultimate limit state. In addition, three-dimensional strains are investigated by reconstructing the root transition region of the blade using solid brick elements in a finite element analysis. The strain components responsible for particular failure characteristics are identified. The structural response of the blade is investigated numerically. Interactive failure phenomena associated with strains, local buckling and material failure are examined in detail. The study shows that local buckling of the sandwich panels with unbalanced construction drives progressive failure of the composite materials and eventually leads to blade collapse owing to significant failure of the load-carrying spar cap. Design modifications of the blade are proposed and validated with the test of a new blade. With respect to the latest DNV GL standard, this study notes a possible method to predict delamination and skin/core debonding failures. This study also recommends the use of three-dimensional solid elements in finite element analysis, especially when the strength and failure of large blades are of concern. Copyright (c) 2017 John Wiley & Sons, Ltd.