On 29 April 2016, an Airbus EC225 Super Puma main rotor detached without warning en-route to Bergen. Control was lost and it crashed and was destroyed. Rotor detachment was attributed to undetected development of metal fatigue in the same gearbox component which caused an identical 2009 accident to a variant of the same helicopter type. Despite this previous accident, the failure mode involved had not been properly understood or anticipated. The investigation identifies significant lessons to be learned related to gearbox design, risk assessment, fatigue evaluation, gearbox condition monitoring, type certification and continued airworthiness, which may also be valid for other helicopter types.