This paper reports the study carried out to define the causes of the in-flight crash of an ultralight aircraft. Two fractured areas were identified after the impact and analysed for that scope. One fracture interested the bolt connecting the engine shaft to the propeller and the second one interested the root of one of the two blade sockets, the other blade was found unbroken. Microstructural and mechanical analysis of samples coming from the bolt and from the blade (respectively CK45 carbon steel and Ni-Cr-Mo steel) do not show any significant metallurgical defects or anomalies. So, the analysis was focused on fractures surfaces of both the bolt and the blade socket. Fatigue crack initiation and propagation at the root of the broken blade socket was identified as the main cause of the failure. The examination of the unbroken blade socket also showed the presence of an extended circumferential crack demonstrating that the stress concentration associated to the notch geometry and the applied stress was critical for the fatigue failure initiation. A deeper investigation about the notch root, also supported by Finite Element Analysis, established that the cause of the failure was the inappropriate value of the radius notch that was lower that the designed one.