Examination of a fractured drive from a Bendix Magneto. 1. Introduction 1.1 A request was received for an examination to be carried out on the subject drive and its magneto after the magneto had malfunctioned in service. 1.2 It is understood that the magneto was operating in Lycoming Engine Type 10-540-E1B5, which was fitted in the starboard position on a Rockwell 500S aircraft. It is also understood that the malfunction was identified by a mag-drop prior to an acceptance flight after the aircraft and engines had been subject to routine checks. 1.3 The magneto to which the subject drive was fitted was to Part.10-349300; the second 3 having been altered to an 8 at some time. 1.4 The examination requested was to identify the mechanism which had resulted in drive separation. 2. Examination Results 2.1 The magneto submitted for examination is shown in Plate 01. 2.2 The fractured parts of the drive were subjected to visual and macro examination and magnetic tests. It was found that the component had been machined from a wrought (rolled?) ferromagnetic steel bar and had been carburised and case hardened during manufacture. Detailed macro examination of the fracture surfaces on the drive showed that all had resulted from overload conditions except those arrowed on Plates 05 and 06. These fracture surfaces were typical of those which result from a low stress, high endurance tension fatigue mechanism and are shown in part on Plates 07, 08 and 09. 2.3 The only damage to the drive dogs was as shown on Plates 02 and 03. 3. Conclusions 3.1 It is considered that fracture of the subject drive resulted from the initiation and progression of tension fatigue cracks in the regions shown on Plates 05 and 06, complete separation occurring when the remaining material was incapable of supporting in service loads. 4. Comment 4.1 Since the preparation of the above report it has been learned that: a) the bearing was found to be worn on stripping the engine. b) a foreign object could have been introduced into the magneto at the time it was last overhauled which became trapped between the armature and stator, momentarily restricting armature rotation. The worn bearing could have been responsible for the fatigue damage present and the foreign object for the drive separation occurring immediately after magneto overhaul.