Examination of a fractured crankshaft from a DeHavilland Gipsy Major Series 10 Mk2 engine. 1. Introduction 1.1 A request was received for an examination to be carried out on the subject crankshaft which had been found in a fractured condition after an in-flight incident. 1.2 It is understood that at the time of the incident the subject engine was fitted to a Chipmunk 22 aircraft which was making a local flight. The propeller and spinner complete with the front portion of the connecting rod, the No.1 piston and connecting rod and the front portion of the crankcase became detached and as a result the aircraft made a crash landing. 1.3 It is understood that the subject engine had completed approximately 1400 hours since it was last overhauled 20 years ago and that it had completed approximately 800 hours in the last 17 years. 1.4 The examination requested was to determine the mechanism which had caused the separation in the crankshaft. 2. Examination and Experimental Results 2.1 The engine is shown in the as examined condition on Plate 01 and its identity label is shown on Plate 02. It was seen by looking into the engine through the hole in the crankcase, that the crankshaft had separated and that the fracture had started at the forward end of the second main bearing journal pin. Cracking had initially progressed aft along a helical path (see Plates 03 and 04). 2.2 The propeller and associated spinner and pieces of the engine (partially shown on Plates 05, 06 and 07) were taken to a laboratory where they were stripped and the part of the piece of the crankshaft containing the fracture face and the No.1 crank was removed for detailed examination. 2.3 Macroscopic examination of the fracture faces on the forward piece of the crankshaft (Plate 07) after cleaning and degreasing in trichloroethane showed that considerable mechanical damage had occurred both before and after separation. It was, however, clearly evident that a high cycle torsional fatigue fracture had initiated in the region indicated by the arrow on Plate 07 (see also Plates 08 and 09). As the torsional crack progressed, lateral bending stresses were induced in the opposite wall of the pin and resulted in multiple approximately longitudinal high stress, low cycle tension fatigue cracks in the inner wall of the pin. These cracks had progressed rapidly and coalesced to produce the plane of separation shown on Plate 10. 2.4 Both portions of the shaft were subjected to crack detection using a DC magnetic flow technique across each crank pin and each main bearing journal pin. No cracks were located (The mated pieces of the crankshaft are shown on Plates 11, 12 and 13). 2.5 Examination of the surfaces of the crack pins, the main bearing pins and associated bearing shells yielded no evidence to indicate that they had been 'tight' or that a breakdown in lubrication had occurred. Pistons 2 to 4 moved freely in their respective bores. However, on stripping the crankshaft it was found that the very low utilisation of the engine had resulted in the accumulation of oil sludge and that this had been retained in all the bores of the crank and bearing pins in the crankshaft by centrifuging when the engine was running. 2.6 The surface of the fractured No.2 main bearing journal pin was degreased and swab etched with 2% nitric acid in alcohol to determine whether there was any evidence of machining abuse in the initiation region. None was found. 'File test' on the outer surfaces of all the crank and bearing pins indicated that all were soft. 3. Conclusions 3.1 It is considered that fracture of the subject crankshaft was the result of high cycle torsional fatigue conditions in service. 3.2 The surfaces of all the crank and bearing pins were soft to a 'file test'. Such items are normally surface hardened. It is not known whether the crankshaft had ever been surface hardened or whether it had been surface hardened and then reground at some time, thus removing the surface hardened layer. If the latter were the case, a reduction in the fatigue resistance of the shaft would be expected.