Incident Overview
A helicopter encountered a rough running engine while on short final approach, prompting the pilot to enter an autorotation. The subsequent landing was hard, resulting in substantial damage to the aircraft.
Crankshaft Failure
The engine failure was traced to a crankshaft fracture. Metallurgical examination revealed that the crankshaft failed due to fatigue cracking, which originated from surface damage and localized overheating. The fatigue propagated through the majority of the crank cheek, indicated by beach marks and crack arrest lines on the fracture faces.
Fatigue Origins
Two fatigue origins were identified on the aft radius surface of the No. 3 main bearing journal, approximately midway between the journal and cheek surfaces. The aft radius was covered by an oil stain. After cleaning, widespread surface damage was evident, extending from the journal surface up to the origin area and completely around the journal. The damage consisted of circumferential scoring, intermittent tearing, and material flow, along with a slight bronze hue. The location of the oil stain atop the damage suggested that the damage occurred during or shortly after overhaul, not as a result of the failure.
Additional Findings
Similar damage and coloration were found on the forward radius of the No. 3 main bearing journal, but no damage was observed on the radii of other journals. Direct visual examination of the No. 3 main bearing journal surface showed heavy circumferential scoring and roughening, but no visual signs of overheating. Most other journals (both main and rod) displayed similar scoring, though to a lesser degree.
Journal Measurements
Measurements of the crankshaft journals showed all main bearing journal diameters between 2.238 and 2.240 inches, and all rod journal diameters at 1.927 inches. According to a distributor for new Franklin engines, standard bearing sizes for the 6A-335 engine are 2.250–2.249 inches for mains and 1.9375–1.9365 inches for rods. Bearings for 0.010- and 0.020-inch undersize journals are available. The measured diameters are consistent with 0.010-inch undersize journals for both mains and rods.
Overhaul History
The crankshaft had been overhauled approximately 165 hours before the failure. The presence of a white layer, along with the undersized journals, indicated that the crankshaft had been renitrided at some point. The Franklin 6V-335 engine was originally certificated in 1956, with the type certificate now held by PZL of Poland.