Accident Description
The helicopter ditched in the ocean following a complete loss of engine power.
Post-Accident Examination
Examination of the engine revealed that the #3 cylinder's exhaust valve had failed. Due to mechanical damage and oxidation of the fracture surfaces, the specific cause of the exhaust valve failure could not be precisely determined. However, it was most likely related to overheating and non-uniform seating loads resulting from severe wear and bell-mouthing of the valve guide. Bent pushrods observed during the examination indicated valve sticking in the guides prior to failure, causing excessive loads in the pushrods.
Valve stems and guides are normal wear items. Mutual wear increased the stem-to-guide clearances. Exhaust valves, particularly the sodium-filled valves used in the accident engine, rely on proper clearance to the guide to maintain good heat transfer properties. The most likely scenario for this failure was that the valve-to-guide clearance became so great as to impede heat transfer from the valve to the head through the guide. The valve then overheated, as indicated by decreased stem hardness and radial head cracks found during metallurgical examination. Off-center valve closure loads, due to bell-mouthing, then fractured the overheated and lower-strength valve head.
Maintenance History
In May 1992, the engine manufacturer issued a service bulletin (SB) recommending engine inspections to determine the wear condition of exhaust valves and guides. The SB detailed procedures to assess wear by wobbling the valve in the head, and recommended performing the inspection every 300 hours on helicopter engines. Engine logbooks showed that the SB was complied with twice: once in January 2005 at 1,921.8 hours, and the last time in March 2005 at 2,204 hours total time. The last inspection was 133.3 hours prior to the accident. The engine manufacturer reported that if performed properly, this inspection method would have been successful in detecting excess valve wear. The presence of extensive lead deposits on the valve stem and in the valve spring keeper groove indicated that stem-to-guide clearances were relatively large, allowing passage of combustion products for an extended period. This suggests that the SB inspections were not performed properly.