LAX05LA146 · Robinson Helicopter Company R22A

The helicopter was ditched in the ocean after a complete loss of engine power. Post-accident examination of the engine revealed that the #3 cylinder's exhaust valve had failed. Because of mechanical damage and oxidation to 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 due to the severe wear and bell-mouthing of the valve guide. The bent pushrods found during the examination also indicated some valve sticking in the guides prior to failure that caused excessive loads in the pushrods. Valve stems and guides were normal wear items. Mutual wear increased the stem to guide clearances and exhaust valves, particularly the sodium filled valves utilized 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 the decreased stem hardness and radial head cracks found during the metallurgical examination. Off center valve closure loads, due to the bell mouthing, then fractured the overheated and lower strength valve head. In May 1992, the engine manufacturer issued a service bulletin (SB) recommending engine inspections to determine the wear condition of exhaust valve and guides. The SB details procedures to assess the wear on the engine by wobbling the valve in the head. The SB recommended performing this inspection every 300 hours on helicopter engines. Engine logbooks only 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 only 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 the passage of combustion products for an extended period of time. This suggests that the SB inspections were not performed properly.

Verbatim NTSB analysis and probable cause from the NTSB dataset