The pilot reported that the Grumman G164A aerial-application airplane experienced a partial loss of engine power while en route to the field to be sprayed near Monte Vista, Colo. He jettisoned the plane’s chemical load following the loss of engine power, but it was unable to maintain altitude and he made a forced landing in a pasture.
During the forced landing, the right main landing gear separated from the fuselage after colliding with an irrigation ditch, and the airplane subsequently nosed over.
A post-accident examination of the engine revealed a fatigue fracture of the No. 3 cylinder exhaust rocker housing. The measured release torque for the fractured No. 3 exhaust rocker assembly was at the upper limit of that specified by the manufacturer; however, the actual release torque before the failure was likely higher than when measured post-accident.
Additionally, a majority of the other rocker assemblies had release torque measurements that exceeded the manufacturer’s specified installation limits.
Although the overhaul procedures used to assemble the rocker assemblies differed from the manufacturer’s overhaul instructions, the investigation was unable to determine if those deviations would have resulted in excessive stress within the rocker housing.
However, the investigation did conclude that the fatigue fracture was likely due to an excessive torque loading on the rocker assembly in conjunction with normal rocker operation. A definitive source of the over-torque condition could not be determined.
The engine had accumulated 171 hours since being overhauled about four months before the accident.
A review of the maintenance records found no history of unresolved airworthiness issues with the engine since the last overhaul.
Additionally, the airplane owner and the primary maintainer of the airplane stated that no maintenance had been performed on any of the rocker assemblies since the last engine overhaul.
A failure of the exhaust rocker housing would result in the corresponding exhaust valve remaining seated during engine operation. Without the typical pathway for the combustion products to be expelled through the engine exhaust, the hot gases would ignite the fuel/air mixture within the engine intake manifold. The likely result of a rocker housing failure would be an immediate and significant loss of engine power.
The NTSB determined the probable cause as the partial loss of engine power due to a fatigue failure of the engine cylinder exhaust rocker housing as a result of an excessively torqued rocker assembly.
NTSB Identification: CEN13LA443
This July 2013 accident report is are provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.