Accident Overview
On July 29, 2013, at 0735 mountain daylight time, a Grumman model G-164A airplane, registered as N5324, was substantially damaged during a forced landing following a loss of engine power near Monte Vista, Colorado. The airline transport pilot was not injured. The airplane was operated by Crop Care Inc. under 14 CFR Part 137 without a flight plan. Day visual meteorological conditions prevailed for the local aerial application flight that departed Monte Vista Municipal Airport at about 0730.
Sequence of Events
The pilot reported a partial loss of engine power while en route to the field to be sprayed. He jettisoned the chemical load, but the airplane could not maintain altitude. A forced landing was made in a pasture, during which the right main landing gear separated after striking an irrigation ditch, and the airplane nosed over. The airframe and wings sustained substantial damage.
Engine History and Examination
The accident engine, a Pratt & Whitney R-1340-AN-1 (serial number ZP-102893), was last overhauled by Covington Aircraft Engines Inc. on March 21, 2013, at about 10,125 hours since new. It was installed on the accident airplane on May 1, 2013, and had accumulated about 171 hours at the time of the accident. Postaccident examination revealed a fracture of the No. 3 cylinder exhaust rocker housing. The NTSB Materials Laboratory determined that the fracture extended from the rocker cap to the rocker shaft and exhibited discolored zones with arced edges consistent with fatigue.
Rocker Assembly Discrepancies
The rocker assembly hardware from the No. 3 cylinder included the shaft, a large castellated nut with a red O-ring seal, and a small castellated nut with a washer and black O-ring seal. The release torque for the small nut measured 250 in-lbs. A review of the manufacturer's overhaul instructions specified that the large nut (with red O-ring) should be installed first, followed by the small nut torqued to 200-250 in-lbs and then tightened to the next slot. However, the overhaul facility used a different procedure: they installed the shaft small-end first, then the small nut torqued to 200 in-lbs, followed by the large nut torqued to 80 in-lbs, but with a red O-ring instead of the copper-covered gasket required for the alternative procedure. The release torque for intake and exhaust rocker assemblies on remaining cylinders varied from 78 to 546 in-lbs, with most exceeding 300 in-lbs. When reassembled per the manufacturer's instructions, release torques ranged between 175 and 220 in-lbs.
Maintenance Records
No history of unresolved airworthiness issues with the engine since the last overhaul was found. The airplane owner and primary maintainer stated that no maintenance had been performed on any rocker assemblies since the overhaul.