WPR17LA174

The commercial pilot reported that, while conducting low-level maneuvering over a field during the agricultural application flight, the engine lost partial power . The pilot leveled the wings, and the engine momentarily regained some power; however, shortly after, it started to lose power again. The pilot subsequently initiated an off-airport landing to a field. During the landing roll, the propeller struck the ground, and the landing gear were torn off. Both wings, the engine mounts, and the bottom of the fuselage sustained substantial damage. Postaccident examination of the engine revealed that three of the supercharger drive gear teeth had broken, one of which was found in the engine oil sump. Examination of the gear teeth fracture surfaces revealed features and topographies consistent with bending overstress separations. Contact patterns and pitting were noted on the pressure faces of the remaining intact gear teeth, which were indicative of heavy loading. No preexisting cracks were found. The thrust bearing exhibited damage consistent with a propeller strike. A representative from an engine overhaul facility that overhauls the accident engine make and model (but did not overhaul the accident engine) reported that it is unlikely that the gear teeth fractured during the accident sequence given that one of the separated gear teeth was found in the oil sump and not in the supercharger blower section because typically when a gear tooth separates during flight, the oil flow will carry the broken fragment down into the sump. The representative reported that the gear teeth usually break due to either a sudden engine stoppage or fatigue or stress cracking. The fatigue or stress cracking can often be the result of a propeller strike and, if left unchecked, can result in the eventual separation of the affected gear teeth. He stated that it is possible for an engine to continue operating relatively normally with only one gear tooth missing; however, multiple broken gear teeth would result in a decrease in power of the supercharger section. Maintenance records were not made available for review during the investigation; therefore, it could not be determined if a propeller strike or sudden engine stoppage was reported before the accident.

On August 1, 2017, about 0655 Pacific daylight time, a Rockwell International S-2R airplane, N4913X, was substantially damaged following a partial loss of engine power and subsequent off-airport landing near Brawley, California. The commercial pilot was not injured. The airplane was registered to and operated by Farm Aviation, Inc., under the provisions of Title 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed for the local flight and no flight plan was filed. The flight departed from a private airstrip in Brawley at 0645. The pilot reported that he departed westbound from the private airstrip with about 180 gallons of chemical and 70 gallons of fuel on board. After climbout, he made a right turn to the east to conduct a low-level reconnaissance of the field he intended to spray. As he began to set up for the eastbound pass on the north side of the field, the engine started to lose power. The pilot leveled the wings, the engine momentarily regained some power, but started to lose power again shortly after. The pilot subsequently initiated an emergency landing to an open field. During the landing roll, the propeller struck the ground and the landing gear was torn off. Both wings, the engine mounts, and the bottom of the fuselage sustained substantial damage. On October 3, 2017, the Pratt & Whitney R1340 radial engine was examined by Federal Aviation Administration (FAA) inspectors. The inspectors reported that the engine was partially disassembled before their arrival. The inspectors reported that one of the supercharger drive gear teeth was separated and found in the engine oil sump. Further disassembly of the supercharger drive section revealed that two additional gear teeth were separated from the supercharger drive. Slight rubbing marks were observed on the supercharger impeller. The thrust bearing exhibited damage consistent after a propeller strike. The oil filter revealed signs of high carbon content and some debris with very little metallic content. Surface rust was noted on internal oil transfer tubes, the camshaft drive gear, and the supercharger drive gear. The spark plugs exhibited normal wear signatures. The inspectors further reported that the carburetor was visually inspected, and no foreign objects were observed in the venturi. The magnetos were externally examined before the gears were rotated by hand. No anomalies were found The separated supercharger gear teeth were examined at the National Transportation Safety Board Materials (NTSB) laboratory, Washington, DC. The fracture surfaces were examined using a stereomicroscope at moderate magnifications. The fractures exhibited features and topographies consistent with bending overstress separations. Contact patterns and pitting were noted on the pressure faces of the remaining intact gear teeth indicative of heavy loading. No indication of preexisting cracking was apparent. During a conversation with arepresentative from an engine overhaul facility, who commonly overhauls Pratt & Whitney R1340 radial engines, he reported that from his experience, the gear teeth most often break off due to either a sudden engine stoppage or due to fatigue or stress cracking. The fatigue or stress cracking can often be the result of a propeller strike and, if left unchecked, can result in the eventual separation of the affected gear teeth. He reported that it is possible for an engine to continue a relatively normal operation with only one gear tooth missing, however multiple broken teeth would result in a decrease in power of the supercharger section. The pilot reported that the engine had accumulated about 783 hours since overhaul, and 74 hours since the last annual inspection on November 16, 2016. No maintenance records were made available for review to determine if there was a report of a previous propeller strike or sudden engine stoppage.