NTSB Factual Report

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 a representative 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.

NTSB Probable Cause

The partial loss of engine power due to the overstress separation of three supercharger drive gear teeth.