Oakes, ND, USA
N4907X
Rockwell S 2R
The pilot was conducting an agricultural application flight in the turbopropeller-equipped airplane when the engine lost total power. The pilot maneuvered the airplane for a forced landing to an uneven field, resulting in substantial damage to the fuselage and wings. Teardown examination of the engine revealed that the teeth of the engine’s helical cam gear were fractured. Several of the gear teeth cracks showed signatures consistent with fatigue failure. The engine assembly had a relatively low time in service (485 hours) since its most recent overhaul. The helical cam gear assembly did not show pre-existing installation anomalies. The type and degree of damage to the helical cam gear was indicative of an uncoupling of the high-speed pinion from the helical cam gear of the engine torque sensor. The uncoupling event resulted in the continuous activation of the negative torque system, which likely drained the oil in the propeller and increased the propeller blade angle. This action would have resulted in an increase in engine operating temperature and subsequent loss of engine power.
On May 25, 2019, about 1430 central daylight time, a Rockwell S2R-10 airplane, N4907X, sustained substantial damage when it was involved in an accident near Oakes, North Dakota. The commercial pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported that he was spraying a field about 10 miles southeast of his home airport. After completing 15 spray runs, the pilot maneuvered the airplane for another spray run when the engine lost total power and the pilot could not maintain altitude. The pilot maneuvered the airplane for a forced landing in a field, during which the fuselage and wings were substantially damaged. The pilot exited the airplane without injuries. The engine had about 485 hours of time in service since its most recent overhaul, and about 12 hours since its most recent annual inspection. The engine was successfully operated in a test cell at idle power. After the engine run, the front section of the engine was disassembled and examined. The helical cam gear was found with its gear teeth fractured and smeared. Several of the gear teeth cracks showed fatigue signatures during metallurgical examination. The helical cam gear assembly did not show any pre-existing installation anomalies and no other pre-impact anomalies were found with the engine.
The failure of the helical cam gear of the engine assembly due to fatigue cracks in the gear teeth, which resulted in a total loss of engine power.
Source: NTSB Aviation Accident Database
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