Rockwell S 2R N4907X accident near Oakes, North Dakota

Probable cause

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for CEN19LA156 for the full record.