On June 17, 1985, an Air Tractor AT-301 (registration N2312U) was involved in an aviation accident near Clovis, NM. Investigators recorded the probable cause as: The loss of engine power due to the absence of the number four cylinder exhaust rocker arm shaft, which led to a forced landing and subsequent nose-over. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
Engine power failure and smoke prompted a forced field landing. The aircraft nosed over due to misalignment with furrows, caused by a missing rocker arm shaft.
What happened
The pilot was conducting an agricultural spray operation when the engine suddenly lost power while pulling up after completing a pass. Smoke began emanating from the right side of the engine compartment. Recognizing the emergency, the pilot attempted to land in an adjacent field. However, the aircraft could not be aligned with the furrows in the field. During the subsequent forced landing, the nose gear collapsed, causing the aircraft to nosed over.
The investigation
A post-accident examination of the engine revealed a critical mechanical failure. The shaft for the number four cylinder exhaust rocker arm was found to be missing from its assembly. This failure likely caused the loss of power and the associated smoke observed during the flight.
Probable cause
The loss of engine power due to the absence of the number four cylinder exhaust rocker arm shaft, which led to a forced landing and subsequent nose-over.