On April 14, 1988, an Air Tractor AT-301 (registration N8841S) was involved in an aviation accident near Hastings, FL. Investigators recorded the probable cause as: Failure of the No. 4 cylinder exhaust valve rocker arm support, which prevented the exhaust valve from opening, resulting in excess pressure and backfire. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During an aerial application flight, the engine lost power shortly after takeoff. The pilot dumped the fertilizer load and landed in a field, but the aircraft nosed over after touchdown. Examination revealed a failed exhaust valve rocker arm support on the No. 4 cylinder.
Incident Summary
While on an aerial application flight, the aircraft experienced a loss of engine power shortly after takeoff. The pilot responded by dumping the load of fertilizer and executing a forced landing in a field. Upon touchdown, the aircraft nosed over and came to rest.
Technical Findings
A detailed examination of the engine was conducted following the accident. The investigation found that the No. 4 cylinder exhaust valve rocker arm support had failed. This failure prevented the exhaust valve from opening as designed. As a result, excessive pressure accumulated inside the cylinder. When the intake valve subsequently opened, the built-up exhaust gases were forced backward through the intake system, causing the engine to backfire.
Operational Aspects
The pilot's actions included dumping the load and selecting a field for landing. The aircraft nosed over after landing, but the source does not provide further details on the condition of the pilot or the aircraft.
Probable cause
Failure of the No. 4 cylinder exhaust valve rocker arm support, which prevented the exhaust valve from opening, resulting in excess pressure and backfire.