On January 2, 1988, a Hiller UH-12E (registration N400AH) was involved in an aviation accident near Vernal, UT. Investigators recorded the probable cause as: Connecting rod failure due to high cycle fatigue. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A commercial pilot and two passengers were surveying a road route when the engine lost manifold and oil pressure, then quit. An autorotation to a snowy road resulted in a hard landing, severing the tail boom and collapsing skids. Investigation revealed connecting rod failure due to high cycle fatigue.
Accident Overview
A commercial pilot and two passengers were conducting a survey of a road route in mountainous terrain. During cruise flight, the pilot observed a drop in manifold pressure, followed by a loss of oil pressure. Subsequently, the pilot reported that the engine ceased operation.
Emergency Descent and Landing
The pilot executed an autorotation to a snow-covered road. The landing was hard, resulting in the main rotor blades severing the tail boom and the landing skids collapsing.
Investigation Findings
A post-accident teardown of the engine revealed evidence consistent with a connecting rod failure. Specifically, the number five rod bearing had disintegrated, and the rod end cap had broken in service. The damaged rod end cap and connecting rod were subjected to metallurgical examination to determine the failure mode. The metallurgist's factual report indicated that the rod exhibited fracture characteristics typical of high cycle fatigue.