On May 10, 1984, a Hiller UH-12E (SOLOY) (registration N90461) was involved in an aviation accident near Delano, CA. Investigators recorded the probable cause as: The fatigue failure of the gusset weld beads below the outer gimbal forward attach point, which bent the collective control rod and forced the collective pitch into the up position. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter suffered an in-flight engine mount failure during crop spraying operations. The collective control jammed in the up position, forcing a hard landing into the treated area. Investigation revealed fatigue cracks in the gusset weld beads.
What happened
The helicopter was engaged in agricultural spraying operations when it experienced an in-flight failure of an engine mount assembly, identified by part number 63181-5. This structural failure caused the collective control rod to bend, which subsequently forced the collective pitch into the up position. Unable to maintain altitude or control descent rate effectively, the helicopter settled directly into the grapes it was in the process of spraying and made a hard landing.
The investigation
A subsequent examination of the aircraft revealed that the gusset weld beads located just below the outer gimbal forward attach point had failed due to fatigue. This mechanical failure was identified as the primary cause of the loss of control over the collective pitch mechanism.
Findings
The incident highlights the critical importance of structural integrity in engine mount assemblies for helicopters operating in high-stress agricultural environments. The fatigue failure of the gusset weld beads led directly to the engine mount malfunction, which prevented proper pilot input and resulted in a hard landing with significant potential for further damage or injury.
Probable cause
The fatigue failure of the gusset weld beads below the outer gimbal forward attach point, which bent the collective control rod and forced the collective pitch into the up position.