Hiller UH 12E N4035A accident near Cape May Court House, New Jersey

Probable cause

The mechanic’s failure to detect excessive wear on the engine’s lateral snubber vibration mount during the latest annual inspection, which resulted in an in-flight engine failure and a subsequent forced landing.

NTSB analysis

The pilot reported that, at 300 ft above the ground while climbing the helicopter out of a marshy area during mosquito control operations, he heard a loud, metallic "bang" and then felt an "immediate significant rhythmic vibration." He immediately performed an autorotation to the nearest clear area. The helicopter collided with trees and then the ground and came to rest on its right side.

A postaccident examination of the engine revealed that the engine's antivibration snubbers were fractured. Subsequent examination of the snubbers revealed that the vibration reported by the pilot was likely due to a fracture of the lateral snubber assembly's vibration mount outboard of the mounting plate. The location of the fracture combined with the deformed outboard edge of the circumferential band indicate that the lateral snubber assembly moved both outboard and inboard relative to the mounting plate and, thus, that the fracture likely did not result from motion in one direction when the helicopter came to rest on its side. Additionally, dark red oxide was found on the steel surfaces that should have been encapsulated by elastomer, which indicates that relatively long-term oxidation of the surfaces had occurred due to environmental exposure. Further, areas of the lateral snubber vibration mount were separated from the elastomer, and only a little elastomer was adhered to the surface. The oxidation and lack of adhered elastomer suggest that preexisting cracks were present at or near the interface between the elastomer and the mounting plate. Circumferential cracks with a slight opening displacement were also observed at the inboard face of the elastomer, indicating that the elastomer was degraded.

According to the helicopter manufacturer, specific inspections of the vibration mount assemblies are required at 50- and 100-hour intervals. An annual/100-hour inspection of the airframe and engine was completed about 6 weeks before the accident; the engine had operated about 18 hours since the inspection. The mechanic who performed the inspection stated that the snubber assemblies appeared airworthy at the time of the inspection. However, given the condition of the lateral snubber vibration mount, the mechanic should have been able to detect the wear on the mount and then replaced the vibration mount assembly.

Text from the NTSB. Open the NTSB docket for ERA14LA405 for the full record.