NTSB Factual Report

On August 25, 2014, about 1026 eastern daylight time, a Hiller UH-12E, N4035A, collided with trees and terrain during a forced landing at Cape May Court House, New Jersey. The commercial pilot was not injured, while one passenger had minor injuries. The helicopter was substantially damaged. The helicopter was operated by Cape May County Mosquito Control. Day, visual meteorological conditions prevailed for the local, public use, aerial observation flight, and no flight plan was filed. The local flight originated at a staging area at Cape May Court House about 0845.

According to the pilot, he was assigned to mosquito observation and evaluation in the local area. While climbing out of a marsh, at 300 feet above the ground, a loud, metallic "bang," followed by "immediate significant rhythmic vibration" was observed. He immediately performed an autorotation to the nearest clear area. The helicopter collided with trees and then the ground. There was no fire.

An inspector with the Federal Aviation Administration responded to the accident site and examined the wreckage. Structural damage to the fuselage and main rotor system was confirmed. The helicopter came to rest on its right side. Approximately 15 gallons of fuel were recovered from the fuel tank, which was not breached. Continuity was established from the main rotor and tail rotor systems to the engine and transmission.

The wreckage was recovered to an aircraft storage facility where the engine and airframe could be examined under the direction of the NTSB. The engine was intact and most of the accessories remained attached and intact. The engine snubber assemblies were impact-separated from their respective snubber mount brackets.

The engine starter was attached to a generator and about 40 amps were applied. The engine ran normally at a low rpm and thumb compression was confirmed on all six cylinders. All rocker arms and valves operated normally during crankshaft rotation. Valve train continuity was confirmed on all of the cylinders and to the accessory drive gears. The magneto leads all exhibited normal sparking.

Both the top and bottom spark plugs were removed and examined. Top plugs numbers 4 and 6 contained light oil as well as bottom plug number 3. All spark plugs exhibited signatures consistent with normal wear.

The cylinders were examined using a borescope. The pistons and cylinder bores exhibited signs of normal combustion and most of the valve faces and seats were in place. The cylinder 6 exhaust valve seat was covered in oil and its position could not be confirmed.

The fuel filter was free of debris and contaminants. The right carburetor was cracked and partially separated and the left carburetor was intact. The master and slave carburetor screens were removed and inspected. Both screens were free of debris and contaminants.

The lower engine snubber assemblies were forwarded to the NTSB Materials Laboratory, Washington, DC, for further examination.

An examination of the lateral snubber assembly's vibration mount revealed that, around approximately ½ of the circumference, elastomer was fractured and separated from the rest of the piece around the outer diameter outboard of the space where the mounting plate was located. Mating pieces of the fractured elastomer remained bonded to the outboard side of the mounting plate around the lower aft quadrant. Another smaller area of fractured and missing elastomer was located on the inboard side of the outer diameter of the elastomer. The outboard edge of the circumferential steel band was exposed in areas where the elastomer was fractured and either missing or had remained attached to the mounting plate, and portions of the exposed edges were deformed outward and inboard. The face of the circumferential steel band where it had mated to the mounting plate was mostly oxidized with dark red oxides and isolated areas of light orange oxides.

The interior face of the lateral vibration mount assembly mounting plate was mostly covered in dark red oxides. Some areas of the interior face had sliding contact damage that disturbed the oxides. Corresponding out-of-plane deformation was also observed on the mounting plate.

An examination of the inboard face of the lateral vibration mount elastomer revealed a white deposit on the exterior surfaces of the elastomer. A sample of the white deposit was analyzed using energy dispersive x-ray spectrometry and wavelength dispersive x-ray spectrometry. Results of the analysis showed the white deposits had a spectrum consisting of a large peak of carbon and a smaller peak of oxygen. A sample of the white deposit was further analyzed using fourier-transform infrared spectroscopy (FTIR). The resulting FTIR spectrum was indicative of a paraffin or polyethylene wax which had a strong match with spectra for several mold release compounds in the FTIR spectrum library. A network of closed cracks was present on the inboard face of the elastomer. Circumferential cracks with a slight opening displacement were also present at the inboard face of the elastomer.

According to the Instructions for Continued Airworthiness for the UH-12E and UH-12E4 series helicopters, the lateral and longitudinal snubber assemblies are life-limited parts with a mandatory replacement at 4,650 hours. In addition, specific inspections of the vibration mount assemblies are required at 50-hour and at 100-hour intervals. The 50-hour inspection includes checking the "mount rubber for delamination cracks and excessive surface checking." The 100-hour inspection includes checking "snubbers for cracks, damage, distortion, excessive play and security; condition of rubber mounts." The engine snubber mounts are also inspected following a sudden stoppage of the main rotor or following a hard landing.

An examination of the longitudinal snubber did not reveal evidence of an in-flight fracture. For additional details of the lateral and longitudinal snubber examinations, refer to the NTSB Materials Laboratory Factual Report 15-063, found in the public docket for this investigation.

According to maintenance records, an annual inspection was completed on the airframe and engine on July 16, 2014. The Hobbs meter reading at the time of the inspection was 3,959.0 hours, and the total airframe time was 4,845.0 hours. The Hobbs reading at the time of the accident was 3,976.8 hours.

NTSB 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.