Accident Overview
On August 25, 2014, at approximately 1026 eastern daylight time, a Hiller UH-12E helicopter, registration N4035A, collided with trees and terrain during a forced landing near Cape May Court House, New Jersey. The commercial pilot was not injured, and one passenger sustained minor injuries. The helicopter was substantially damaged. The flight was operated by Cape May County Mosquito Control as a local, public-use aerial observation flight under day visual meteorological conditions. No flight plan was filed. The flight originated from a staging area at Cape May Court House at about 0845.
Pilot Statement
According to the pilot, he was conducting mosquito observation and evaluation in the local area. While climbing out of a marsh at 300 feet above ground level, a loud metallic "bang" was heard, followed by an immediate significant rhythmic vibration. The pilot performed an autorotation to the nearest clear area. The helicopter struck trees and then the ground. There was no fire.
Wreckage Examination
An FAA inspector examined the wreckage at the accident site. 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 moved to an aircraft storage facility for further examination under NTSB direction. The engine was intact, with most accessories attached. The engine snubber assemblies were impact-separated from their mount brackets.
Engine Inspection
The engine starter was attached to a generator and about 40 amps were applied. The engine ran normally at 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 cylinders and to the accessory drive gears. Magneto leads exhibited normal sparking.
Both top and bottom spark plugs were removed and examined. Top plugs numbers 4 and 6 contained light oil, as did bottom plug number 3. All spark plugs showed signatures consistent with normal wear. Cylinders were examined via borescope; pistons and cylinder bores showed normal combustion signs, and most 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. The right carburetor was cracked and partially separated; the left carburetor was intact. Both carburetor screens were free of debris.
Snubber Examination
The lower engine snubber assemblies were sent to the NTSB Materials Laboratory for detailed examination. The lateral snubber assembly's vibration mount showed that approximately half of the elastomer circumference was fractured and separated from the rest of the piece around the outer diameter outboard of the mounting plate location. Mating pieces of 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 on the inboard side of the outer diameter. The outboard edge of the circumferential steel band was exposed where elastomer was missing or attached to the mounting plate; portions of exposed edges were deformed outward and inboard. The band's face where it mated to the mounting plate was mostly oxidized with dark red oxides and isolated light orange oxides.
The interior face of the lateral vibration mount assembly mounting plate was mostly covered in dark red oxides, with sliding contact damage disturbing the oxides in some areas. Corresponding out-of-plane deformation was observed on the mounting plate.
White deposits were found on the exterior surfaces of the lateral vibration mount elastomer. Analysis using energy dispersive x-ray spectrometry and wavelength dispersive x-ray spectrometry showed a large peak of carbon and a smaller peak of oxygen. Fourier-transform infrared spectroscopy indicated the deposit was a paraffin or polyethylene wax, with a strong match to spectra for mold release compounds. A network of closed cracks was present on the inboard face of the elastomer, and circumferential cracks with slight opening displacement were also present.
Maintenance Records
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 mandatory replacement at 4,650 hours. Specific inspections are required at 50-hour and 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, security, and condition of rubber mounts. Engine snubber mounts are also inspected following a sudden main rotor stoppage or hard landing.
An annual inspection was completed on the airframe and engine on July 16, 2014. The Hobbs meter reading at that time was 3,959.0 hours, and the total airframe time was 4,845.0 hours. At the time of the accident, the Hobbs reading was 3,976.8 hours.
An examination of the longitudinal snubber did not reveal evidence of an in-flight fracture.