WPR12LA039

The loss of main rotor power due to a main rotor transmission failure from a lack of lubrication.

According to the pilot, the helicopter was about 500 ft above ground level (agl) when he saw the master caution warning light illuminate; he then saw that the low transmission oil pressure light was also illuminated. He immediately maneuvered away from power lines and set up an approach to land. While descending, the helicopter yawed left and right, and, about 10 ft agl with the airspeed almost at 0, the main rotor speed slowed and then stopped, and the helicopter subsequently fell to the ground. The pilot reported that the engine continued to operate for about 15 seconds after impact. An on-scene examination revealed that both main rotor blades remained intact with no leading edge damage. Examination of the helicopter revealed a band of discoloration around the circumference of the transmission case near the main rotor reduction gear assembly; localized paint peeling was also noted. There were no oil stains or residue on the transmission case. During the subsequent transmission disassembly, the mast was found coated in metal debris along its length, and the mast bearing was found coated in oil. Molten metal pieces were found in the upper planetary gears, ring gear, and sun gear, consistent with insufficient lubrication. The lower planetary gears sustained significant damage from the molten metal pieces. There was no damage to the transmission oil pump, oil was present in the oil pump cavity, and some debris and scoring were evident on the housing. Examination of the passages within the transmission oil manifold revealed medium-to-high density particles in several of the passages, but none of the passages were blocked; particles were also found in the chip detector. The particles were identified as being generally composed of metals commonly used in gears, housings, and bushings. The damage to the transmission indicated that it likely seized as a result of an overtemperature condition due to a lack of lubrication. Due to the severity of the damage, the reason for the lack of lubrication could not be determined.

An aircraft inspection was performed on the engine at Air Transport in Phoenix, Arizona, on May 8, 2012. The helicopter sustained impact damage to the entire fuselage. The cockpit roof had been crushed in toward the floor board more so on the left forward portion of the airframe. The transmission mounts separated from the airframe. In order to remove the transmission from the helicopter, the hydraulic line was cut; hydraulic fluid streamed out of the line. The oil tank sustained impact damage; there was no oil in the tank when observed from the sight-glass on the side of the tank or the filler port. A portion of the k-flex coupling remained attached at the engine output drive shaft. Rotation of the output drive shaft produced rotation of the power turbine spool with no binding evident. Aircraft sheet metal was present in the engine inlet housing with damage to the variable inlet guide vanes. The inlet guide vane actuator could not be manipulated due to debris in the inlet. There was also damage to the first stage compressor blades, and metal spray on the second stage power turbine stator vanes. There was no metallic debris adhering to the accessory gearbox chip detector and oil was in the gearbox. The transmission was disassembled. When the accessory plate was removed, there were large pieces of metal in the cavity along with oil. The oil pump was removed and disassembled; there was no damage to the pump, and oil was present in the cavity with some debris and scoring was evident in the housing. The oil pump drive shaft spline and shaft were intact. The gears driving the hydraulic pump and the tail rotor drive were intact. Visual inspection of the transmission showed heat signatures on the planetary gear casing, but there was no other obvious external damage. The freewheeling unit was not free to move in the normal drive direction. The planetary casing contained 3 oil jets; each oil jet had two retaining bolts. Each oil jet was missing one of two retaining bolts. The swashplate mast assembly was removed; the mast bearing was oil coated, and the mast itself had metal debris the length of the mast. However, there was no scoring of the mast. When the top case cover of the transmission was removed, molten metal pieces were located in the upper planetary gear area between the planetary gears and the ring gear. The planetary gears and the ring gear appeared to be intact. The upper planetary case was removed exposing the sun gear, which contained a large amount of metal transfer in the gears, and the gear teeth were smeared in a clockwise direction, which indicated that the transmission was under power at the time of the accident. Investigators also reported that the lower planetary gear assembly had sustained significant damage. The sun gear quill assembly was removed to gain access to the main input drive quill. The housing area was filled with debris and the main input quill alignment housing had cracked on both sides. The oil jets remained intact, and the lower mass bearing rotated. A castellated nut was found in the lower housing area along with oil. The lower planetary gear retaining cap nut was also located in the main input drive housing. The oil lines and pressure regulator were free of debris. The transmission oil manifold and chip detector were shipped to the National Transportation Safety Board (NTSB) Materials Laboratory, in Washington, D.C., for further examination. The materials engineer that examined the components noted that the components were received wrapped in cloth, and when the cloth was unwrapped, loose particles were identified. A borescope was used to examine the passages and interior of the transmission oil manifold; there was no evidence of blockage in the accessible passages. There were some particles on the interior surfaces, which were removed using a cotton swab for additional examination. The materials engineer reported that the particles were similar in size and appearance, but varied in color. Two of the largest particles from the chip detector were measured; the first particle measured 0.20 inch long with an aspect ratio of 4, and the second particle measured 0.09 inches with an aspect ratio of 1. The materials engineer identified and measured two of the largest particles from the transmission oil manifold. The first particle measured 0.1 inch long with an aspect ratio of 1.5, the second particle measured 0.07 inch long with an aspect ratio of 1. The composition of the particles was analyzed using energy dispersive x-ray spectroscopy. Results from the analysis revealed the particles had different compositions, which included compositions consistent with low alloy steel, a magnesium alloy, and a brass alloy. A detailed report is attached to the public docket for this accident. The internal configuration of the oil system component was documented through the use of radiographic images and the resultant computed tomography (CT) images were examined and analyzed by the NTSB. A review of the images determined that there were no indications of any completely blocked passages. It did determine; however, that there were medium and high density materials in several locations within the component, and that there were high density particles in several areas around the relief valve. The images also revealed no indications of damage to either the thermocouple or pressure sensor/switch. A detailed report is attached to the public docket for this accident. A representative from Bell helicopter reviewed photographic evidence of the transmission, and reported that it appeared that the transmission seized as a result of an over-temperature due to a lack of lubrication. The representative further indicated that both the main and tail rotor blades had stopped prior to impact, which indicated that the transmission oil system had stopped providing lubricant. According to UH-1H flight manual Emergency Procedures section, when the pilot receives a Master Caution light, they are directed to check the CAUTION panel and to land as soon as possible. For a transmission oil pressure and/or transmission oil hot lights, the pilot's corrective action in both cases is to land as soon as possible, and to reference paragraph 9-19. Paragraph 9-19 stated in part that if a transmission oil-hot or low pressure lights illuminate, the limits have been exceeded. It directs the pilot to land as soon as possible and to perform an emergency shutdown of the engine after landing. FAA inspectors responded to the accident site. The helicopter impacted flat desert terrain; the ground scar was about 15 feet. The helicopter came to rest on its right side on an estimated heading of 350 degrees. The landing gear skid crossover tubes had been shoved into the underside of the helicopter, which crushed the lower section of the fuselage and flight control rods and linkages. The aft crossover tube remained attached to the skids; however, it separated from the fuselage. All of the fuel bladders remained in their respective mounting locations with the exception of the left aft fuel bladder, which came to rest on top of the belly of the fuselage. Fuel was noted as leaking from the fuselage; the separated fuel bladder remained uncompromised and half full of fuel. The tail rotor separated from the tail boom, and came to rest about 180 feet from the main wreckage. The tail rotor drive shaft between the 42-degree gearbox and the 90-degree gearbox had separated and a section of the drive shaft was located under the vertical stabilizer. The main rotor blades, transmission assembly, and mounts were separated from the fuselage; however, the assembly remained in the helicopter in its normal relative position. Both main rotor blades were intact with damage to the lower surface of the blades consistent with ground damage; there was no leading edge damage evident. The main rotor transmission remained intact but separated from the fuselage. The drive