Los Banos, CA, USA
N36MV
HILLER UH-12E
The pilot of the helicopter had just completed an aerial application flight and was returning to land. As he approached the landing area, he began to apply power, but the engine did not respond. He continued to twist the throttle, but it felt loose with no resistance and the engine speed did not change. He aborted the landing approach and performed a run-on landing in a field; during the approach the engine speed began to decrease and the helicopter landed hard, causing substantial damage to the tail boom and lower fuselage. Postaccident examination revealed that the throttle control attach fitting located in the aft section of the collective control had become separated. The nuts and bolts attaching the fitting were not located. The lack of witness marks on the throttle fitting indicates that either the attachment bolt failed or rapidly unscrewed from its associated nut. The disconnection resulted in a loss of engine control and subsequent partial loss of engine power. The last annual inspection occurred 11 months and 250 flight hours prior to the accident.
HISTORY OF FLIGHT On February 2, 2010, about 1320 Pacific standard time, a Hiller UH-12E helicopter, N36MV, landed hard near Los Banos, California. S and S Helicopters was operating the helicopter under the provisions of Title 14 Code of Federal Regulations Part 137 as an aerial application flight. The certificated commercial pilot was not injured. The helicopter sustained substantial damage. The local flight departed from a field near Los Banos. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that he had just completed an aerial application over a wheat field, and was returning to land. As he approached the landing area at an altitude of about 30 feet above ground level, he began to apply power, but the engine did not respond. He continued to twist the throttle, but it felt loose with no resistance, and the engine speed did not change. He aborted the landing approach, and turned the helicopter towards a larger open field. He circled the field about three times while attempting to diagnose the problem. He then elected to perform a run-on landing; during the landing approach, the engine speed began to decrease, and the helicopter landed hard. During the landing sequence, the tailboom separated from the aft bulkhead, and the lower fuselage sustained crush damage. Post accident examination revealed that the throttle control fitting, located at the aft end of the collective stick assembly, had become separated from the cam lever rod bearing. According to the Hiller service manual applicable to this helicopter, the throttle control fitting is connected to the cam lever rod with an AN23-16A bolt, NAS43DD3-4 spacer, AN960PD10L washer, and a NAS679A3 nut. None of these fittings were located on the engine deck, and examination of the control arm mating surfaces revealed no gouges or fretting signatures. According to maintenance logbook entries, the helicopter underwent an annual inspection on February 25, 2009, at an airframe total time of 6,815 flight hours. According to the FAA inspector who responded to the accident, the helicopter’s total flight time at the time of the accident was 7,064 hours.
A partial loss of engine power due to a failure of the throttle interconnect hardware, which resulted in a hard landing.
Source: NTSB Aviation Accident Database
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