Primary finding
Probable cause
A flight control system malfunction while the helicopter was maneuvering at low altitude due to the failure/separation of the rotor blade control tube.
Investigator assessment
Analysis narrative
While maneuvering about 50 feet above ground level during an aerial observation flight, one blade associated with the control rotor assembly separated from the helicopter, and a violent vibration started. Unable to maintain control, the pilot made a forced landing on the underlying sloping terrain, and the helicopter rolled over. During the subsequent inspection of the rotor system, one of the two control rotor blade assemblies was found separated from the rotor head. A rotor blade control tube had cracked and broken off outboard of the two attachment bolts. The component had been operated 21 hours since receiving its last inspection. The reason for the cracking and separation of the control tube could not be determined.
Source record
Factual narrative
October 30, 2008, about 0840 mountain daylight time, a Hiller UH-12E, N61436, experienced a flight control system malfunction while maneuvering about 5 miles west-southwest of Powder River, Wyoming. Unable to maintain control of the helicopter, the pilot made a hard forced landing on the slope of a hillside. Upon touchdown, the helicopter rolled over and was substantially damaged. The commercial certificated pilot received a minor injury, and the passenger was not injured. The helicopter was owned and operated by Wyoming Air Corporation, Casper, Wyoming. Visual meteorological conditions prevailed at the time of the aerial observation (predator control) flight, and no flight plan was filed. The flight was performed under the provisions of 14 Code of Federal Regulations Part 91, and it originated from a refueling vehicle near Powder River, about 0830. The pilot reported to the National Transportation Safety Board investigator that the accident occurred while he was maneuvering about 50 feet above ground level and looking for coyotes. Suddenly, one blade associated with the control rotor assembly separated from the helicopter and a violent vibration started. The pilot reported that the vibration precluded him from being able to read the airspeed indicator, and he only had "minimal control" of the helicopter. During the subsequent examination of the helicopter, one of the two control rotor blade assemblies (Hiller part number 36209-3) was found separated from the rotor head. The rotor control blade assembly tube was observed cracked and broken off outboard of the two bolts attaching it to the cuff assembly. The helicopter had been operated about 21 hours since receiving its last annual inspection.