Accident Overview
On September 6, 2009, at 0805 central daylight time, a Bell UH-1B helicopter, registration N204JB, experienced a tail boom separation during an aerial application flight near Arvilla, North Dakota. The flight was conducted under Title 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed and no flight plan was filed. The local flight originated from Morten Airport (62ND), Larimore, North Dakota, shortly before the accident.
Sequence of Events
The pilot reported hearing a “crack” and a “bang” when pulling up at the end of the field during a spray run. The helicopter began spinning to the right as it gained altitude. The pilot rolled off the throttle and was able to clear a row of trees at the end of the field. The helicopter impacted terrain on one side of the tree line, and the tail boom came to rest approximately 100 feet away on the other side of the tree line. A piece of the aft fuselage bulkhead was located approximately 100 feet from the tail boom.
Examination Findings
Examination of the tail boom revealed that the upper left attach point fitting was separated from the tail boom. The bolt along with the forward portion of the fitting remained attached to the fuselage. The bolt from the lower left attach point fitting was missing; the bolt hole was slightly elongated and the bulkhead was slightly buckled. Half of the upper right attach point bolt was missing. The lower right attach fitting was in place, and a portion of the fuselage bulkhead was separated from the fuselage bulkhead and remained with the fitting.
Components including pieces of the tail boom’s upper left attach fitting, the upper right attach bolt, and a portion of the lower right attach fitting and bolt were examined at the Bell Helicopter laboratory under the supervision of the Safety Board. The examination revealed that the tail boom’s upper left attach fitting contained fatigue fractures originating at the upper and lower rivet holes, with corrosion present along the fractured surface. The tail boom’s upper right attachment bolt exhibited a fatigue fracture with multiple origins. The tail boom’s lower right attach fitting fractured due to overload.
Maintenance History
According to the helicopter operator and maintenance records, the tail boom involved in this accident was installed on the helicopter on February 20, 2006. This tail boom, serial number BBBF-0002, part number 204-200-022-3A, had been removed from a Bell UH-1F. The UH-1F tail boom is 23.8 inches longer than the UH-1B tail boom originally installed on the helicopter. Longer main rotor blades were also installed at the same time. The pilot/owner, who holds airframe and powerplant mechanic certification with inspection authority, performed his own maintenance. According to Bell Helicopter, no load data exists for a UH-1B helicopter with a UH-1F tail boom installation, and no Supplemental Type Certificate is available for this installation.