NTSB Factual Report

On September 6, 2009, at 0805 central daylight time, a Bell UH-1B, N204JB, experienced a tail boom separation during an aerial application flight in Arvilla, North Dakota. The pilot received serious injuries. The helicopter received substantial damage to the fuselage and tail boom. The flight was being operated under Title 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed and no flight plan was filed. The local flight originated from the Morten Airport (62ND), Larimore, North Dakota, shortly before the accident.

The pilot reported that he heard a “crack” and a “bang” when he pulled up at the end of the field during the spray run. He stated the helicopter began spinning to the right as it gained altitude. He rolled off the throttle and was able to clear a row of trees at the end of the field. The helicopter impacted the terrain on one side of the tree line, and the tail boom was located 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 of the tail boom revealed 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.

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 removed from the helicopter. These components were examined at the Bell Helicopter laboratory under the supervision of the Safety Board. The examination revealed the tail boom’s upper left attach fitting contained fatigue fractures that originated at the upper and lower rivet holes. Corrosion was also present along the fractured surface. The tail boom’s upper right attachment bolt contained a fatigue fracture which had multiple origins. The tail boom’s lower right attach fitting fractured as a result of overload.

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 that was originally on the helicopter. Longer main rotor blades were also installed on the helicopter at the same time. The pilot/owner, who is an airframe and powerplant mechanic with inspection authority, performed his own maintenance. According to Bell Helicopter, there is no load data available for UH-1B helicopters with a UH-1F tail boom installation, nor is there an Supplemental Type Certificate available for this installation.

NTSB Probable Cause

The aircraft owner/mechanic/pilot's installation of an improper tail boom assembly and the resulting fatigue failure of the tail boom's attach fittings.