History of Flight
On July 6, 2017, at about 1130 central daylight time, a Bell 47D1 helicopter, registration N153B, experienced main rotor blade separation and contact with a ground vehicle during the initiation of an aerial application flight near Big Lake, Minnesota. The flight was operated under 14 Code of Federal Regulations Part 137 by Oleen Ag Air. The airline transport pilot was uninjured. Day visual meteorological conditions prevailed, and no flight plan was filed.
According to the pilot, he had completed four previous aerial application flights and landed for a fifth refill. After being refilled with pesticide, he increased rotor RPM from idle to 3,100. He noted a smooth engine and synchronized needles. Before pulling collective, he felt a sudden pull and heard a loud snap. He looked to his right and saw that the water tank on his load truck had been punctured and the driver's side cab dented. He observed that the main rotor blades were missing and the engine was still running, so he shut it off and exited.
Aircraft and Pilot Information
The 49-year-old pilot held an FAA airline transport pilot certificate with multi-engine airplane and rotorcraft helicopter ratings. He reported 16,300 total flight hours, with 20 hours in the same make and model. The helicopter, serial number 7, was manufactured in 1947 and initially issued a normal category airworthiness certificate, later configured for agriculture under a restricted category certificate. The last annual inspection was on May 15, 2017, at 11,902.5 total hours. The helicopter was powered by a 200-horsepower Lycoming VO-435-A1F engine driving two wooden main rotor blades (part No. 47-110-120-30). The blades were constructed of laminated wood covered with lacquered fiberglass cloth, with a stainless-steel leading edge cap. The blades had serial numbers 9545 and 9546 and were installed on June 14, 2011, with 108.5 hours since overhaul.
Damage and Wreckage
The main rotor blades splintered, and debris was scattered from the helicopter's 2 o'clock position counterclockwise to 8 o'clock. The transmission case was fractured, and the rotor mast was found about 120 ft behind the helicopter's 7 to 9 o'clock position. A servicing truck parked nearby sustained damage to the driver's side cab and water tank, consistent with a main rotor blade impact plane at ground level height.
Investigation Findings
An FAA engineer examined the wreckage and noted overload failure on separation surfaces, with no evidence of fatigue on those parts. However, detailed examination of the metal core assembly (part No. 47-110-122-1) from one main rotor blade by the NTSB Materials Laboratory revealed a fatigue fracture. The core assembly, a welded steel structure, fractured through the bar at the outboard edge of the lug. A corroded region at the trailing edge showed fatigue progression about 0.25 inch toward the leading edge, with multiple origins and ratchet marks. The remaining fracture was consistent with brittle overstress. Surface corrosion was present on both sides of the core assembly, with primer missing or discolored. The wood from the blades showed no evidence of fungal decay, and construction met design specifications.
Additional Information
The helicopter type certificate holder noted that wooden main rotor blades are no longer manufactured, with an estimated 200 sets remaining in operation. The holder recommended looking for signs of overload consistent with a rotor strike, but debris had been moved before investigators arrived. No fatigue origin was visible externally, and the maintenance manual did not require inspection of the core assembly. The NTSB found no comparable accident data to assess the type certificate holder's recommendations.