BELL 47D1 near Big Lake, MN — 2017-07-06

Probable cause

The failure of the metal core assembly within a wooden main rotor blade due to undetected corrosion and fatigue, which caused the wooden blade to separate from the rotor mast.

NTSB narrative

The airline transport pilot was preparing to depart in the helicopter on his fifth aerial application flight of the day. While on the ground, he increased engine rpm from idle to 3,100 rpm. Before making a collective input, the pilot felt a "sudden pull" and heard a "very loud snap." The pilot then saw that a company truck, which was parked nearby, had just sustained damaged and that the helicopter's two main rotor blades were missing. He shut down the engine; exited the helicopter; and found the main rotor blades, which were constructed of wood, splintered and scattered on the ground from about the helicopter's 2 o'clock position counterclockwise to its 8 o'clock position. The helicopter's transmission case was fractured, and the rotor mast had separated from the transmission and came to rest about 120 ft behind the helicopter's 7 to 9 o'clock positions. The damage to the company truck was consistent with impact from one or both main rotor blades. The main rotor blades were installed on the helicopter about 6 years before the accident. Examination of the recovered sections of the wood from the main rotor blades found that the blades were constructed according to blade design specifications and that there was no evidence of fungal decay in the wood. One main rotor blade had a metal core assembly separation near its root. Examination of two adjoining sections of the assembly revealed a fracture through the bar at the outboard edge of a lug. Visual examinations of the fracture faces revealed a corroded region at the trailing edge of the bar with fracture marks and arrest lines that were consistent with fatigue progression from the trailing edge. Ratchet marks indicated multiple fatigue origins in the area. The fatigue origin area had propagated about 0.25 inch toward the leading edge of the blade. Given this information, the main rotor blades most likely separated from the helicopter due to the failure of the blade's metal core assembly as a result of undetected corrosion and fatigue. The metal core assembly's fatigue origin area within the wooden blade would not have been visible to a pilot or mechanic, but x-ray nondestructive techniques could likely have detected the fatigue crack within the wooden blade. However, the Bell 47D1 Federal Aviation Administration (FAA)-approved maintenance manual and related FAA service bulletins did not include procedures to inspect the condition of the main rotor blade metal core assembly. The wooden main rotor blades are no longer manufactured, and an estimated 200 sets of these blades were still in operation at the time of this investigation.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Inspect main rotor blades for fatigue and condition before flight.

NTSB coding

• Part(s) separation from AC · Standing-engine(s) operating

Evidence available

• Photos
• 11 docket documents

Docket documents11

• Evidence Control Formform
• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Release of Aircraft Wreckage, NTSB Form 6120.15form
• Statement of Party Representatives to NTSB Investigationinterview
• Maintenance Records - Logbook Excerptother
• Memorandum for Record – Communication Summary J Propstother
• Party Submission - Scott's Helicopter Letterother
• Photos - Supplied to the NTSBphotos
• Materials Laboratory Factual Report 17-092report
• Reports from Federal Agencies - Summary of Insp. Richards Findingsreport
• Reports from Federal Agencies - US Forest Service -- Inspection of Wooden Propeller Blade Sectionsreport