Additional Participating Entities:
Federal Aviation Administration / Flight Standards District Office; Minneapolis, Minnesota
Scott's - Bell 47 Inc; Le Sueur, Minnesota
United States Forest Service; Madison, Wisconsin
Aviation Accident Factual Report - National Transportation Safety Board: https://app.ntsb.gov/pdf
Investigation Docket - National Transportation Safety Board: https://dms.ntsb.gov/pubdms
Aviation Accident Factual Report - National Transportation Safety Board
Location: Big Lake, MN
Accident Number: CEN17LA264
Date & Time: 07/06/2017, 1130 CDT
Registration: N153B
Aircraft: BELL 47D1
Aircraft Damage: Substantial
Defining Event: Part(s) separation from AC
Injuries: 1 None
Flight Conducted Under: Part 137: Agricultural
On July 6, 2017, about 1130 central daylight time, a Bell 47D1 helicopter, N153B, had main rotor blade separation and contact with a ground vehicle during the initiation of an aerial application flight near Big Lake, Minnesota. The airline transport pilot was uninjured. The helicopter sustained substantial main rotor blade damage. The helicopter was registered to the pilot and operated by Oleen Ag Air as a Title 14 Code of Federal Regulations Part 137 aerial application flight. Day visual meteorological conditions prevailed in the area about the time of the accident, and the flight was not operated on a flight plan. The local flight was originating at the time of the accident.
According to a statement from the pilot, he had completed four aerial application flights and landed the helicopter on the ground to be refilled for his fifth flight. He, in part, stated, "At approximately 11:30 am, I was parked on the ground. After being refilled with pesticide, I increased RPM from idle to 3,100 RPM. I noted a smooth engine and synchronized needles with the main rotor and the tail rotor. Before pulling any collective, I felt a sudden pull and heard what sounded like a very loud snap. Then, I looked to my right and saw that the water tank on my load truck had been punctured, and that the driver's side cab had also been dented." The pilot observed that the main rotor blades were missing and the engine was still running. He shut the engine off and exited the helicopter.
The rotor blades splintered and the splintered debris was collected and relocated before Federal Aviation Administration (FAA) inspectors could examine the accident site.
Pilot Information
Certificate: Airline Transport; Commercial
Age: 49, Male
Airplane Rating(s): Multi-engine Land; Single-engine Land
Seat Occupied: Left
Other Aircraft Rating(s): Helicopter
Restraint Used: Unknown
Instrument Rating(s): Airplane; Helicopter
Second Pilot Present: No
Instructor Rating(s): Airplane Single-engine; Instrument Airplane
Toxicology Performed: No
Medical Certification: Class 2 Without Waivers/Limitations
Last FAA Medical Exam: 07/01/2016
Occupational Pilot: Yes
Last Flight Review or Equivalent: 03/03/2016
Flight Time: 16300 hours (Total, all aircraft), 20 hours (Total, this make and model), 15890 hours (Pilot In Command, all aircraft), 150 hours (Last 90 days, all aircraft), 80 hours (Last 30 days, all aircraft), 5 hours (Last 24 hours, all aircraft)
The 49-year-old pilot held an FAA airline transport pilot certificate with a multi-engine airplane land rating. He also held instrument commercial pilot privileges in single engine land airplanes and in rotorcraft helicopters. He held an FAA second-class medical certificate issued on July 1, 2016, with no limitations. He reported accumulating 16,300 hours of total flight time and 20 hours of flight time in the same make and model as the accident airplane.
Aircraft and Owner/Operator Information
Aircraft Manufacturer: BELL
Registration: N153B
Model/Series: 47D1
Aircraft Category: Helicopter
Year of Manufacture: 1947
Amateur Built: No
Airworthiness Certificate: Restricted; Normal
Serial Number: 7
Landing Gear Type: Skid;
Seats: 2
Date/Type of Last Inspection: 05/15/2017, Annual
Certified Max Gross Wt.:
Time Since Last Inspection:
Engines: 1 Reciprocating
Airframe Total Time: 11902.5 Hours as of last inspection
Engine Manufacturer: Lycoming
ELT:
Engine Model/Series: VO-435-A1F
Registered Owner: Pilot
Rated Power: 200 hp
Operator: Pilot
Operating Certificate(s) Held: Agricultural Aircraft (137)
Operator Does Business As:
Operator Designator Code: KJPG
N153B was a Bell 47D1 helicopter, with serial number 7. The helicopter was manufactured in 1947 and was issued a normal category airworthiness certificate. Its helicopter's type certificate data sheet indicated it seated three and had a maximum gross weight of 2,200 pounds. The helicopter was configured for agriculture and pest control and was issued a restricted category airworthiness certificate. The last helicopter annual inspection was conducted on May 15, 2017. At the time of that inspection, the helicopter had a total time of 11,902.5 hours.
The helicopter was equipped with a 200-horsepower Lycoming VO-435-A1F engine with serial No. L-1047-31C. The engine drove two wooden main rotor blades with blade part No. 47-110-120-30.
The main rotor blades are constructed of laminated wood and covered with lacquered fiberglass cloth. For protection, a stainless-steel cap is installed on the leading edge. Fixed trim tabs located near the outboard end of the trailing edge of each blade may be adjusted (by bending) to track blades. Each blade is secured to its grips by a special bolt passing through the root of the blade and metal plates which are attached to the top and bottom of each blade root. These metal plates extend over the trailing edge of the blade to provide attachment lugs for the blade drag brace.
The main rotor blade had a finished length of 200 +/- 0.125 inches and a maximum chord plane length of 13.75 inches. The blade design called out the use of pine, birch, spruce, and balsa wood species for its construction. The blade design incorporated an internal steel core assembly that had a specified bar length of 199.62 inches and width of 1.62 inches. A lug was attached to the inboard portion of the core assembly, which had a maximum length of 9.62 inches and a combine bar and lug width of 4.75 inches. According to the operator, the main rotor blade serial Nos. were 9545 and 9546. The logbook endorsement indicated that the blades were installed on the helicopter on June 14, 2011 and at that time, they had accumulated 108.5 hours since overhaul.
Meteorological Information and Flight Plan
Conditions at Accident Site: Visual Conditions
Condition of Light: Day
Observation Facility, Elevation: KCFE, 967 ft msl
Observation Time: 1155 CDT
Distance from Accident Site: 11 Nautical Miles
Direction from Accident Site: 210°
Lowest Cloud Condition: Clear
Temperature/Dew Point: 28°C / 18°C
Lowest Ceiling: None
Visibility: 10 Miles
Wind Speed/Gusts, Direction: 6 knots, 170°
Visibility (RVR):
Altimeter Setting: 29.94 inches Hg
Visibility (RVV):
Precipitation and Obscuration: No Obscuration; No Precipitation
Departure Point: Big Lake, MN
Type of Flight Plan Filed: None
Destination: Big Lake, MN
Type of Clearance: None
Departure Time: 1130 CDT
Type of Airspace:
At 1155, the weather at the Buffalo Municipal Airport, near Buffalo, Minnesota, was Wind 170° at 6 kts; visibility 10 statute miles; sky condition clear; temperature 28° C; dew point 18° C; altimeter 29.94 inches of mercury.
Wreckage and Impact Information
Crew Injuries: 1 None
Aircraft Damage: Substantial
Passenger Injuries: N/A
Aircraft Fire: None
Ground Injuries: N/A
Aircraft Explosion: None
Total Injuries: 1 None
Latitude, Longitude: 45.313333, -93.717500 (est)
An FAA inspector examined the wreckage, and forwarded images of the wreckage to the National Transportation Safety Board (NTSB) investigator in charge. Additionally, the pilot supplied a sketch of the accident site. The examination, images, and sketch revealed that wood split from the main rotor blades was scattered randomly on the property. The separated wood splinters were scattered roughly from the helicopter's 2 o'clock position counter-clockwise to its 8 o'clock position. The transmission case was fractured, and its rotor mast was not present. The mast came to rest about 120 ft behind the helicopter's 7 to 9 o'clock position. The ground near the helicopter did not exhibit any witness marks consistent with the helicopter impacting terrain or being moved.
Other Damage
A servicing truck was parked nearby. The truck, to include the driver side of the cab and water tank, was damaged in one impact plane consistent with the size and shape of a main rotor blade. The height of the impact plane was consistent with the height of the rotor blade when the helicopter at or near ground level.
Tests And Research
An engineer with the FAA Airplane Certification Office examined the recovered wreckage. The separation surfaces that he examined showed evidence of failure in overload. The engineer did not see anything that pointed to fatigue on any of the parts. One of the main rotor blades shattered and he described it like a baseball bat when hit by a ball in the wrong spot. The other main rotor blade showed similar but much less damage. The mast appeared to have ripped out of the transmission towards the left (aft looking forward). The engineer reviewed accident pictures and noted that the fuel truck was parked on the right side of the helicopter, and the pilot does not have much for visibility over his right shoulder as the pilot's view blocked by a holding tank. Another picture showed that the fueling hose was laid out on the ground.
The recovered sections of the wood from the main rotor blades were shipped to United States Department of Agriculture, Forest Service, Forest Products Laboratory, near Madison, Wisconsin, for detailed examination. The examination revealed that the blades were manufactured from plywood and defect-free lumber, in accordance with the rotor blade design. The plywood was made from birch veneers and laminated with an exterior grade adhesive. The wood used in the remainder of the laminations was determined to be Sitka spruce. No evidence of fungal decay activity was observed in the plywood or wood laminations. Visual and microscopic examination of sections from the blades confirmed that no decay activity was present.
One main rotor blade had a metal core assembly separation near its root. Two adjoining sections of that separated main rotor blade core assembly were removed and sent to the NTSB Materials Laboratory where a Senior Metallurgist conducted a detailed examination and produced a Materials Laboratory Factual Report No. 17-092. The core assembly, part No. 47-110-122-1, was a welded steel structure consisting of a tapered lug welded to the trailing edge of a longer bar. Engineering drawing specified a grade of plain carbon steel with an ultimate tensile strength between 55,000 and 100,000 psi.
The metal core assembly was fractured through the bar at the outboard edge of the lug. Visual examinations of the fracture faces revealed a corroded region at the trailing edge of the bar with fracture markings and arrest lines indicative of fatigue progression from the trailing edge. Ratchet marking also indicated multiple origins in the area. The fatigue had propagated toward the leading edge about 0.25 inch.
The initial portion of the fatigue was heavily corroded and the fracture surface obscured while the later areas were not. The remaining fracture was consistent with brittle overstress propagation with small shear lips around the periphery of the bar. Rockwell hardness (HRB) measurements on a cut cross core assembly section averaged 93.4 HRB, which was consistent with a carbon steel having an ultimate tensile strength of approximately 98,000 psi. Cross sectional examination revealed a ferrite and pearlite microstructure consistent with the specified material.
The upper surface of the metal core assembly displayed significant surface corrosion with almost all the yellow green primer missing or discolored. The lower surface had less attack but had corrosion removing and undercutting the primer in many places particularly in the fracture area.
The fatigue origin area had some surface corrosion, but no large pits were noted and remnants of the primer remained. Large pits and material removal were seen in the adjacent areas of the upper surface of the bar. The laboratory report is appended to the docket associated with this investigation.
Additional Information
The wooden main rotor blades are no longer manufactured. A wooden main rotor blade repair station representative was asked how many main rotor blades were in operation. He estimated that there are about 200 sets of Bell 47 main rotor blades remaining in current operation.
The fatigue origin area was within the wooden blade and was not visible.
The helicopter type certificate holder forwarded excerpts from the FAA approved helicopter maintenance manual. The excerpts were reviewed, and they included direction to inspect wooden blades for condition of finish, loose areas or blisters, damage that punctures or tears fabric, trim tabs and leading-edge cracks, dents, distortion, and punctures, and wood checks in the butt area. Additionally, the index of service bulletins were reviewed. Neither the maintenance manual nor the service bulletins contained direction to examine the main rotor blade core assembly for its condition.
The helicopter type certificate holder, in a letter to the NTSB investigator in charge, recommended "looking closer at the transmission for signs of overload common with a rotor strike" as opposed to a liberated rotor blade. The type certificate holder reported that "a good witness feature for this type of case could be the metal ballast box located just inboard of the blade tip." The type certificate holder stated that an assessment of where the main rotor blade debris actually came to rest would be a great indicator of the blade trajectory. However, the debris had been moved before any investigators had arrived.
The type certificate holder also reported that the red blade's ballast box was fully intact, showing no signs of deformation. However, the yellow blade's ballast box was not found in the recovered helicopter wreckage, inspected in the state in which it has been purchased and delivered from the insurance company.
The NTSB aviation accident database and associated dockets were reviewed and there were no transmission or ballast box images available in the investigation dockets where a Bell 47 had a main rotor blade strike, which precludes the type certificate holder's recommended comparison.
14 CFR Part 137: Agricultural
Accident occurred Thursday, July 06, 2017 in Big Lake, MN
Aircraft: BELL 47D1, registration: N153B
Injuries: 1 Uninjured.
This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed. NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
On July 6, 2017, about 1200 central daylight time, a Bell 47D1 helicopter, N153B, had main rotor blade separation and contact with a ground vehicle during the initiation of an aerial application flight near Big Lake, Minnesota. The airline transport pilot was uninjured. The helicopter sustained substantial main rotor blade damage. The helicopter was registered to the pilot and operated by Oleen Ag Air as a 14 Code of Federal Regulations Part 137 aerial application flight. Day visual meteorological conditions prevailed in the area about the time of the accident and the flight was not operated on a flight plan. The local flight was originating at the time of the accident.
According to initial information from the pilot, the main rotor shaft separated while the helicopter was standing with the engine running prior to departure on the flight.
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