History of Flight
On October 6, 2016, at about 1645 central daylight time, a Fairchild Hiller FH-1100 helicopter, registration N4035G, impacted terrain near Lino Lakes, Minnesota, following an in-flight separation of the main rotor assembly. The helicopter was destroyed; the airline transport pilot and a passenger sustained fatal injuries. The aircraft was registered to Helicopter Connection LLC and operated by the pilot under 14 CFR Part 91 as a personal flight. Day visual meteorological conditions prevailed, and no flight plan was filed. The local flight originated from Anoka County-Blaine Airport (ANE) near Minneapolis, Minnesota, about 1620.
According to a pilot-rated passenger who had flown with the accident pilot earlier that day, the accident pilot had not flown the helicopter for about a year and asked the passenger to "ride along" as a safety pilot. Both performed a preflight inspection that revealed no anomalies. They departed about 1000 for a 5-minute flight then returned and went to lunch. After lunch, they departed for a 45-minute local flight. After landing, the passenger offered to help move the helicopter into the hangar, but the pilot indicated he might fly later.
Later that day, witnesses observed the helicopter flying north. One witness reported the helicopter rocking back and forth before it "spun sideways" and "a bunch of parts" separated. Others heard a "clunk" or "pop" sound. One witness saw the main rotor blades "seize" then "snap off," followed by tail rotor departure. The helicopter then "dropped out of the sky." Multiple witnesses saw parts separating during descent.
Personnel Information
The 48-year-old pilot held an airline transport pilot certificate with airplane multi-engine land rating and commercial privileges in airplane single-engine land, airplane single-engine sea, and rotorcraft-helicopter. He also held a flight instructor certificate with airplane single- and multi-engine and instrument airplane ratings, and a flight engineer certificate with turbojet rating. He held an FAA special issuance first-class medical certificate dated August 16, 2016, with limitations for corrective lenses and not valid after February 28, 2017. The pilot reported 15,000 total flight hours and 400 hours in the six months prior. His last logbook entry was September 4, 2015, the date of his commercial rotorcraft-helicopter checkride, at which he had 55.5 total helicopter hours, with about 38 hours in the accident make and model.
The pilot's helicopter flight instructor, who provided instruction from April to August 2015, stated the pilot initially had trouble transitioning from fixed-wing to helicopter, which is common, but later handled it as well as other students. The instructor provided ground instruction on teetering rotor systems and noted the pilot responded correctly during training for turbulent conditions. During power-loss simulations, the pilot was initially slow to lower the collective but eventually demonstrated proper autorotation proficiency.
The instructor reported that all instruction occurred near Lake Charles, Louisiana. After passing his checkride, the pilot trailered the helicopter to the Minneapolis area. During the trip, a door opened and cracked the windshield; the pilot had just replaced it shortly before the accident.
Aircraft Information
The helicopter was issued an FAA standard airworthiness certificate on October 20, 1982, for normal category operations. It was powered by an Allison (Rolls Royce) M250-C20B engine with a two-bladed teetering main rotor system. The engine had a rated 420 shaft horsepower; takeoff power was 274 shp for 5 minutes, and maximum continuous power was 233 shp. Maximum gross weight was 2,750 lbs, accommodating a pilot, another pilot or passenger in the cockpit, and three passengers in the cabin. The flight manual prohibited acrobatic flight and advised against abrupt control movements in turbulence. The most recent annual inspection was completed on June 18, 2015, at 501.7 total time in service.
In January 2004, the manufacturer issued Alert Service Letter 23-5, noting instances of internal and external mast corrosion even when properly sealed. Corrective action required removing the transmission top case with mast attached and shipping to the factory for non-destructive inspections and special coating. This process was approved only at the factory and must be repeated at each transmission overhaul or every 10 years.
Meteorological Information
At 1645, weather recorded at ANE, about 4 miles southwest of the accident site, included wind from 010° at 6 knots, visibility 10 statute miles, overcast clouds at 6,000 feet, temperature 15°C, dew point 6°C, and altimeter 29.95 inches of mercury.
Wreckage and Impact Information
The main wreckage came to rest on its right side about 4 nautical miles and 52° magnetic from ANE. The surrounding area was discolored and charred from a postimpact ground fire. Other wreckage sections showed no pre- or postimpact fire evidence.
The first piece of debris, a composite section, was found about 1,675 ft south of the main wreckage. A debris path extended to the main wreckage containing floor mats, interior material, exhaust components, a tailboom section, engine cowl, exterior metal with the rotating beacon, a seat cushion, and a tail section including the tail rotor and gearbox. Separated main rotor blades and hub were found in a pond about 500 ft south of the main wreckage. All major components were accounted for.
The cockpit and cabin were destroyed by impact and postimpact fire. Cyclic, collective, and tail rotor control continuity could not be established due to substantial damage, but observed discontinuities were consistent with overload or thermal damage.
The engine, transmission, and tail rotor driveshafts exhibited separations consistent with torsional overload and overload. Circumferential witness marks were on the tail rotor driveshaft exterior. The main transmission showed thermal melting, soot discoloration, and deformation. The mast separation surface had overload fractures and could not be rotated by hand.
The main rotor blades and hub had overload fractures on the separation surface. Components found outside the main wreckage lacked soot or thermal damage. The engine had several missing compressor blades; remaining blades were bent opposite rotation.
Sections of the main rotor mast from the transmission and hub were sent to the NTSB Materials Laboratory for examination.
Additional Information
The NTSB database was queried for previous mast bumping accidents in Fairchild-Hiller FH 1100 helicopters. Investigations FTW68A0085, NYC83FA102, LAX83FA362, IAD98FA049, and DFW07FA198 included mast bumping findings. Investigation CHI00FA266 noted internal corrosion in the main rotor mast. These reports are appended to the docket.
The FAA Helicopter Flying Handbook discusses low-G conditions and mast bumping, describing scenarios where low-G maneuvers can lead to mast bumping and structural failure. The handbook advises pilots to apply aft cyclic and adhere to rotorcraft flight manual limitations.
Medical and Pathological Information
An Anoka County Coroner arranged an autopsy by the Midwest Medical Examiner's Office, Ramsey, Minnesota. Toxicological samples were taken. The cause of death was multiple blunt force injuries, manner of death ruled accidental. The FAA Bioaeronautical Sciences Research Laboratory tested the pilot for carbon monoxide, ethanol, and drugs, all negative.
Tests and Research
Retained sections of the rotor mast were examined by the NTSB Materials Laboratory. The mast showed deformation and fractures on slant angles consistent with overstress fracture under bending and torsion. Deformation and impact marks adjacent to the fracture were consistent with mast bumping. No evidence of preexisting cracks or corrosion was found.