Fairchild Hiller FH 1100 N4035G accident near Lino Lakes, Minnesota

Probable cause

The separation of the main rotor assembly due to mast bumping.

NTSB analysis

The airline transport pilot was conducting a local flight with one passenger. The accident flight was the pilot's third flight in the helicopter on the day of the accident; before that day, he had not flown the helicopter in nearly one year. One witness stated that he saw the helicopter rocking back and forth before it "spun sideways" and "a bunch of parts" departed the airframe. Some reported hearing a "clunk" sound, and others reported hearing a "pop" sound. Another witness saw the main rotor blades "seize," then "snap off," followed by the tail rotor departing the helicopter. The witness stated that the helicopter then "dropped out of the sky." The fuselage impacted in an open field, and a postcrash fire erupted. The separated main rotor blades and hub were found in a pond about 500 ft south of the main wreckage.

Examination of the rotor mast showed deformation and fractures consistent with overstress under bending and torsion loads but no evidence of preexisting cracks or corrosion. The observed deformation of the mast was consistent with a mast bumping event. Mast bumping can occur in low acceleration of gravity (G) flight conditions, causing the rotor blade to exceed its flapping limits and resulting in the main rotor hub bumping into the rotor shaft. This often results in structural failure of the rotor shaft and a subsequent separation of the main rotor.

Due to the extensive thermal damage to the wreckage, only a limited examination could be conducted; however, no mechanical malfunctions or anomalies were noted that would have precluded normal operation.

The pilot had accumulated about 15,000 flight hours in airplanes but only had about 55 hours flight time in helicopters, most of which were accumulated more than 1 year before the accident. Although the pilot had received instruction on how to avoid mast bumping, given his low helicopter experience relative to his airplane experience, it is possible that he made a large, abrupt flight control input that resulted in a low-G flight condition and led to the observed mast bumping. However, the pilot's control inputs are unknown, and the initiating event for the mast bumping could not be determined.

Text from the NTSB. Open the NTSB docket for CEN17FA012 for the full record.