Robinson Helicopter R44 N7196H accident near Iliamna, Alaska

Probable cause

The main rotor transmission mount design, which resulted in an in-flight vibration/oscillation, and damage to the helicopter during an emergency descent and hard landing. Contributing to the accident was the lack of information from the manufacturer regarding this known flight oscillation, and loading the helicopter beyond the forward center of gravity limit by the pilot.

NTSB analysis

The commercial pilot was on a public use helicopter flight in support of a state agency under Title 14, CFR Part 91. The pilot reported that shortly after takeoff he felt a vibration in the anti-torque pedals he had not felt before, and he noticed a slight yawing motion. He said the vibrations turned to oscillations, in both yaw and pitch, to the degree he felt the helicopter was going to come apart. He reported he made a descent while fighting to maintain control. The pilot said he touched down with 5-10 knots of forward airspeed, and subsequently the main rotor contacted the tailboom. He said after the accident he recalculated the weight and balance, and determined that the helicopter was loaded under the gross weight, but forward of the forward center of gravity (CG)limit about 1.1 inch. Research by the NTSB investigator-in-charge disclosed that operators of the same model helicopter were aware of similar events, and that the condition had been dubbed as "chugging." Further research revealed that similar accidents had been reported. Following a similar accident in Ireland the manufacturer reported to authorities that there were no further vibration issues, and that they would not be issuing a service bulletin. The manufacturer said they were familiar with the phenomenon known as chugging, and that they had conducted flight tests related to the phenomena. The tests determined that an oscillation may develop at high gross weight, about 90-100 knots, and that the oscillation was more of a "bucking" movement due to the fore-and-aft movement of the rotor mast. According to the manufacturer, the tests determined that chugging could occur within the normal CG range, most typically at or near a gross weight with a CG near the forward limit. The manufacturer believes that the oscillation is not destructive to the helicopter, and that the oscillation can be mitigated by the application of power. The manufacturer attributes the oscillation to the degree of firmness of the transmission mounts. Currently there are no manufacturer service bulletins or FAA Safety Alerts referencing the phenomena, and there is no manufacturer provided pilot training or guidance pertaining to chugging and its remedy.

Text from the NTSB. Open the NTSB docket for ANC09GA040 for the full record.