Iliamna, AK, USA
N7196H
ROBINSON HELICOPTER R44
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.
On May 12, 2009, about 1330 Alaska daylight time, a Robinson R-44 helicopter, N7196H, sustained substantial damage during a hard landing, about 57 miles northwest of Iliamna, Alaska. The helicopter was being operated by the State of Alaska, Anchorage, Alaska, as a visual flight rules (VFR) public use flight under Title 14, CFR Part 91, when the accident occurred. The commercial pilot and the two passengers were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on May 12, the director of maintenance for the operator said the flight was made by the Alaska State Troopers, Fish and Wildlife Service, who were doing a game management patrol. He said the pilot told him that he had taken off from a ridge, and climbed about 300 feet above the ground. The helicopter was flying about 90 knots indicated airspeed when the pilot felt an unusual medium-frequency vibration in the controls. He said the pilot decided to make an immediate precautionary landing. During the descent the pilot told him the vibration increased. He said the helicopter landed hard, and the main rotor blades struck the tail boom. The pilot did not report any unusual mechanical problems prior to the accident. On May 15, the helicopter was examined by the IIC at the State Trooper facility in Anchorage, Alaska. The IIC was accompanied by representatives from the FAA, and the State Troopers. No preaccident mechanical anomalies were discovered, and a review of the helicopter's logbooks showed no discrepancies. Small dents were noted in the cabin top fore and aft of the main rotor mast fairings. In a written report to the NTSB dated May 21, the pilot wrote that about one and one-half minutes into the flight he felt a vibration he had not felt before. he said the vibration was mostly in the pedals, and then a slight yawing motion developed. He said about the same time his passenger said "Is that normal?" referring to the vibration. The pilot told the passenger it was not, and that he was going to land immediately. He said the vibrations turned to oscillations, in both yaw and pitch, to the point he felt the helicopter was going to come apart. He said an emergency landing was his only option. He reported that he picked a spot on the ground and made a controlled descent, while fighting to maintain control of the helicopter. He said his only concern was to get the helicopter on the ground in one piece, and subsequently he touched down with 5-10 knots of forward airspeed. During the landing the pilot said the main rotor contacted the tailboom. According to the pilot, after the accident he recalculated the weight and balance for the helicopter, and determined that the helicopter was loaded under the gross weight for the helicopter, but slightly forward of the forward center of gravity limit, about 1.1 inch. Additional Information An Internet search of helicopter sites by the IIC revealed that operators of the Robinson R44 helicopter were aware of similar events, and that the condition had been dubbed "chugging." A search for similar accident/incidents by the IIC revealed that on December 16, 2006, a Robinson R44 helicopter, United Kingdom registration, G-CEFR, near Ballymena, Ireland, had a similar vibration resulting in an emergency landing, and damage to the helicopter. On August 27, 2007 the manufacturer reported to Ireland's Aviation Accident Investigation Board (AAIB) that they were no longer encountering the vibration problem during production flight tests, and therefore they did not intend to issue a service letter about the problem. (Ireland CAA/AAIB EW/G2006/12/08) On March 15, 2007, a Robinson R44 helicopter, N196DM, near Opa Locka, Florida, had a similar vibration, which resulted in the pilot making an emergency landing on unsuitable terrain, damaging the helicopter. (NTSB MIA07LA059) On September 30, 2007, a Robinson R44 helicopter, N8353K, near Jackson Center, Ohio, had a similar vibration, which resulted in the pilot making an emergency landing, and damaging the helicopter. (NTSB CHI07LA309) The IIC contacted Robinson Helicopter, and asked if they were familiar with the term "chugging" as it related to Robinson helicopters, particularly the model R44. During a telephone conversation with the manufacturer's accident investigator on May 27, the investigator said that he was familiar with the phenomenon known as "chugging," and that the manufacturer had conducted flight tests related to the phenomena. He said the tests determined that an oscillation may develop during operation of the helicopter at high gross weight, about 90-100 knots, and that the oscillation was more of a "bucking" motion due to the fore-and-aft movement of the rotor mast. He said that the tests showed the tendency to enter the oscillation regime was exacerbated by a forward CG (within the CG envelope) and a 30 degree banked turn to the left. He further indicated that the helicopter may also begin to oscillate in a right turn, but it entered the oscillation regime more easily in a left turn. According to the investigator, the tests also showed that chugging could occur within the normal CG range, and most typically at or near a gross weight condition. According to the manufacturer, it was determined that the oscillation is not divergent (destructive to the helicopter), and that the helicopter can be landed safely. He said that the oscillation can be mitigated by the application of power, and the condition flown out of. He said that the only damage he had seen to the test helicopter was on one occasion, when the pilot landed while the helicopter was still "bucking," and that the damage was manifested as dents/marks on the cabin-top, made by the fore-and-aft movement of the main rotor-shaft fairings. The Robinson investigator said that the manufacturer attributes the oscillation to the firmness, or lack of firmness, of the transmission mounts, and that the manufacturer changes the mounts on helicopters that exhibit a tendency toward chugging during post manufacturing test flights. The investigator said he did not know the standard by which mount firmness was measured. He said he believed that approved helicopter service centers were aware of chugging, although currently there are no service alerts/bulletins referencing the phenomena. The investigator said to his knowledge there were no factory provided alerts/bulletins, pilot training, or Pilot Operating Handbook (POH) entries, pertaining to the chugging and the remedies to resolve it.
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.
Source: NTSB Aviation Accident Database
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