Aircraft Involved In An Alaska Accident Involving Main Rotor Transmission Design

The NTSB has made a series of recommendations to the FAA concerning Robinson R44 helicopters. The recommendations come following an NTSB investigation involving one of the aircraft in Alaska in May of 2009. In that accident, an R44, N7196H, sustained substantial damage to the main rotor and tailboom during a hard landing, about 57 miles northwest of Iliamna, Alaska. The commercial pilot and the two passengers were not injured. The helicopter was being operated by the State of Alaska, Alaska State Troopers, Anchorage, Alaska, in VFR conditions.

File Photo

In a written statement, the pilot reported that, about 1.5 minutes into the flight, he felt a vibration he had not felt before. He said the vibration was mostly in the pedals, then a slight yawing motion developed. The pilot said the vibrations became 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. During a telephone conversation on May 27, 2009, an accident investigator for Robinson Helicopter told the NTSB IIC that he was familiar with the anomaly reported by the pilot, which the manufacturer referred to as mast rocking.2 He stated that the oscillation was more of a “bucking” motion due to the fore-and-aft movement of the rotor mast and that the manufacturer had found the onset of the oscillation regime was exacerbated by a forward CG (although still within the CG envelope) and a 30º banked turn to the left; he indicated that the helicopter may also begin to oscillate in a right turn but entered the oscillation regime more readily in a left turn.

The Robinson Helicopter investigator also stated that the manufacturer determined that the oscillation is not divergent (that is, the main rotor blades do not diverge from their normal plane of rotation) and can be mitigated by adding power. Pilots can then land the helicopter safely.

The NTSB determined that the probable cause of the Iliamna accident was “the main rotor transmission mount design, which resulted in an in-flight vibration/oscillation and damage to the helicopter during the subsequent 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.” At least three similar events preceded the Iliamna accident.

In its recommendations, the NTSB says the FAA should require Robinson Helicopter to resolve the root cause of the mast-rocking vibration in the main rotor assembly to ensure that all applicable R44 helicopters are free of excessive vibrations in all flight regimes, as required by 14 Code of Federal Regulations Section 27.251, "Vibration." It should further require Robinson Helicopter to maintain a database of all reported incidents of mast rocking in the main rotor
assembly of R44 helicopters.

The board also says that the FAA should require Robinson Helicopter to issue a service letter to all approved service centers describing the mast-rocking vibration that can occur in the main rotor assembly of R44 helicopters and instructing service centers to report all incidents of mast rocking to the manufacturer, require Robinson Helicopter to amend the R44 helicopter flight manual to inform pilots of the potential for mast-rocking vibration in the main rotor assembly and how to safely exit the condition, and require that the Robinson Helicopter R44 pilot training program be revised to provide pilot instruction in the recognition and mitigation of in-flight mast-rocking vibrations in the main rotor assembly.

FMI: www.ntsb.gov