Accident Overview
On May 12, 2009, at approximately 1330 Alaska daylight time, a Robinson R-44 helicopter, registration N7196H, sustained substantial damage during a hard landing about 57 miles northwest of Iliamna, Alaska. The helicopter was operated by the State of Alaska as a visual flight rules (VFR) public use flight under Title 14, CFR Part 91. The commercial pilot and two passengers were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect.
Pilot’s Account
The flight was part of a game management patrol conducted by the Alaska State Troopers Fish and Wildlife Service. According to the director of maintenance, the pilot reported taking off from a ridge, climbing about 300 feet above the ground, and flying at approximately 90 knots indicated airspeed when he felt an unusual medium-frequency vibration in the controls. The pilot decided to make an immediate precautionary landing, but during the descent the vibration increased, leading to a hard landing during which the main rotor blades struck the tail boom. The pilot later stated in a written report that about one and a half minutes into the flight, he felt a vibration he had not experienced before, mostly in the pedals, followed by a slight yawing motion. The vibrations escalated into oscillations in yaw and pitch, leading him to initiate an emergency landing. He described a controlled descent while struggling to maintain control, touching down with 5–10 knots of forward airspeed, which caused the main rotor to contact the tail boom.
Investigation Findings
Post-accident examination of the helicopter at the State Trooper facility in Anchorage, Alaska, by the NTSB investigator-in-charge (IIC), along with representatives from the FAA and the State Troopers, revealed no preaccident mechanical anomalies. A review of the helicopter's logbooks showed no discrepancies. Small dents were noted on the cabin top fore and aft of the main rotor mast fairings. The pilot recalculated the weight and balance after the accident and determined that the helicopter was loaded slightly forward of the forward center of gravity (CG) limit, by about 1.1 inch, though it remained under the gross weight limit.
Manufacturer’s Input
The IIC contacted Robinson Helicopter and discussed the phenomenon known as “chugging,” which involves oscillations in the rotor mast. The manufacturer’s accident investigator stated that flight tests had determined that an oscillation may develop during operation at high gross weight and around 90–100 knots, characterized by a “bucking” motion due to fore-and-aft movement of the rotor mast. The tendency to enter this oscillation was exacerbated by a forward CG (within the CG envelope) and a 30-degree banked turn to the left, though it could also occur in right turns. The manufacturer noted that chugging can occur within the normal CG range, most typically near gross weight. Tests indicated the oscillation is not divergent and can be mitigated by applying power; the helicopter can be landed safely. However, landing while still oscillating could cause dents on the cabin top. The manufacturer attributed chugging to the firmness of the transmission mounts and changes mounts on helicopters that exhibit the tendency during post-manufacturing flight tests. As of the investigation, there were no service alerts, bulletins, pilot training, or Pilot Operating Handbook entries addressing chugging.
Similar Incidents
The IIC found at least three similar incidents: on December 16, 2006, a Robinson R44 (UK registration G-CEFR) near Ballymena, Ireland, experienced a similar vibration resulting in an emergency landing and damage; on March 15, 2007, N196DM near Opa Locka, Florida, had a similar vibration leading to an emergency landing on unsuitable terrain; and on September 30, 2007, N8353K near Jackson Center, Ohio, had a similar vibration resulting in an emergency landing and damage.