VALDEZ, AK, USA
N119PH
Aerospatiale SA-319B
The commercial certificated pilot was landing a wheel/ski-equipped helicopter at a remote spot on a snow-covered ridgeline, about 4,200 feet msl. The pilot said that as the landing gear wheels touched the ridgeline, the helicopter started to shake in ground resonance. He decided to lift off, and the vibration became severe. The pilot said he lowered the collective pitch control and began a descent toward lower terrain. During the descent, the pilot said the helicopter nearly became uncontrollable until he heard one of the main rotor spacing cables snap. The previous 3 per revolution vibrations smoothed out, and became a 1 per revolution vibration. The pilot landed the helicopter on a glacier moraine. After landing, the pilot discovered that the broken spacing cable damaged the inboard trailing edge of one main rotor blade. The tail boom assembly was buckled and torn at the bottom attach point to the fuselage. A postaccident examination of the helicopter revealed that an attaching bolt, used to secure the yellow blade spacing cable to the red blade rotor grip fitting, was missing and the yellow blade damper was compressed. The missing bolt from the spacing cable fitting has a hollow shaft design which enables the bolt to shear when excessively loaded. Under extreme unbalanced conditions, the separation of the spacing cable permits an unbalanced rotor blade additional horizontal movement. Following the accident, the main rotor dampers were sent to an overhaul facility by the operator's insurance company. The overhaul facility reported that examination of the dampers did not reveal any observed malfunction.
On April 7, 2003, about 1115 Alaska daylight time, a wheel/ski-equipped Aerospatiale SA-319B helicopter, N119PH, sustained substantial damage when a severe vibration developed during an attempted landing at a remote mountain ridgeline, about 26 miles east-northeast of Valdez, Alaska. The helicopter was being operated as a visual flight rules (VFR) on-demand passenger flight under Title 14, CFR Part 135, when the accident occurred. The helicopter was operated by Performance Helicopters LLC., Wasilla, Alaska, in support of a heli-ski company. The commercial certificated pilot, and the five passengers, were not injured. Visual meteorological conditions prevailed. VFR company flight following procedures were in effect. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on April 7, the pilot reported he was landing at a remote spot on a snow-covered ridgeline, about 4,200 feet msl. The pilot said that as the landing gear wheels touched the ridgeline, the helicopter started to shake in ground resonance. He decided to lift off, and the vibration became severe. The pilot said he lowered the collective pitch control and began a descent toward lower terrain. During the descent, the pilot said the helicopter nearly became uncontrollable until he heard one of the main rotor spacing cables snap. The previous 3 per revolution vibrations smoothed out, and became a 1 per revolution vibration. The pilot landed the helicopter on a glacier moraine. The director of maintenance for the operator reported that he inspected the helicopter at the scene. He said that the inboard trailing edge of one main rotor blade was damaged by a broken spacing cable. He also indicated that one main rotor damper appeared to be fully compressed. The other two main rotor dampers appeared to be mid-range. The director of maintenance reported that the helicopter was previously operated by the French military, and maintained by the manufacturer. A conformity inspection was conducted by Federal Aviation Administration personnel on October 7, 2002, and the helicopter was issued a standard certificate of airworthiness. The helicopter's dampers had accrued about 70 hours since a time continued repair/bench check at an overhaul facility. The main rotor head had accrued about 964 hours since an overhaul by the manufacturer. The helicopter utilizes a three blade, fully-articulated, main rotor system where each rotor blade is attached to the rotor hub through a series of hinges, which allow each blade to move independently of the others. Each rotor blade can be feathered, or rotated about its span-wise axis to change the pitch angle of the rotor blade. A horizontal flapping hinge allows the blade to move vertically up and down. A vertical drag hinge allows the blade to accelerate and decelerate horizontally. Dampers are incorporated to prevent excessive back and forth movement around the drag hinges. The accident helicopter also used spacing cables between the three main rotor blade grips to prevent excessive horizontal movement. The Federal Aviation Administration describes ground resonance as an aerodynamic phenomenon associated with fully-articulated rotor systems. It develops when the rotor blades move out of centrifugal phase with each other and cause the rotor disc center of gravity to become unbalanced, coupled with the natural frequency of the aircraft rocking on its landing gear. It may be produced by a gust of wind, a sudden control motion, or a hard landing. Helicopters equipped with wheel-type landing gear and fully-articulated rotor systems, are more prone to ground resonance than other rotor designs. Severe ground resonance can result in the destruction of the helicopter. The helicopter was examined by the NTSB IIC at the operator's facility on April 14, after it was recovered. The examination revealed that the tail boom assembly was buckled and torn at the bottom attach point to the fuselage. One main rotor blade damper, identified as the damper for the yellow blade, appeared to be fully compressed. An attaching bolt, utilized to secure the yellow spacing cable to the red blade rotor grip fitting, was missing. The damaged main rotor blade was not examined, as it had not arrived at the operator's facility. The missing bolt from the yellow spacing cable fitting has a hollow shaft design which enables the bolt to shear when excessively loaded. Under extreme unbalanced conditions, the separation of the spacing cable permits an unbalanced rotor blade additional horizontal movement. Following the accident, the main rotor dampers were sent to an overhaul facility by the operator's insurance company. The overhaul facility reported that examination of the dampers did not reveal any observed malfunction. At 1115, an Aviation Routine Weather Report (METAR) at Valdez was reporting in part: Wind, 140 degrees (true) at 9 knots; visibility, 10 statute miles; clouds and sky condition, few at 4,300 feet, 7,500 feet broken; temperature, 36 degrees F; dew point, 12 degrees F; altimeter, 29.32 inHg.
The pilot's encounter with a ground resonance condition during the landing touchdown, which resulted in the shearing of a main rotor spacing cable bolt, and buckling of the tail boom.
Source: NTSB Aviation Accident Database
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