DEN05LA056
DEN05LA056

On February 3, 2005, at approximately 0800 mountain standard time, a Hughes 369HS helicopter, N9118F, operated by W. Enterprises, Inc., was substantially damaged while landing on a ridge near Cascade Peak, northeast of Provo, Utah. Visual meteorological conditions prevailed at the time of the accident. The local charter flight was being conducted under the provisions of Title 14 CFR Part 135 without a flight plan. The commercial pilot and two passengers were not injured. The flight originated at Park City, Utah, at approximately 0700.

According to the pilot, he was transporting two employees from the Utah Department of Transportation, to a ridge near Cascade Peak to assess the avalanche danger in Provo Canyon. As he applied power, just before touching down, he felt an "unusual" vibration in both tail rotor pedals and the helicopter's airframe. He stated that he was at approximately 4 feet above the ground. The pilot said that he increased the power and brought the helicopter up to a 10 foot hover to reposition approximately 20 feet away. While repositioning, the vibrations and noise became worse. As he attempted the second landing, he heard a "clanking" sound. He immediately lost all tail rotor authority and the helicopter began to spin to the right. He decreased the power and executed an autorotation. The helicopter continued the spin to the right, approximately 200 degrees, as it settled into a 1foot deep layer of snow. The pilot stated that, since the helicopter was still upright and stable, he decided to continue to run the engine at flight idle for 2 minutes before completing the engine shutdown.

A visual examination revealed a 3 inch by 4 inch tear on the aft right side of the fuselage where a fractured piece of the tail rotor drive shaft had punctured though it. Further examination revealed that the tail rotor would spin freely and that the tail rotor drive shaft had fractured. No tail rotor blade damage was noted.

A witness, who was one of the passengers, stated that they approached the landing zone heading in a southwest to northeast direction. He stated that they were also landing on a down slope. As they approached the landing zone, he noticed the nose of the helicopter "elevate slightly" and then he felt something in the rear of the helicopter "hit" the snow. He said that the pilot lifted the helicopter up about 10 to 15 feet as it began to spin. When they touched down, they were facing to the west. He stated that he noticed a mark in the snow where the pilot first attempted to land. The mark was approximately 30 feet away and was about 10 feet in length. He stated that he watched the pilot walk away from the helicopter, in the direction of the mark, and he watched him walk directly through the mark in the snow. The witness stated that he followed the pilot's tracks and took several photos looking to the northeast direction.

On March 7, 2005, a manufacturer's representative from Boeing examined the helicopter. He stated that the aircraft's maintenance logs appeared to be in accordance with the applicable manufacturer service and maintenance manuals. The aircraft records indicated that the maintenance was in compliance with all applicable Manufacturer Service Notices/ Letters/Bulletins, and all applicable FAA Airworthiness Directives, and "nothing exceptional was noted in the aircraft maintenance logs that would be considered causal to the accident." He stated that, the data at the time of the mishap, indicated that the aircraft had been operating within the weight and balance constraints, that there was sufficient power available for the conditions and configuration to perform the planned mission, and that the aircraft was being operated within published operational parameters.

The Boeing representative stated that, during an interview with operator, several references to a "history of events concerning tail rotor system and tail empennage vibrations" were noted, although there was little documentation that reflected maintenance being accomplished on those systems. Furthermore, the documentation did not clarify or substantiate the operator's claims. He stated that there was an entry in the maintenance log dated January 14, 2005, which reflected the replacement of the upper vertical stabilizer due to cracks in the skin on the leading edge and for a fractured strut bracket, but there was no reason provided for the cause of the damage to the vertical stabilizer. It was noted that the lower vertical stabilizer had an "arc shaped" contact mark approximately 6 inches up from the bottom of the fin. However, this damage was reportedly from a previous incident.

The Boeing representative stated that there was no visible indication of a sudden stoppage of the main rotor system. A visual inspection of the main rotor blades showed that they were intact and undamaged. The tail rotor drive shaft (PN: 369A5518, SN: 6917) was fractured into several sections. The tail rotor blades (p/n: 500P33000-501, s/n: P262, and P263) appeared undamaged and the full-length abrasion strips on each blade were in place and secure. Although, tail rotor blade s/n: P262 had a "blemish" on the abrasion strip on its exterior side at approximately 5 inches from the blade tip, the cause of the blemish was not determined.

According to the Boeing representative, the tail rotor drive shaft had a "rotational/torsion fracture" near fuselage station 170.0, and exhibited a degree of "shaft wind-up" normally associated with a "sudden stoppage to the tail rotor system." There was no "characteristic damage" to the tail rotor blades that would normally be present with a tail strike event. However, he added that there "have been instances where blades have struck water or soft snow and [did] not exhibit physical damage." No further faults or malfunctions of any other helicopter system or component was noted that could have contributed to this accident.

On April 4, 2005, four sections of the fractured tail rotor drive shaft were examined at the NTSB Materials Laboratory in Washington D. C. According to the metallurgist, the fractured surfaces exhibited "extensive torsional deformation on both sides of the aft fracture." The indicated direction of torsion, as viewed looking aft, was "as if the forward portion of the shaft rotated clockwise," relative to the aft portion of the shaft. This fracture was largely circumferential and was at a location where the exterior surface of the shaft contained mechanical rubbing and rotational contact damage. Several other areas of rotational damage were noted on the shaft surface between the forward and aft fractures. A detailed examination of the fracture surfaces in the shaft revealed clean fractures on a 45-degree slant plane, "typical of recent overstress fracture under tension or tearing loads." No evidence of preexisting fracture such as fatigue cracking was noted.

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