Aviation Accident Summaries

Aviation Accident Summary ANC13TA076

Tok, AK, USA

Aircraft #1

N70NW

BELL UH-1B

Analysis

The pilot had just completed a water drop with a long-line and water bucket when he noticed a degradation of the helicopters directional control. He returned to the airport, released the bucket, and attempted a "stuck-pedal" run-on landing. During the attempted landing, the helicopter began to drift left toward trees. Unable to maintain directional control, the pilot initiated a go-around, and the main rotor blades then struck the treetops. After realigning with the runway, he executed a "slow-speed" run-on landing to a gravel area adjacent to the runway. During touchdown, the helicopter bounced multiple times, and it then came to rest upright. The helicopter sustained substantial damage to the fuselage, vertical fin, and tail rotor blades. A postaccident examination of the tail rotor assembly revealed that the tail rotor crosshead bearings split inner rings inboard shoulder was spalled and completely worn away, which allowed the outboard set of bearing balls to run over the pitch change rods seating flange for the split inner rings inboard shoulder. The pitch change rod was then free to migrate outboard, and it machined a hole through the end of the crosshead. The movement of the pitch change rod decreased the effectiveness of the left antitorque pedal and increased the effectiveness of the right antitorque pedal.

Factual Information

On August 8, 2013, about 1430 Alaska daylight time, a restricted category Bell UH-1B helicopter, N70NW, sustained substantial damage during a forced landing, after a loss of tail rotor effectiveness at the Tok Junction Airport, Tok Alaska. The helicopter was being operated by Northern Pioneer Helicopters, Big Lake, Alaska, as a 14 CFR Part 133 external load, aerial firefighting flight when the accident occurred. The certificated commercial pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed, and the U.S. Forestry Services flight following procedures were in effect. The local flight departed Tok Junction Airport, about 1400. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on August 9, the pilot reported that the helicopter was dispatched to Tok Junction in support of firefighting operations for the U.S. Forestry Service. The pilot said that during the accident flight, he had just completed a water drop using a water bucket and a 100-foot long-line attached to the helicopters belly-mounted cargo hook. Immediately after releasing the load of water at a predetermined site, he noticed a degradation of directional control. He said that he was able to maintain control of the helicopter, and he subsequently returned to the Tok Junction Airport with the 100-foot long-line and water bucket still attached to the helicopter. As the helicopter approached the airport, the pilot jettisoned the 100 foot long-line and water bucket before attempting an emergency, stuck pedal run-on landing. During the emergency landing approach, the helicopter began to drift to the left, towards a stand of trees. Unable to maintain directional control, he initiated a go-around, and the main rotor blades struck the tops of the trees. After realigning the helicopter with the runway, he performed a "slow-speed" run-on landing to a gravel area adjacent to the runway. During touchdown, the helicopter bounced multiple times and came to rest upright. The helicopter sustained substantial damage to fuselage, vertical fin and tail rotor blades. The helicopter was equipped with after-market tail rotor blades. An on-scene examination of the tail rotor assembly by the operator revealed that the pitch change rod, and the crosshead bearing retention nut were protruding outboard through the tail rotor crosshead. The entire tail rotor gearbox assembly was removed from the accident helicopter for further examination. On January 14 and 15, 2014, under the supervision of the NTSB IIC, the tail rotor pitch change assembly was examined at Bell Helicopters field investigation laboratory, Hurst, Texas. The examination revealed that the tail rotor crosshead bearings inboard shoulder of the split inner ring was spalled, and completely worn away, allowing the bearings outboard set of bearing balls to run over the pitch change rods seating flange for the split inner ring inboard shoulder. The pitch change rod was then free to migrate outboard machining a hole through the end of the crosshead. As a result of the pitch change rod moving outboard, a decrease in the amount of maximum tail rotor blade pitch change occurred.

Probable Cause and Findings

The failure of the tail rotor crosshead bearing, which resulted in a loss of tail rotor control.

 

Source: NTSB Aviation Accident Database

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