BELL UH-1B near Tok, AK — 2013-08-08

Probable cause

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

NTSB narrative

The pilot had just completed a water drop with a long-line and water bucket when he noticed a degradation of the helicopter's 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 bearing's split inner ring's inboard shoulder was spalled and completely worn away, which allowed the outboard set of bearing balls to run over the pitch change rod's seating flange for the split inner ring's 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.

Analysis

Primary failure mode

Loss of control

First missed decision gate

Pilot could have aborted the water drop to maintain control.

NTSB coding

• Loss of tail rotor effectiveness · Enroute-cruise

Evidence available

• 2 docket documents

Docket documents2

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Statement of Party Representatives to NTSB Investigationinterview