ROBINSON HELICOPTER R44 near Fort Glenn, AK — 2015-09-19

Probable cause

The separation of the tail rotor gearbox from the tailboom attachment point due to overstress loads imposed by a tail rotor blade imbalance as the result of impact with an undetermined object during low-level flight operations.

NTSB narrative

The commercial helicopter pilot was conducting an aerial mustering flight with cattle on a remote island. The pilot reported that, while in a hover, he heard "metallic grinding noises" from behind his position in the front right seat; however, the helicopter was "stable with engine power," the rotor rpm was within normal limitations, and the had "normal responding controls and tail rotor authority." The pilot immediately landed the helicopter on a sandbar, completed the shutdown process, and exited from the helicopter without further incident. While conducting a postflight inspection, the pilot observed that the tail rotor gearbox had separated from the tailboom mounting assembly and was lying on the ground. An examination of the tail rotor gearbox mounting flange bolts and the tail rotor blades revealed signatures consistent with overstress. No signatures of pre-existing damage were observed during the examination. During the low-level profile flown during the aerial mustering operation, it is likely that the tail rotor blades impacted an object which resulted in a severe imbalance of the blades and subsequent separation of the tail rotor gearbox. During the impact and separation sequence, the tail rotor blades likely impacted the vertical stabilizer where the red tail rotor blade "danger" decal is mounted. An examination of the components could not determine what object was impacted. No evidence of impact with foreign objects such as birds, vegetation, or cattle was observed, although such an impact could not entirely be ruled out since neither of the tail rotor blade tips was recovered.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Pilot could have conducted a more thorough preflight inspection for mechanical issues.

NTSB coding

• Part(s) separation from AC · Maneuvering-hover

Evidence available

• Photos
• 5 docket documents

Docket documents5

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Statement of Party Representatives to NTSB Investigationinterview
• R-44 Tail Rotor System Question Emailother
• Accident Photographsphotos
• NTSB Materials Laboratory Factual Reportreport

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