BELL 407 near San Antonio, TX — 2015-11-03

Probable cause

The failure of the No. 3 tail rotor adapter and shaft due to a loss of torque on the retaining nut. Contributing to the accident was the lack of a requirement to inspect the tail rotor assembly for security in the helicopter manufacturer's maintenance manual and the operator's approved aircraft inspection program.

NTSB narrative

The commercial pilot was conducting an emergency medical services flight. The pilot reported that, when he climbed the helicopter to a 3-ft hover, he heard a loud bang, and the helicopter began an uncommanded right yaw. The pilot applied full left pedal, but this did not arrest the yaw. The pilot then lowered the collective, and the helicopter touched down and then rotated 270 degrees to the right before it came to a stop. The tail rotor was examined, and it could be manually rotated, but a grinding noise could be heard coming from the damaged No. 3 hanger bearing. The main rotor blades did not rotate in synchronization with the tail rotor blades. The No. 3 tail rotor shaft adapter and shaft splines were completely ground down, the adapter and shaft had failed, and there was evidence of excessive torsion beyond the intended allowable tolerances. The aluminum adapter washer hole was elongated, consistent with a loose through bolt, and the drive shaft through bolt had worn threads. There was a 1/8-inch gap between the adapter and the base of the hanger bearing, consistent with the adapter not being fully in contact with the bearing and potential stress within the adapter and shaft splines. The retaining nut had migrated almost .025 inch outward and was very loose, consistent with the loss of torque, which would have allowed movement between the tail rotor adapter and shaft and led to their eventual failure. A review of the helicopter maintenance manual and operator's approved aircraft inspection program revealed that neither had a requirement for the tail rotor assembly to be inspected for security.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Inspection of tail rotor assembly before flight could have prevented failure.

NTSB coding

• Aircraft structural failure · Maneuvering-hover

Evidence available

• Photos
• 6 docket documents

Docket documents6

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• FAA Inspector's Statementinterview
• Able Engineering, Summary of Preliminary Investigationother
• Certification of Party Representative to NTSB Investigationother
• Metallurgical Report, Analytical Research and Testing Laboratories w/photosphotos
• Photosphotos

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