SCHWEIZER 269C near Schaumburg, IL — 2010-09-20

Probable cause

The in-flight separation of the left aft cluster fitting assembly following the loss of torque on the lock nut due to a manufacturing defect.

NTSB narrative

While the helicopter was hovering prior to departure for a routine training flight, a bolt and its associated hardware for the left aft cluster fitting of the tail boom assembly departed the helicopter. Subsequently, the instructor initiated a forced landing and the helicopter came to rest upright. Examination of the bolt and lock nut revealed incomplete threads at the pinch point of the locking stake due to a manufacturing defect. The bolt and lock nut assembly were installed as part of an aft cluster fitting modification that was performed about 4 months and 216 operational hours prior to the accident. As a result of the accident, the Federal Aviation Administration issued an Emergency Airworthiness Directive for owners to check the lock nuts.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Inspection of the aft cluster assembly could have identified potential issues before the flight.

NTSB coding

• Aircraft structural failure · Maneuvering-hover

Evidence available

• 7 docket documents

Docket documents7

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• NTSB Materials Laboratory Chief Concurrence Statementinterview
• Statement of Party Representatives to NTSB Investigationinterview
• FAA Emergency AD 2011-01-52other
• Maintenance Recordsother
• Schweizer Service Letter L-177other
• IMR Test Lab Reportreport

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