What happened
On the morning of 23 April 2004, a Bell/Garlick UH-1B Iroquois helicopter, registration ZK-HSF, was performing a ferry flight toward Gore, New Zealand. The aircraft was being moved to facilitate scheduled maintenance work. While flying near Mokoreta in Southland, the helicopter experienced a sudden and violent in-flight break-up. Witnesses on the ground reported seeing a loud bang followed by pieces of the aircraft falling from the sky, after which the helicopter spiraled into the ground. The pilot, who was the sole occupant, sustained 1 fatal injury upon impact, and the aircraft was completely destroyed.
The investigation
Investigators examined the wreckage and the aircraft's maintenance history to determine the cause of the structural failure. The inquiry focused on the condition of the main rotor head and the history of the flight operations. Evidence was gathered regarding recent flight vibrations and engine temperature readings noted by the pilot the previous day. The investigation also scrutinized the maintenance records of the tension-torsion (TT) straps, which are critical, finite-life components within the rotor hub assembly.
Findings
Technical analysis revealed that a main rotor blade separated from the aircraft because of a fatigue failure of a tension-torsion (TT) strap. It is believed that the fatigue cracking in this component was triggered by a main rotor overspeed event that had occurred at an unspecified time prior to the accident. Crucially, this overspeed event had not been reported by the pilot, which prevented necessary maintenance inspections from being performed. Additionally, the investigation found that the TT straps lacked proper identification markings, a deficiency that had been noted in maintenance records but went unaddressed by various parties.
Safety action
Following the accident, the Civil Aviation Authority (CAA) issued Airworthiness Directives to ensure that all finite-life components are properly tracked by serial number and that any components exceeding their service life are removed from service. Furthermore, recommendations were made to improve pilot education regarding the necessity of reporting rotor overspeed events and to increase awareness among maintenance engineers regarding the identification of critical life-limited parts.