What happened
On April 13, 2008, a Eurocopter AS355 F1 helicopter, registered ZK-IAV, departed Wellington International Airport for a private flight to the Marlborough Sounds. Shortly after takeoff, while climbing through 1,400 feet, the pilot experienced a loud bang followed by intense vibrations. The aircraft's main rotor lost balance and track, causing the helicopter to pitch up and roll sharply to the right.
As the vibration became severe enough to make cockpit instruments unreadable, the pilot struggled to maintain control. Unable to reach the airport due to a forced descent, the pilot identified an open field near Mount Victoria. After declaring a Mayday, the pilot performed an emergency run-on landing. The aircraft came to a stop after traveling approximately 35 meters. There were no injuries to the pilot or the passenger.
The investigation
Investigators examined the rotor head assembly and discovered that a 4-centimeter thick segment of elastomer from a spherical thrust bearing had separated and become lodged between the starflex hub and the rotor blade sleeve. This separation caused the main rotor blade to move outboard, creating a significant imbalance. The investigation also looked into the history of the components, noting that while the metal parts of the bearings had a finite life, the elastomer portion was monitored on an "on-condition" basis without a set calendar expiration.
Findings
- The primary cause of the incident was the failure of a spherical thrust bearing due to undetected internal corrosion and de-bonding of the elastomer segment.
- The failure was likely initiated by unobserved surface damage to the elastomer.
- The pilot's ability to recover the aircraft was heavily dependent on the immediate availability of a suitable landing site.
- Existing inspection protocols failed to detect the internal degradation of the bearing prior to the in-flight failure.
Safety action
Following the investigation, the Commission issued recommendations to the Director of Civil Aviation to address two critical safety concerns: the potential inadequacy of current inspection procedures for these bearings and the need to evaluate whether a mandatory calendar life limit should be imposed on such critical components to prevent age-related failures.