What happened
On 23 January 2015, a Robinson R44 helicopter, registration ZK-HPC, was performing agricultural spraying operations near Waikaia. While executing a 180-degree turn at the end of a spray run, the pilot experienced an intense and unusual vibration. Although the pilot maintained control of the aircraft, the severity of the shaking prompted an immediate landing near the site. Upon inspection by ground personnel, a large crack was identified on the lower surface of one of the main rotor blades, extending from the trailing edge toward the leading edge.
The investigation
The investigation involved a detailed metallurgical analysis of the damaged blades. The blades were sent to the Robinson Helicopter Company factory in the United States, where experts used scanning electron microscopy to examine the fracture. The investigation also included a comparison with other blades used in similar agricultural roles and flight testing by the manufacturer using strain gauges to measure stresses during simulated spraying maneuvers. Investigators also reviewed the impact of the aircraft's modification, as the helicopter was operating in a Restricted Category due to the installation of an agricultural spray system.
Findings
Technical examinations confirmed that the crack originated from metal fatigue at the chord transition of the blade. The investigation found no evidence of manufacturing defects or material flaws. However, several contributing factors were identified:
- The helicopter was being used for agricultural spraying, a role for which the aircraft was not specifically designed.
- The operational profile involved high-stress maneuvers and engine power levels that likely exceeded allowable limits.
- The heavy loads associated with spraying, especially when operating near the maximum all-up weight, subjected the blades to stresses not anticipated by the manufacturer.
- The use of the aircraft in a modified state for high-cycle agricultural work likely accelerated the fatigue process.
Safety action
Following the incident, the manufacturer, Robinson, issued a service bulletin (SB-89) to improve safety margins by reshaping the trailing edge profile and increasing the chord transition radius. The FAA also issued an airworthiness directive requiring inspections or modifications for older blade revisions. Additionally, the Commission recommended that the Director of Civil Aviation consult original equipment manufacturers when evaluating modifications that might significantly alter the intended operational use or fatigue life of aircraft components.