What happened
On 17 February 2019, an Airbus Helicopters AS350 helicopter, registered ZK-HEX, was assisting in forest fire suppression operations in the Nelson region. The helicopter, with the pilot as the sole occupant, was equipped with a Cloudburst CB1000MF monsoon bucket suspended on an 8.5-meter lifting line. While returning to a pond after dropping water, the lifting line made contact with the tail rotor, disabling it and causing a loss of directional control. The pilot initiated a forced landing, jettisoned the bucket, and performed an autorotation. The helicopter landed heavily in a forested area, sustaining substantial damage. The pilot received minor injuries.
Why it happened
The monsoon bucket featured a tubular, segmented, stainless-steel ring held in place by hook-and-loop fastener tabs at the top to maintain rigidity when empty. During the accident flight, one or more tabs likely came undone, allowing the ring to become loose and the bucket to lose shape. This caused a sudden change in aerodynamic stability, leading the bucket to trail behind the helicopter and the lifting line to contact the tail rotor. The reason for the tabs loosening could not be determined. Additionally, the 8.5-meter lifting line length likely increased the risk of the external load contacting the tail rotor.
Damage to aircraft
The helicopter struck the ground heavily with minimal sideways movement. The impact separated the front sections of both landing gear skid tubes and deformed the front cabin structure. Main rotor blades were substantially damaged and appeared to have struck the tail boom and foliage. The rear tail boom and tail rotor assembly separated from the airframe. One tail rotor blade was broken at the root; the other showed no external damage. Yellow synthetic sheathing from the lifting line was found on the tail rotor assembly. The tail rotor drive shaft failed at its forward coupling. The jettisoned bucket was found collapsed approximately 100 meters south of the wreckage.
Aircraft and equipment information
The helicopter was built in 1982 as an AS350 BA variant and later converted to an FX2 variant with a Honeywell LTS101 700 D-2 engine. It had 5,816.43 flight hours and was maintained per the operator's approved program. The monsoon bucket, purchased new in January 2013, had 300 hours of use. The bucket manufacturer had developed design improvements to reduce the likelihood of the hook-and-loop tabs coming undone, but these were not mandatory. The aircraft manufacturer later published a Safety Information Notice on appropriate line lengths for monsoon bucket operations, which the operator incorporated into its procedures.