What happened
On February 3, 2023, a Garlick Helicopters UH-1D helicopter was engaged in aerial firefighting operations in the Galvarino commune, Araucanía Region, Chile. The crew, consisting of a pilot in command and a flight instructor, was performing water drops to combat forest fires. Following the release of a water load over a warehouse, the aircraft experienced a sudden loss of control. The helicopter's tail boom fractured and detached from the fuselage, followed by a failure of the main rotor system. The aircraft plummeted into the terrain, where it impacted the ground and was consumed by fire. The accident resulted in two fatalities, as both the pilot and the instructor died at the scene.
The investigation
DGAC Chile investigators examined the wreckage and maintenance records of the aircraft. The investigation confirmed that the helicopter was operating with a valid airworthiness certificate and that all maintenance had been performed according to the approved program by an authorized maintenance center. The aircraft's engine and rotor components had undergone recent inspections. Witnesses and other pilots on site reported that the aircraft did not strike any obstacles, such as trees or the water bucket, prior to the structural failure. The investigation focused on the physical evidence of overstress found on the static stops, the main rotor hub, and the severed main rotor mast.
Findings
- The primary cause of the accident was the loss of control due to a sudden cyclic control maneuver, which induced a phenomenon known as mast bumping.
- This mast bumping occurred when the rotor blades' flapping exceeded the limits of the system, causing the rotor hub to strike the mast.
- The resulting structural overstress led to the separation of the tail cone and the subsequent failure of the main rotor mast.
- The aircraft was performing high-intensity firefighting maneuvers, which involve load changes and flight profiles susceptible to zero or negative G-loads.
- The flight manual specifically warns against abrupt control inputs that can lead to such instability.
Safety action
- The investigation recommends that helicopter operators utilizing two-blade, semi-rigid rotor systems reinforce training regarding the risks of mast bumping, particularly during low-G maneuvers common in aerial firefighting.
- It is advised that safety findings be disseminated through workshops and seminars to emphasize the dangers of abrupt cyclic inputs during water drop operations.