What happened
On July 16, 2024, an Airbus Helicopters AS 350 B2 was performing external load operations, specifically forest liming, near Wiesenbach, Germany. During the mission, the pilot experienced a significant loss of engine power while attempting to pick up a 600 kg load of lime approximately 8 meters above the ground. The sudden power loss was accompanied by a decrease in main rotor RPM and a low-rotor-RPM warning.
The pilot immediately initiated an autorotation and targeted a clearing with low vegetation. The helicopter landed in a nose-down attitude on soft forest soil. While the aircraft sustained light damage to the landing gear and airframe, there were no injuries to the pilot.
The investigation
The BFU investigation focused on the fuel system and the refueling chain. Earlier that day, the pilot had noted engine performance issues during a previous flight, which led to the discovery of brownish, contaminated fuel during a drain procedure. Despite attempts to rectify the issue by draining the tanks and refilling with fresh fuel, the power loss recurred during the second flight phase.
Investigators examined the aircraft's fuel filters, including the Airframe Fuel Filter and the Fuel Control Unit (FCU) filter. Laboratory analysis conducted by the WIWeB institute revealed that the filters were heavily contaminated with a gel-like substance. Furthermore, the investigation traced the contamination back to a mobile refueling unit (a "tank bus") used by the operator.
Findings
- The primary cause of the engine power loss was a fuel supply disruption caused by contamination with Superabsorbent Polymer (SAP).
- The SAP, a copolymer based on sodium acrylate/acrylic acid, was found within the mobile refueling unit's water separator and filter. This material, designed to absorb water, had swelled into a gel-like mass upon contact with water, effectively clogging the aircraft's fuel filters.
- The presence of water in the fuel was likely introduced during the recent cleaning of the primary fuel tanker's compartments.
- Contributing factors included a lack of specific training for personnel regarding the risks of SAP contamination and inadequate procedures for ensuring fuel quality during mobile refueling operations.
- Significant fungal growth was also detected in the fuel, though it was not determined to be the primary cause of the filter blockage.