What happened
On September 3, 2001, at approximately 07:11, a Bell 206B-3 helicopter, registration B-31135, operated by Ling Tian Airlines, departed from Taichung Shuannnan Airport to perform insulator washing duties for Taiwan Power Company. The flight crew performed several successful sorties between 07:32 and 08:40. Following the third sortie, the aircraft landed at a temporary work site to refuel. During this period, the fuel level was replenished from 21 gallons to 35 gallons using a mobile refueling truck.
At approximately 08:45, during the fourth flight leg toward power tower number 11, the engine experienced an abnormal sound and appeared to flame out. The pilot attempted to maneuver the aircraft, but while passing over residential areas, the helicopter struck power lines. The impact caused the aircraft to disintegrate and crash in a bamboo grove on Jun-gong Road. Both crew members on board sustained fatal injuries.
The investigation
The Taiwan Transportation Safety Board (TTSB) examined the wreckage, the fuel from the aircraft, and the fuel from the mobile refueling truck. Investigators analyzed the engine's warning lights, specifically noting that the engine flameout warning light and the fuel pump low-pressure warning light showed filament curling, indicating they were illuminated at the moment of impact. The investigation also included a technical review of the Bell 206B-3 fuel tank design, specifically the placement of the fuel tank drain valve.
Findings
- The primary cause of the accident was engine failure caused by massive water contamination in the fuel system. Laboratory analysis of the fuel recovered from the aircraft showed a water content of 96% to 98%.
- The fuel in the mobile refueling truck's tank was found to have a water content of 99.9%, indicating that the truck operator had failed to effectively perform necessary water drainage procedures prior to refueling.
- The aircraft's fuel tank design contributed to the risk, as the drain valve was not located at the lowest point of the tank. Due to the aircraft's streamlined shape, a small amount of water (approximately 0.307 gallons) could remain trapped in the tank and could not be drained.
- The crew's maintenance and flight operations lacked standardized procedures for identifying water content in fuel; while they were trained to drain tanks, they lacked specific methods to verify the presence of water.
- The rear occupant was not wearing a seatbelt, which contributed to the severity of the injuries during the impact as the occupant was thrown forward into the pilot.