What happened
On September 28, 1981, at approximately 10:27 local time, an SA 315B "Lama" helicopter operated by Air Zermatt AG suffered a complete loss of power during its final approach to the Zermatt heliport. The aircraft was involved in a training flight where the chief pilot was instructing a recently hired colleague on handling external loads. During the fourth rotation of the day, while maneuvering near the avalanche gallery below the new shooting range at an altitude of roughly 30 meters, the turbine engine flamed out without prior warning.
The pilots had observed fuel quantities ranging between 80 and 100 liters on the indicator just minutes before the incident. Despite the red reserve warning light never illuminating during the flight, the engine ceased functioning due to actual fuel exhaustion. The helicopter impacted the ground with significant force, resulting in severe structural damage to the airframe. Both pilots sustained serious spinal injuries, though no passengers or third parties were harmed.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the mechanical and electrical systems of the helicopter, specifically focusing on the fuel quantity measurement system manufactured by Jaeger. Post-accident inspection revealed that the fuel tank contained only 0.4 liters of kerosene, while the indicator needle rested at zero. Laboratory tests on the five-pin connector of the fuel sender unit uncovered severe contamination and oxidation caused by corrosive metal salts, particularly on pins 2, 4, and 5.
The investigation highlighted that previous maintenance actions were insufficient to address underlying issues. Approximately one week prior to the accident, two other pilots had independently reported intermittent flickering of the reserve warning light when fuel levels were above 70 liters. The workshop addressed this by blowing out the connector, but a thorough inspection was not conducted. Furthermore, insulation measurements taken after the crash showed inadequate resistance at 500 volts, confirming that moisture and oxidation had compromised the electrical integrity of the system.
Findings
The primary cause of the accident was the engine flameout due to fuel exhaustion during approach. Contributing factors included:
- Faulty Fuel Indication: The condition of the electrical connector on the fuel sender caused significant errors in the fuel quantity display. High transition resistance and leakage currents due to corrosion led to false high readings, preventing the reserve warning light from activating.
- Inadequate Maintenance: Previous maintenance efforts failed to thoroughly diagnose and resolve the reported intermittent warning light issues, allowing the oxidation problem to persist.
- Lack of Independent Warning System: The helicopter lacked a backup fuel quantity warning system independent of the primary indicator, which would have provided a critical safety margin given the operational necessity of flying with minimal fuel reserves.
Safety action
The STSB recommended that authorities evaluate whether all helicopters should be required to install an independent residual fuel warning system. This recommendation was based on the operational reality that helicopters often fly with low fuel quantities and rely heavily on accurate, reliable warning systems for safety.