Accident Overview
On 19 August 1990, an Aerospatiale AS350 B1 "Ecureuil" helicopter, registration HB-XSO, operated by Heli TV SA, was involved in an accident during a firefighting mission near Sabbione, Ticino, Switzerland. The helicopter was destroyed, and the pilot sustained slight injuries.
Flight Details
The helicopter departed Lugano-Agno airfield at 0831 local time with approximately 295 liters of kerosene (55% of capacity) to fight a forest fire in the Bavona valley. The pilot flew with a "bamby bucket" water container. After about 90 minutes of flight, the low-fuel warning light began to flash intermittently during descents, indicating a remaining fuel quantity of 20–22%. The fuel truck scheduled to refuel had not yet arrived at the designated refueling point near Sabbione. The pilot continued operations until the fuel gauge indicated about 10% (54 liters), then decided to land after the next rotation.
While on a downwind leg approaching the refueling area, the pilot heard the main rotor RPM decreasing. He noted that reducing the collective pitch increased RPM, while increasing pitch caused RPM to drop. He initiated a 180° autorotation toward the refueling point. The final approach was short due to limited height, with a speed of 40–60 knots. The empty water bucket was still attached to the cargo hook. During the flare, the tail rotor protection spur struck the ground, causing the entire tail boom to separate. The helicopter spun around, touched down on its skids, and rolled onto its right side.
Injuries and Damage
The pilot was slightly injured and exited the helicopter unaided after activating the fire shutoff valve. The helicopter was destroyed. No other injuries or third-party damage occurred.
Investigation Findings
- The pilot held a valid professional pilot license and was authorized for external load operations.
- No evidence of medical impairment before or during the flight.
- Flight, duty, and rest times were not adhered to; however, no causal link to the accident was established.
- The helicopter was airworthy and properly maintained.
- After the accident, the fuel gauge indicated 5% (27 liters), but only 1.05 liters could be drained from the tank. The gauge continued to show 5% even after emptying.
- Examination of the fuel measuring system revealed that it had been repaired three months earlier due to a malfunction. After repair, the gauge showed approximately 5% when empty but functioned correctly above 10%.
- The pilot continued flying for about 10–11 minutes after the low-fuel light became steady.
- The flight manual states that approximately 18 minutes of fuel remain at maximum continuous power after the low-fuel light illuminates.
- Fuel consumption during this type of operation was about 160–180 liters per hour. Total engine running time from Lugano to flameout was about 100 minutes.
Cause
The accident was due to a partially unsuccessful autorotation landing following an engine flameout caused by fuel exhaustion.
Contributory factors:
- Continuation of the flight despite the steady low-fuel warning light.
- A technical defect causing erroneous fuel gauge indication in the lower range.
A possible contributing factor was the late arrival of the fuel truck.