What happened
On October 28, 2000, a STOL Aircraft Co. UC 1 Twin Bee, registration HB-LSK, was performing a VFR flight from Reichenbach to Como, Italy, and back. The flight included stops at Lake Como and Locarno. During the return leg, while crossing the Grimsel Pass, the pilot noticed the main tank fuel gauges dropping toward zero. Upon attempting to switch to the auxiliary tank, the pilot discovered the reserve tank was already empty, as the fuel had inadvertently flowed into the main tank via gravity.
Believing there was still sufficient fuel remaining based on initial calculations, the pilot continued the flight toward Reichenbach. While flying over Grindelwald at approximately 9,000 feet, both engines failed simultaneously due to fuel exhaustion. The pilot attempted an emergency landing at Interlaken but, unable to reach the airfield, performed a forced landing on a wooded meadow near Wilderswil. During the impact, the landing gear was torn off, and the left wing struck a tree, causing the aircraft to spin 90 degrees. The accident resulted in three injuries among the passengers, while the pilot remained unhurt. The aircraft sustained heavy damage.
The investigation
The investigation examined the fuel system, the refueling process at Reichenbach, and the pilot's flight preparations. Investigators found that the refueling process was flawed; the pilot believed 215 liters of Avgas had been added, but the actual amount was only approximately 114.4 liters. This discrepancy occurred because the fuel pump meter had not been reset to zero, meaning a previous 100-liter fueling session was still reflected in the total.
Technical analysis of the HB-LSK revealed that while the transfer pump switch functioned correctly, the warning light for the pump was defective, failing to indicate the status of the fuel transfer. Furthermore, the investigation noted that the aircraft's weight and balance were within limits, as the actual lower fuel load prevented the aircraft from being overweight at takeoff.
Findings
- The primary cause of the accident was an engine failure resulting from fuel exhaustion.
- The pilot performed an incomplete flight preparation and failed to conduct adequate fuel checks before the return leg.
- The flight was continued despite a clearly insufficient fuel reserve.
- A defective transfer pump warning light prevented the pilot from identifying the fuel state.
- The refueling infrastructure at the departure airfield was improperly designed, leading to inaccurate fuel volume readings.