What happened
On May 5, 1967, an Agusta Bell 47 G-3B helicopter, registration HB-XCD, was conducting a training flight and aerial photography mission near Herzogenbuchsee. During the return flight toward Bern-Belp, the pilot noticed a rapid and abnormal decline in the fuel level readings. Despite the fuel gauges showing nearly empty levels, the pilot decided to continue the approach to the airfield rather than performing an immediate precautionary landing.
As the aircraft descended toward the airfield, the engine suddenly failed due to fuel exhaustion. The loss of engine power led to a rapid decrease in rotor RPM. While attempting to cross the Aare river, the helicopter struck trees on the western riverbank. The impact destroyed the aircraft and caused severe injuries to the occupants, including the death of the passenger.
The investigation
The investigation focused on the cause of the engine failure and the mechanical condition of the fuel system. Investigators examined the fuel tank configuration, which featured interconnected tanks with a total usable capacity of 222 liters. The investigation also reviewed the maintenance history of the aircraft, noting that the helicopter had undergone significant repairs following previous incidents involving the tail rotor.
Technical analysis of the fuel system revealed that the engine failure was caused by a significant fuel leak. The investigation also scrutinized the pilot's decision-making process following the discovery of the dropping fuel levels and the aircraft's flight path during the final moments of the descent.
Findings
- The primary cause of the engine failure was a faulty installation of a fuel drain valve, which led to substantial fuel loss. The use of improper screws and the lack of safety wiring allowed the valve assembly to loosen.
- The pilot employed inappropriate flight tactics after observing the rapid decrease in fuel levels. Instead of executing an immediate landing to verify the fuel status, the pilot attempted to complete the approach to the airfield.
- The pilot's recent hiatus from helicopter training (five months) likely contributed to a delayed and insufficient reaction to the sudden engine failure and the subsequent need to transition to autorotation.
- The investigation noted that the pilot's attempt to maintain a low altitude to avoid potential fire hazards actually hindered the ability to perform a safe autorotative landing.