What happened
On April 26, 1975, an Enstrom F-28A helicopter, registration HB-XEB, was engaged in a multi-leg flight involving several stops. After an initial flight from Buttwil, the aircraft flew to Küsnacht for a passenger pickup and then proceeded toward Engelberg. During the descent toward Luzern-Beromünster, the pilot noted the fuel gauge was below one-quarter and decided to land at the airfield to refuel. Upon landing, the pilot learned that the fuel station was closed for lunch and would not reopen until 14:00.
Driven by the need to hand the aircraft over to another pilot by 14:00, the pilot calculated that the remaining fuel was sufficient for a short flight back to Buttwil. Approximately three minutes after departing Beromünster, while flying over the western shore of Lake Baldegger, the engine failed. The pilot immediately initiated an autorotation and performed a forced landing on a meadow near Hitzkirch. The aircraft struck the ground with its tail rotor guard first, causing the main rotor blades to strike the tail boom and sever the tail rotor. The helicopter slid across the terrain, rotating approximately 150 degrees.
The investigation
The investigation examined the fuel system, the pilot's decision-making process, and the aircraft's mechanical state. Investigators found that the fuel gauge was unreliable; the needle reached its maximum reading at approximately 103 liters, even though the total capacity was 125.9 liters. Furthermore, the automatic shut-off valve used during refueling prevented the pilot from knowing the exact amount of fuel added. The investigation also noted that the pilot was unaware that the aircraft had flown for 44 minutes prior to his flight without being refueled, which invalidated his fuel consumption calculations.
Findings
- The pilot's decision to continue the flight despite low fuel levels was critically flawed.
- The fuel gauge provided misleading information due to its lack of precision and the way the gauge reached its limit.
- The pilot failed to use the onboard wooden dipstick to verify the actual fuel level before departing Beromünster.
- The pilot's obligation to hand over the aircraft at a specific time influenced his risky decision to fly with low fuel.
- The damage sustained during the landing was caused by the slope of the terrain and the pilot's limited experience with the aircraft type.
Safety action
- The investigation identified inadequate flight preparation and poor decision-making as primary contributing factors to the fuel exhaustion.