1975 Enstrom F-28A Fuel Exhaustion Accident at Hitzkirch (HB-XEB)

Flight History and Accident Sequence On April 26, 1975, the Enstrom F-28A helicopter, registration HB-XEB, was refueled at Buttwil airfield. The fuel tanks were filled until the overfill automatic shut off the flow. A flight instructor and a student subsequently flew the helicopter for 44 minutes. The pilot involved in the accident then took over the aircraft, which remained at the fuel pump. He asked the instructor about the helicopter's technical condition, specifically regarding the engine oil supply, but the conversation did not address the fuel level or the time elapsed since the last refueling. After boarding, the pilot checked the fuel gauge, which indicated approximately 7/8 full. The helicopter departed at 1123 MEZ, flying to Speck-Fehraltdorf airfield before proceeding to an off-airport landing at Küsnacht, where a passenger boarded. The flight continued over the Rigi to Engelberg, reaching an altitude of 8,000 to 9,000 feet. The pilot then turned west toward Brünig and headed for Buttwil. During the descent on the crosswind side of Luzern-Beromünster airfield, the fuel gauge indicated approximately 1/4 full, prompting the pilot to land there to refuel. The helicopter landed at 1320 MEZ. The passenger disembarked with the rotor still turning and inquired about the location of the fuel pump to signal the helicopter there. He returned to inform the pilot that the attendant was absent and fuel would not be available until after the lunch break at 1400 MEZ. The pilot was scheduled to hand over the helicopter to another pilot at 1400 MEZ. He calculated that the fuel gauge showed slightly more than 1/8 full on the ground. Based on a previous flight time of approximately 1.5 hours and a maximum flight time of about 2.25 hours in training operations at a consumption rate of 55 liters per hour, he estimated a reserve of 0.75 hours. He interpreted the fuel indication as sufficient for about 0.5 hours, while the planned overflight would take 6 to 7 minutes. Concluding the decision was positive, he opted to continue the flight. At this point, the pilot had been airborne for 88 minutes. Approximately three minutes after takeoff, at about 300 meters above the western shore of Lake Baldegg, the engine stopped. The pilot immediately initiated an autorotation. The emergency landing occurred at 1326 MEZ after a slight flare in the direction of 140 degrees onto a meadow sloping approximately 2.5 degrees in the direction of flight. The helicopter first touched down on the tail rotor guard and then on the skids. Two of the three main rotor blades struck the rear fuselage and severed the tail rotor. During the skid, the helicopter yawed approximately 150 degrees to the left and came to rest in a normal position. The accident site was located at coordinates 660.615/229.770, at an elevation of 478 meters, in the municipality of Hitzkirch. ## Damage and Findings The occupants were uninjured, and the helicopter sustained heavy damage. No significant third-party damage was reported. The pilot, born in 1948, held a valid private pilot license for helicopters with a radiotelephony extension and was authorized to conduct the flight. He had obtained permission for the off-airport landing at Küsnacht. There was no indication that he was in poor health at the time of the accident. The Federal Air Office records showed no prior incidents. The pilot's total helicopter experience was 81.24 hours, with 42.57 hours in the preceding 90 days, all on the accident aircraft type. The helicopter was approved for traffic. At the time of the accident, the weight and center of gravity were within prescribed limits. The weather in the accident area featured a Bise situation, with 3/8 to 4/8 cumulus clouds with a base around 2,000 meters, 1/8 to 3/8 cirrus clouds at 8,000 to 10,000 meters, visibility of 10 to 15 kilometers, and winds from the northeast to east at 10 to 15 knots with light gusts. The fuel system consisted of left and right tanks mounted in the fuselage, connected by an equalizing line. A float in the right tank controlled a moving-coil instrument in the cockpit via a potentiometer. The horizontal display area was 25 mm long, divided into quarters by five marks, with the leftmost mark labeled E (Empty) and the rightmost F (Full). There was no low-fuel warning. The unusable fuel quantity was specified as 2 lbs (approximately 1.2 liters) in the Type Certificate Data Sheet. At the accident site, approximately 1 liter was found in both tanks combined. Post-accident tests at Buttwil airfield showed that when the battery was switched on, the gauge needle jumped to a position 1 mm to the right of the E mark. From 8.2 liters of fuel, the needle moved approximately linearly toward the F mark as the quantity increased. The overfill automatic activated at 102.8 liters. Further filling to a maximum of 125.9 liters did not move the needle further. This static measurement did not account for flight attitude, vibrations, voltage fluctuations, or gauge hysteresis. The helicopter was equipped by the manufacturer with a wooden dipstick marked in quarters, located in the cockpit, which allowed for a precise check of the fuel level. The AIP Switzerland listed the operating hours for Luzern-Beromünster airfield as 0700 to 1200 hours and 1400 hours to sunset (MEZ). ## Assessment The pilot decided to land at Luzern-Beromünster to refuel shortly before reaching his destination, indicating he was aware that the available fuel quantity was tight for a safe continuation of the flight to Buttwil. In his considerations regarding a possible continuation to Buttwil, a crucial element was missing: he did not know that the helicopter had already flown for 44 minutes before he took over and had not been refueled. This made a reliable estimate of consumption based on flight time impossible. The accuracy of the fuel gauge was not sufficient to safely utilize the last part of the fuel supply. It was therefore correct that the pilot decided on a precautionary landing after noting the gauge dropped below 1/4. A check of the fuel supply before takeoff at Beromünster could have been reliably performed using the dipstick, but the pilot did not consider this option. The characteristic of the fuel gauge, which reaches its limit at approximately 103 liters, was misleading. Interrupting the refueling when the overfill automatic activated was impractical, as it left the pilot unaware of the actual fuel quantity after refueling. After landing at Luzern-Beromünster due to the tight fuel supply, the pilot's decision to continue the flight was objectively wrong. The commitment to hand over the helicopter to another pilot at 1400 MEZ influenced the decision to continue. After the engine failure in flight, the pilot's actions were appropriate: he immediately initiated an autorotation and flew to a suitable meadow for landing. The damage upon touchdown was attributed to the terrain slope and the pilot's relatively limited helicopter experience.