Accident Summary
On June 25, 1999, an Enstrom F28 A-D helicopter was conducting a commercial sightseeing flight from Rotenburg/Wümme, Germany. The helicopter took off at approximately 17:08 local time for a planned 30-minute flight with one pilot and one passenger on board. About 12 minutes into the flight, at an altitude of roughly 1200 feet and a speed of 70 knots, the pilot noticed uncontrollable yawing movements. He initiated an autorotation landing onto a grain field near Verden/Aller. During the landing, the tail rotor struck the ground, causing the helicopter to spin before coming to rest. The aircraft sustained heavy damage, and there was minor crop damage. No injuries occurred.
Investigation Findings
Technical examination revealed that after the forced landing, only 1.5 liters of fuel remained in the tanks, and the fuel lines to the engine were empty. The investigation concluded that the accident was initiated by an engine failure due to fuel starvation. Several consistent witness reports indicated that the engine failure was not sudden but intermittent.
The pilot initially attributed the accident to a technical defect in the tail rotor. After landing, the fuel gauge indicated 60 lbs (38 liters), but measurements during the investigation showed that the gauge was inaccurate, sometimes displaying nearly double the actual amount. The readings varied depending on the time since refueling. The inaccuracy was either unnoticed or undocumented before the accident. The pilot was unaware of the gauge's unreliability, having limited flight experience on the Enstrom F-28 model.
The helicopter's fuel gauge was known to be generally inaccurate and unreliable. The model's translucent plastic fuel tanks allowed visual estimation of fuel level, but on this particular helicopter, the plastic had become discolored with age, making visual estimation impossible. This condition was either unnoticed or not documented during the annual inspection and 100-hour check on May 6, 1999. No dipstick was available at the company.
During the preflight inspection, the pilot claimed to have visually checked the tank level and compared it with the fuel gauge, which indicated 120 lbs (76 liters). He stated that both readings matched. However, the investigation determined that the actual fuel at that time was likely only 30-40 lbs (19-25 liters), indicating a significant estimation error.
Probable Cause
The accident was caused by an engine failure due to fuel starvation, which resulted from an incorrectly performed pre-flight check by the pilot. A faulty fuel gauge and the deteriorated condition of the fuel tank contributed to the incident.