What happened
On June 25, 1999, an Enstrom F28 A-D helicopter was conducting a commercial sightseeing flight departing from Rotenburg / Wümme. Approximately 12 minutes into the flight, while cruising at 1,200 feet with a speed of 70 knots, the pilot experienced sudden, uncontrollable yawing motions. The pilot attempted to stabilize the aircraft by initiating an autorotation toward a grain field. During the landing, the tail rotor struck the ground, causing the helicopter to spin and resulting in heavy damage to the aircraft. There were no injuries to the pilot or the passenger.
The investigation
The BFU investigation focused on the cause of the sudden yawing and the subsequent engine failure. Technical examination of the aircraft following the crash revealed that only 1.5 liters of fuel remained in the tanks, and the fuel lines to the engine were empty. While the pilot initially suspected a tail rotor malfunction, investigators established that the engine failure was the primary driver of the event.
Investigators found that the fuel gauge on this aircraft model is notoriously unreliable and prone to significant errors. In this specific instance, the gauge indicated 6-0 lbs (approximately 38 liters) remaining, whereas the actual amount was much lower. The investigation also noted that the plastic fuel tanks had become cloudy due to age, making it impossible to visually estimate the fuel level from the outside. Furthermore, the operator did not have a dipstick available to manually verify fuel levels, and the aircraft lacked a secondary, independent fuel warning system.
Findings
- The primary cause of the accident was an engine failure due to fuel exhaustion.
- The pilot performed a pre-flight check by visually inspecting the tank, which he believed matched the gauge reading of 120 lbs (76 liters); however, the actual fuel level was likely between 30 and 40 lbs (19 to 25 liters).
- The intermittent nature of the engine failure—rather than a sudden shutdown—caused the aircraft to yaw in alternating directions, which misled the pilot into believing there was a tail rotor issue.
- Contributing factors included an inaccurate fuel gauge and the inability to visually monitor fuel levels through the clouded, aged plastic tanks.
- The lack of a manual measuring stick or a secondary fuel warning system prevented the detection of the low fuel state.