What happened
On April 2, 2018, a Bell 206L-1 helicopter was performing a short landing pattern at the Uetersen/Heist (EDHE) airfield to refuel for a planned ferry flight. After a routine pre-flight inspection, the pilot started the engine and departed from runway 27, following a Cessna 172.
During the downwind leg, as the pilot increased pitch to accelerate and climb near the threshold of runway 09, the engine experienced a malfunction characterized by a "stuttering" sensation. The pilot immediately turned back toward the airfield and notified the tower, but the engine subsequently failed completely. Realizing the airfield was no longer reachable, the pilot initiated an autorotation. To avoid a forward impact into the trees, the pilot performed a 18/0-degree yaw maneuver at the last moment, causing the aircraft to drop vertically into a wooded area. The helicopter sustained heavy damage to the airframe, and while there were no injuries to the pilot, the impact caused significant damage to the surrounding forest.
The investigation
The BFU examined the aircraft and the engine at the crash site and subsequently at a maintenance facility. Investigators found that the tail rotor shaft was twisted and severed, and the main rotor blades had sustained multiple impacts. While the engine showed no external signs of damage and the internal components (N1 and N2 driveshafts) were functional, investigators noted several irregularities regarding maintenance markings. Specifically, there was a notable absence of torque seal (witness marks) on many engine bolts.
Upon inspecting the fuel system, investigators discovered a leak at the connection between the fuel line and the fuel filter. The screw connection was found to be only finger-tight. Subsequent testing by a maintenance facility and the engine manufacturer confirmed that even a low torque application could lead to a leak under operating pressure, and the engine performed normally during a test stand run in the USA.
Findings
- The engine failure was caused by a leak at the fuel line connection between the fuel filter and the engine fuel pump.
- The screw connection at the fuel filter was found to be insufficiently tightened.
- There was a lack of torque seal/witness marks on various engine fasteners, which would have allowed for the visual detection of loosened hardware.
- Maintenance records did not indicate when the fuel line connection at the fuel filter was last disturbed or tightened.