What happened
On 11 September 2007, a Eurocopter EC155B1 helicopter, registration OY-HSL, was performing a passenger flight from the "Nobel George" oil platform in the North Sea toward Den Helder Airport. The aircraft was operating under instrument flight rules at an altitude of 5,000 feet with the autopilot engaged.
During the cruise phase, the helicopter experienced a sudden and rapid change in heading. This aerodynamic disturbance caused the autopilot to disengage automatically and triggered several cockpit warnings, including master warnings and alarms related to roll, pitch, yaw, and collective parameters. The crew, unaware of any structural damage, performed various system checks. Finding no obvious mechanical anomalies, they continued the flight manually. Upon landing at Den Helder Airport, air traffic control informed the crew that a section of the aircraft's vertical tailfin was missing. There were no injuries among the two crew members or the nine passengers on board.
The investigation
The Dutch Safety Board analyzed data from the flight data recorder and cockpit voice recorder to reconstruct the flight path. The investigation established that the sudden heading deviation occurred at approximately 0 9:17 hours. Following the event, the aircraft's upper tailfin part was found washed ashore on the island of Terschelling and subsequently recovered.
Technical examination of the remaining lower tailfin revealed a fatigue crack on the forward attachment fitting. While the aft fitting showed signs of failure, the damage was consistent with an overload caused by the initial failure of the forward component. The investigation also noted that similar cracks had been discovered during inspections of other EC155B aircraft, despite the components meeting manufacturer material specifications.
Findings
- The primary cause of the loss of the upper tailfin was a fatigue crack located on the forward attachment fitting of the lower tailfin section.
- The failure of the forward fitting led to the separation of the upper tailfin, which subsequently placed an unsustainable overload on the aft attachment fitting, causing it to fail.
- The crew was unaware of the structural failure during the flight, as the symptoms were initially interpreted as an autopilot malfunction.