What happened
On May 17, 1999, a Eurocopter SA 365N Dauphin 2, registration LN-OLT, was conducting an air ambulance mission from Tromsø to Sifjord. While cruising at approximately 800 feet near Brokskar, the aircraft encountered strong turbulence and a horizontal wind vortex. This sudden atmospheric disturbance caused the helicopter's nose to pitch downward sharply. During this maneuver, the horizontal stabilizer suffered a structural failure, with both sides of the component ruptably breaking away from the tail boom.
In response to the pitch-down, the pilot applied rearward cyclic control. This movement, combined with the effects of the Stability and Control Augmentation System (SCAS), caused severe rotor blade flapping, which resulted in the main rotor blades making contact with the main gearbox cowling. The crew, unaware of the extent of the structural damage, continued the flight to Sifjord. It was only upon landing that the crewman discovered that large sections of the horizontal stabilizer and both vertical fins were missing.
The investigation
Investigators from the Accident Investigation Board Norway (AIBN) examined the wreckage and recovered portions of the stabilizer from the shore of Kvaløya. The investigation focused on the aerodynamic loads experienced during the encounter and the structural integrity of the stabilizer. Technical analysis included wind simulations and an evaluation of the aircraft's certification standards under FAR 29.
Findings
- The primary cause of the accident was the rupture of the horizontal stabilizer due to aerodynamic loads encountered during a period of turbulence and wind vortices.
- The investigation found that the wind conditions, while strong enough to cause the failure, were likely within the established FAR 2-9 certification limits, suggesting that the design and certification requirements may not adequately account for the specific aerodynamic complexities of this aircraft type, such as dynamic stall or the effects of additional vertical fins.
- While Eurocopter's analysis suggested the failure resulted from extreme vertical gusts, the AIBN could not rule out the possibility that the structure had been previously weakened by aging or internal damage not visible during standard composite inspections.
Safety action
Following the investigation, several safety recommendations were issued to the Norwegian Civil Aviation Authority, including:
- Evaluating the need for revised airspeed limits in the Aircraft Flight Manual for flight in turbulent conditions.
- Reviewing maintenance and inspection procedures, specifically the effectiveness of the "coin tapping" method for detecting internal composite damage.
- Assessing whether the implementation of certain Service Bulletins regarding rotor-to-fuselage clearance should be made mandatory.