What happened
On November 23, 2008, a Eurocopter AS355F1, registration OO-HSB, was performing a Helicopter Emergency Medical Service (HEMS) mission near Zuien/enkerke, Belgium. The aircraft, operated by Heli Service Belgium, was dispatched to assist with a medical emergency at a holiday center.
Shortly after takeoff, while cruising at approximately 350 feet, the crew experienced a continuous low rotor RPM warning. The pilot observed a significant power imbalance: the left engine was producing only 10% torque, while the right engine was operating at maximum continuous power. As the aircraft began to lose altitude, the pilot attempted to regain RPM by performing a flare and reducing collective pitch.
Unable to maintain altitude and facing a continuous loss of power, the pilot executed an emergency landing in a nearby field. The helicopter approached the landing site with a tailwind. Despite attempting to flare to reduce speed, the aircraft struck a ditch at high velocity. The impact caused the helicopter to skid and eventually tip onto its right side, resulting in the destruction of the airframe and the separation of the tail rotor.
The investigation
The AAIU(Be) investigation focused on the engine performance, meteorological conditions, and the aircraft's preparation for flight. Investigators examined the engine components and found damage to the first compressor stage of the left engine.
Meteorological data confirmed that the aircraft was operating in icing conditions, characterized by temperatures below +5°C and visible dampness. The investigation also reviewed the aircraft's preparation, noting that while the pilot had cleared most snow from the fuselage, the rotor blades and engine intake grids had not been wiped or protected with covers or socks. Furthermore, the investigation analyzed the impact of the aircraft's systems, specifically the activation of the engine anti-icing and the cabin heater, which both utilize engine bleed air and reduce available power.
Findings
- The primary cause of the accident was the failure to follow established procedures for protecting the aircraft against icing conditions.
- The left engine likely ingested snow or ice, which caused damage to the compressor and a subsequent loss of power.
- The activation of the engine anti-icing system and the cabin heater further reduced the engine's ability to maintain altitude.
- The aircraft was left outdoors in freezing conditions without intake covers or blade socks, allowing for ice accumulation on the engine grids and rotor blades.
- The emergency landing was complicated by a tailwind and the presence of a ditch in the landing area, which led to the aircraft's rollover.
Safety action
- The AAIU(Be) recommends that Heli Service Belgium implement a sensitization program for its pilots regarding the strict adherence to flight manual and company procedures during cold weather operations.
- The importance of recurrent training for winter operations was emphasized to ensure pilots are prepared for the complexities of flying in icing conditions.