What happened
On June 26, 1998, an Eurocopter AS 350 B2 operated by Héli-Inter Guyane was performing a transport flight over the Amazon rainforest. After dropping off passengers and cargo at Crique Tortue, the aircraft was en route to Cayenne-Rochambeau airport. Shortly after 14:00 local time, the pilot experienced an engine failure.
Following the loss of power, the pilot issued a distress call, stating the aircraft was crashing approximately ten nautical miles from Cayenne. The pilot attempted an emergency autorotation, but the aircraft's flight path involved two turns, which limited available maneuvering options. The helicopter struck the dense forest canopy with a high vertical descent rate and subsequently impacted the ground with significant force. The impact caused the cabin floor to deform severely and resulted in the death of the pilot.
The investigation
The investigation focused on the condition of the Arriel 1D1 turbomechanism and the flight dynamics during the emergency. Investigators examined the wreckage, noting that while the engine exterior appeared undamaged, internal inspections via endoscope revealed significant damage. Specifically, the compressor blades had been damaged by the ingestion of branches, and the turbine blades showed signs of metal loss and overheating, including blue discoloration and broken blades.
Analysis of the radio communications confirmed a "low rotor RPM" alarm was active during the pilot's Mayday call. The investigation also reviewed the aircraft's maintenance history, finding that the engine had been maintained according to manufacturer specifications. The wreckage analysis showed the main rotor blades were grouped in a manner characteristic of low rotor speed and high power application by the pilot attempting to manage the descent.
Findings
- The primary cause of the accident was a contained engine failure resulting from prior overheating, which was not detectable under the aircraft's operating conditions.
- The low altitude of the flight and the dense, inhospitable terrain of the Amazon rainforest left the pilot with very few landing options.
- The execution of the autorotation was compromised by a low rotor RPM and the pilot's attempt to maneuver through the trees, which led to a high-energy impact with the canopy.
- The heavy vegetation provided minimal deceleration, allowing the aircraft to continue its descent into the ground with high vertical velocity.