What happened
On November 8, 2007, a Hughes 369D helicopter, registration OH-HLA, was performing a specialized branch-sawing mission in Kangasniemi, Finland. The aircraft was equipped with a heavy external saw weighing 369 kg, which extended 25 meters below the helicopter.
During the mission, weather conditions shifted from light snow to heavy sleet, resulting in several centimeters of accumulation. After an intermediate landing for hot refueling, the pilot resumed the sawing operation. While hovering nearly stationary to cut branches, the engine suddenly failed. Due to the heavy external load and the pilot's control inputs, the aircraft did not enter a controlled autorotation. Instead, the helicopter pivoted around the saw boom, causing it to yaw back toward its original approach path, strike trees, and ultimately crash into the ground. The pilot, the sole occupant, sustained minor injuries but was able to exit the cabin independently.
The investigation
The investigation examined the engine's performance, the aircraft's configuration, and the operator's procedures. Investigators found that the Allison 250 engine was susceptible to flame-outs if enough snow or ice was ingested, a known risk documented by the manufacturer.
Technical analysis revealed that a fatigue crack had broken a pipe supplying anti-ice air to the compressor guide vane. Additionally, the investigation found that the engine's warning system and the automatic reignition system were disconnected. The investigation also scrutinized the operator's flight manuals and weather restrictions, noting discrepancies between official company policy and actual field practices.
Findings
- The primary cause of the accident was flying in unfavorable weather conditions involving snowfall, which led to the engine ingesting wet snow accumulated on the air intake.
- A significant contributing factor was that the engine failure warning system and the automatic restart system were not activated because the 'ENG OUT' thermal circuit breaker had been pulled.
- The company's operational weather restrictions were inconsistent and did not align with actual flight practices.
- There were deficiencies in the company's oversight of personnel procedures and a failure to ensure flight manuals were up to date for specific aircraft configurations.
- The pilot's decision to continue operations during heavy sleet exceeded the company's internal safety requirements.