Accident Overview
A helicopter operating on a United States Forest Service (USFS) prescribed fire mission lost control while in a slow flight phase out of ground effect. The aircraft subsequently impacted heavily wooded terrain, where trees were approximately 100 to 110 feet tall. The prescribed fire was supported by the application of aerial ignition spheres dispensed from a cabin-mounted plastic sphere dispenser (PSD) machine. According to USFS operating procedures, PSD missions are typically flown at altitudes of 50 to 300 feet above the highest vegetation at airspeeds between 20 and 40 knots.
Sequence of Events
During the mission, a radio distress call was heard on the mission frequency. The call was "Mayday, Mayday, Mayday, we are going down." USFS personnel reported that the voice making the distress call appeared to be that of the ignition specialist on board, not the pilot. No further communications were heard from the helicopter.
Investigation Findings
Post-accident examinations of the engine, fuel control, and power turbine governor did not reveal conclusive evidence of mechanical failure. Similarly, examination of the airframe did not find evidence of a control system failure. Fuel calculations determined that the helicopter should have had about 36.9 gallons of usable fuel on board at the time of the accident.
Evidence at the accident site showed some rotation signatures of dynamic drive components, although a power level could not be determined. The USFS inspection of the PSD machine found no evidence of a malfunction that could have contributed to the accident. The helicopter was within weight and balance limits for the type of operation. The reason for the loss of control could not be determined.