Background
On 18 November 1999, a Eurocopter AS 350 B2 helicopter was involved in an accident near Reuth. The helicopter was conducting forestry flights following a two-day period of winter weather with snowfall.
Flight Sequence
After the pilot performed a pre-flight inspection, the helicopter was refueled from a tanker truck. The engine was started, and following a warm-up period of approximately 12 minutes, the helicopter took off with an empty external load container. Shortly after departure, a member of the ground crew observed a black-and-white smoke cloud emanating from the engine exhaust pipe. The crew member reported this observation to the pilot via radio, and the pilot responded, "I have engine failure." At that time, the helicopter was at an altitude of 25 to 30 meters above a forested area. The pilot released the external load and initiated an autorotation toward a suitable emergency landing site at the forest edge. During the approach to the landing area, the helicopter passed through trees and shrubs before impacting the ground hard on open terrain near the forest edge.
Investigation Findings
The on-site technical examination of the helicopter found no evidence of a technical defect. The bypass valve of the cell-side main fuel filter had not opened. Fuel samples taken from both the tanker truck and the helicopter were analyzed for contamination and revealed a variety of solid foreign materials, undissolved water, and microbiological contamination. The engine was examined under the supervision of the French accident investigation authority at the manufacturer's facility; no accident-relevant technical defects were found, and no contamination was present within the engine fuel control unit.
The investigation considered the possibility that snow ingestion could have caused the engine failure. When helicopters are parked outside during snowfall, snow can accumulate in unfavorable locations inside or outside the cowlings. If not detected and removed before flight, such accumulations can dislodge during flight due to high compressor suction or changes in attitude. Depending on engine power, even small amounts of snow can extinguish the engine flame. This scenario could not be completely ruled out in this case, especially given the witness report of a black-and-white smoke cloud from the exhaust. However, no other evidence supported this theory. The pilot stated that the helicopter had been parked with all covers installed and that he carefully removed any snow residues during the pre-flight check.
Conclusions
The cause of the engine failure could not be determined. After the failure, the accident was unavoidable because the helicopter was at low altitude over forested terrain and a suitable emergency landing site was not reachable. The contact with obstacles during the approach caused severe damage to the helicopter and led to an uncontrollable flight state, resulting in the hard impact.