Accident Description
At approximately 1630 mountain daylight time, a helicopter (serial number 610993D) was engaged in transporting a driller's helper to connect water hose sections on a steep mountainside. After completing the first hose connection, the pilot initiated a climb-out. During the climb, the pilot perceived abnormal cyclic control response. He instructed the helper to "hang on," then descended rapidly and attempted to land on the steep talus slope. The main rotor blades struck the hillside, severing the tail boom and detaching. The helicopter rolled onto its right side and slid downslope, tearing off the cabin roof and the front of the transmission cowl. The helper, who was not wearing a lap belt, was thrown onto the rocky slope. The helicopter then rolled onto its left side and continued sliding backward. The pilot, wearing a helmet, was hanging inverted and his head was dragged against the rocks. The helicopter finally came to rest on its side.
Injuries and Rescue
The injured helper attempted to assist the seriously injured pilot out of the front seat, managing only to unfasten the pilot's lap belt. The helper crawled upslope and called for help from the driller. When he returned, the pilot had crawled out of the cabin. First-aid assistance arrived shortly thereafter. The driller reportedly turned off the helicopter's electric start (fuel) pump. Both occupants were evacuated by aircraft to hospitals for treatment.
Weather and Helicopter Condition
The accident site was at approximately 5,000 feet above sea level. Weather conditions were overcast with light snow, visibility of ½ mile, temperature between minus three and minus four degrees Celsius, and wind from the south at five mph. Snow had been falling throughout the day. The helicopter had been operating in light falling snow. The pilot had not selected engine anti-ice ON, contrary to the Flight Manual requirement for operations when OAT is below 5°C and visible moisture is present. The helicopter was not equipped with the optional engine air inlet deflector kit. The pilot reported no snow buildup on the exterior surface, but plenum inspections for snow accumulation were not conducted because the helicopter was refueled while running during work.
Engine Examination
Examination of the helicopter revealed no mechanical discrepancies or evidence of fuel contamination. The fuel load was between 100 and 150 pounds of Jet B. The Allison 250 C20B engine compressor had been damaged by debris and was contaminated with a fine gray powder identified as bentonite, a drilling mud product. Bentonite was found in the particle separator, around the bleed valve outlet, and throughout the axial and centrifugal compressor sections and combustion chamber. The manufacturer had not previously encountered bentonite, but noted that similar deposits from atmospheric pollution had caused a loss of compressor efficiency of up to 14%. However, an engine power check conducted 45 hours earlier had exceeded specifications. The bentonite was not used at the accident site, suggesting ingestion elsewhere. The effect on engine performance was not determined. After replacement of the compressor, the engine ran successfully without power fluctuations. A bleed valve was slow to close, attributed to post-occurrence contamination.
Pilot Experience and Actions
The pilot had over 13,000 total flying hours, including over 8,000 hours on helicopters and over 2,600 hours on McDonnell-Douglas 369 helicopters in similar operations. He was considered highly experienced. During the flight, he had removed the left door for vertical reference work and was not using the shoulder harness. The engine auto-relight system was ON, and engine anti-ice was OFF. The estimated helicopter weight was 2,048 pounds, with a hover ceiling out of ground effect performance of 15,800 feet. The pilot decided to attempt a forced landing on the slope because he believed flight control problems might worsen if he tried to fly downslope to flatter ground. His helmet effectively reduced the severity of his head injuries. The passenger would likely have sustained fewer injuries if he had worn his lap belt.
Causal and Contributing Factors
The helicopter experienced a sudden loss of engine power (flame-out) during climb-out, likely as the result of the dislodging of a snow deposit in the air inlet plenum. A contributing factor was that the optional engine air intake deflector kit had not been installed on this helicopter. The engine compressor contamination with bentonite would not be expected to cause a flame-out. The sudden loss of engine power was most likely due to snow ingestion, consistent with experiences of other operators of this model. Subsequent to the occurrence, a safety advisory was issued to Transport Canada recommending review of MD 369 Series requirements for flight in falling or blowing snow, and assessment of the safety benefit of making the air deflector kit mandatory.