Aviation Safety Letter 2/2004

Collision with Mountain

Collision with Mountain

On September 28, 2002, a de Havilland DHC-3 Otter took off from Lac de l'Avion, Quebec, near Natashquan Airport, at approximately 10:50 eastern daylight time (EDT) on a flight to a hunting camp 57 mi. to the north, along the Aguanish River. The pilot and three passengers were on board. Upon arriving at the destination at approximately 11:35, the aircraft flew over part of the neighbouring forest before crashing upside down on rugged ground. The pilot survived and the passengers were fatally injured on impact. This synopsis is based on the Transportation Safety Board of Canada (TSB) Final Report A02Q0130

The pilot had logged about 7 980 hr of flying time, with almost 7 800 hr on aircraft equipped with floats or skis. As the company chief pilot, he was responsible for professional standards for the flight crew under his authority. Weather conditions during the first 40 nautical miles (NM) allowed for a direct course. However, for the rest of the distance, clouds frequently came down to the mountain tops, forcing the pilot to make a few detours. The weather conditions at Natashquan Airport improved from 10:18 to 11:43, and the cloud base rose from 600 ft to 5500 ft above ground level (AGL); however, in mountainous terrain, it was possible for clouds to remain longer.

The mountain bordering the north side of the Aguanish River at the hunting camp is very steep. The pilot had not been to the location for a year and flew over the camp to assess the landing area. He also noticed moose tracks on the bank and initiated a turn to the left to show the passengers. The turn was done at approximately 95 mph. It seemed to the pilot that, during the turn, close to the mountain, the aircraft drifted toward the mountain. After almost completing a 360° turn, the pilot felt vibrations that he associated with wake turbulence. Because the aircraft seemed to want to sink, he applied full power. The left wing hit the tops of several trees, and the aircraft flipped before crashing upside down on the slope of the mountain.

The aircraft cut a 460-ft-long swath through the trees along a constant left curve. At first, only the treetops were involved, but the aircraft quickly dropped, cutting the trees progressively closer to the ground. The aircraft's speed fell and the left wing tore off. The take-off weight was within prescribed limits, and the aircraft was equipped and maintained in accordance with existing regulations. The TSB determined that the engine was producing high power. The continuity of the control cables could still be established - despite the separation of the left wing - because the cables did not break. The pilot had not noticed anything unusual about the aircraft's operation before the accident.

The manufacturer was contacted to determine the consequences of wake turbulence on a flight trajectory if an aircraft of this type crossed its own wake turbulence after making a 360° turn. According to the experts, even in still air, it is hard to cross one's own wake turbulence. Furthermore, even if this had occurred, the bumpiness felt would have been minimal and immaterial.

The onboard GPS had the capacity to save the last five flights in memory. Analysis of this data showed that for the first 32 NM, the aircraft followed a constant course. However, the aircraft followed the Aguanish River to its destination for the last 25 NM. A more detailed analysis of the last three points recorded by the GPS indicated that the aircraft's bank was between 18° and 35° for most of the 360° turn.

Analysis - As the pilot progressed to the final destination, the ceiling dropped. He chose to follow the course of the Aguanish River to reach the destination because clouds were touching the tops of the mountains. At the destination, he made a 360° turn. Because the mountain was sloped at 40° and clouds obscured the top, it seems that the pilot had trouble judging the horizontal and vertical distance between himself and the mountain. He noticed only at the last moment that he had drifted much too close to the mountain.

As he was approaching the mountainside, the pilot felt vibrations, probably from the first impact with the treetops. Although he increased engine power, he could not get out of the predicament because of the low ceiling, the proximity of the mountain, and a bank angle that he could not increase. Because of the geographic and weather conditions, the pilot probably had trouble judging his horizontal and vertical distance with respect to the mountain, and the aircraft crashed.

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