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Out-of-limit weight-and-balance and optical illusions contributed to a 2016 Beaver accident near Laidman Lake, British Columbia

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Richmond, British Columbia, 24 January 2018 – In its investigation report (A16P0180) released today, the Transportation Safety Board of Canada (TSB) found that an out-of-limit weight-and-balance condition and optical illusions associated with low-altitude flight in snow-covered, sloping terrain likely contributed to the aerodynamic stall and loss of control of a de Havilland DHC-2 Beaver aircraft near Laidman Lake, British Columbia. As a result, the pilot was fatally injured, and two passengers sustained serious injuries. The other two passengers sustained minor injuries. There was no post-impact fire, but the aircraft was substantially damaged.

On 10 October 2016, at approximately 0820 Pacific Daylight Time a privately operated de Havilland DHC-2 Beaver aircraft on amphibious floats left Vanderhoof Airport for Laidman Lake, both in British Columbia, for a hunting expedition. At about 12 nautical miles (nm) away from the destination, the pilot turned the aircraft to fly over a mining exploration site located on higher terrain east of the lake. The aircraft continued to fly at a constant altitude over the rising terrain for about four minutes until it was just about 100 feet above the trees. As the aircraft was now too low and too slow to climb further, the pilot banked it steeply to the left toward lower terrain. The aircraft rolled abruptly from side to side, then struck the trees and ground. The emergency locator transmitter (ELT) on-board activated on impact, the signal was detected, and a search and rescue operation was initiated.

The investigation determined that at the time of the occurrence, the aircraft was 682 pounds over its maximum weight, its cargo was not secured, and its centre of gravity exceeded the aft limit by 3.1 inches. This out-of-limit weight-and-balance condition increased the aircraft's stall speed and degraded its climb performance, stability, and slow-flight characteristics. During impact, the forward shifting of the unsecured cargo and the partial detachment of the rear seats resulted in injuries to the passengers.

The investigation also determined that, as the aircraft approached the mountain ridge, the high overcast ceiling and uniform snow-covered vegetation were conducive to optical illusions associated with flight in mountainous terrain. These illusions likely contributed to the pilot misjudging the proximity of the terrain and inadvertently adopting an increasingly nose-up attitude, without detecting the declining airspeed, before banking the aircraft to turn away from the hillside. As the angle of bank increased during the turn, the stall speed also increased and the aircraft entered an accelerated stall. These conditions, coupled with the low altitude, likely prevented the pilot from regaining control of the aircraft before it struck the ground.

The absence of a stall warning system deprived the pilot of the last line of defence against an aerodynamic stall and the subsequent loss of control of the aircraft. In 2017, the Board recommended that all commercially operated DHC-2 aircraft in Canada be equipped with a stall warning system. Although the response to Recommendation A17-01 has not been assessed yet, today's report underscores the benefits of equipping all DHC-2s with a stall warning system to reduce the risk of injuries or death from a stall at low altitude.

out-of-limit weight-and-balance condition and optical illusions associated with low-altitude flight in snow-covered, sloping terrain likely contributed to the aerodynamic stall and loss of control of a de Havilland DHC-2 Beaver aircraft near Laidman Lake, British Columbia. As a result, the pilot was fatally injured, and two passengers sustained serious injuries. The other two passengers sustained minor injuries. There was no post-impact fire, but the aircraft was substantially damaged.

On 10 October 2016, at approximately 0820 Pacific Daylight Time a privately operated de Havilland DHC-2 Beaver aircraft on amphibious floats left Vanderhoof Airport for Laidman Lake, both in British Columbia, for a hunting expedition. At about 12 nautical miles (nm) away from the destination, the pilot turned the aircraft to fly over a mining exploration site located on higher terrain east of the lake. The aircraft continued to fly at a constant altitude over the rising terrain for about four minutes until it was just about 100 feet above the trees. As the aircraft was now too low and too slow to climb further, the pilot banked it steeply to the left toward lower terrain. The aircraft rolled abruptly from side to side, then struck the trees and ground. The emergency locator transmitter (ELT) on-board activated on impact, the signal was detected, and a search and rescue operation was initiated.

The investigation determined that at the time of the occurrence, the aircraft was 682 pounds over its maximum weight, its cargo was not secured, and its centre of gravity exceeded the aft limit by 3.1 inches. This out-of-limit weight-and-balance condition increased the aircraft's stall speed and degraded its climb performance, stability, and slow-flight characteristics. During impact, the forward shifting of the unsecured cargo and the partial detachment of the rear seats resulted in injuries to the passengers.

The investigation also determined that, as the aircraft approached the mountain ridge, the high overcast ceiling and uniform snow-covered vegetation were conducive to optical illusions associated with flight in mountainous terrain. These illusions likely contributed to the pilot misjudging the proximity of the terrain and inadvertently adopting an increasingly nose-up attitude, without detecting the declining airspeed, before banking the aircraft to turn away from the hillside. As the angle of bank increased during the turn, the stall speed also increased and the aircraft entered an accelerated stall. These conditions, coupled with the low altitude, likely prevented the pilot from regaining control of the aircraft before it struck the ground.

The absence of a stall warning system deprived the pilot of the last line of defence against an aerodynamic stall and the subsequent loss of control of the aircraft. In 2017, the Board recommended that all commercially operated DHC-2 aircraft in Canada be equipped with a stall warning system. Although the response to Recommendation A17-01 has not been assessed yet, today's report underscores the benefits of equipping all DHC-2s with a stall warning system to reduce the risk of injuries or death from a stall at low altitude.

TSB

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