20th of November 2014, Icing, Blog #727
- V2Aviation
- Nov 20
- 3 min read
After a ~12-hour delay, a Cessna 208B was scheduled to operate a flight from Yellowknife Airport (CYZF) to Fort Simpson Airport (CYFS), both in the Northwest Territories, Canada. The delay was caused by freezing drizzle that was reported in Fort Simpson the night before.
With 2200 pounds of fuel, a pilot and five passengers, the aircraft lifted off from Yellowknife at approximately 0642lt (local time), with an IFR clearance to 8000 feet above sea level (asl) direct to Fort Simpson. During the climb to 8000 feet, the flight encountered some icing. After levelling off, cruise power was set, with a maximum airspeed of 120 knots.
At 06:59 LT, the pilot contacted the Edmonton, Alberta, Area Control Centre (ACC) and requested a lower altitude, as he was unable to maintain 120 KIAS. This was approved by ATC with a descent to 6000 feet asl. During the descent, speed continued to decline, even with the application of maximum continuous power. The pilot subsequently requested a return to Yellowknife, which ATC approved at 3000 ft asl. The pilot experienced periods of elevator buffeting and uncommanded forward pitch movements during the turn. He continued the descent to maintain 110 KIAS. Flap position remained fully up, as the pilot was concerned that movement would further affect tailplane effectiveness. After being handed over to Yellowknife tower, the pilot informed ATC that the flight was in severe icing and was unable to maintain altitude. A Mayday was declared.
At 0720, when the aircraft was 300 feet above ground level (agl), the pilot experienced a series of wing drops and an associated rate of descent of 1200 feet per minute at 100 to 110 KIAS. In response, the power lever was pushed fully forward, exceeding the maximum continuous rating of 1865 foot-pounds of torque. No flap selections were made.
While still in darkness, the aircraft contacted the frozen surface of Great Slave Lake at 0721, and continued moving for 2300 feet before it struck a rock outcropping with the nose and left main landing gear. The aircraft came to rest approximately 600 feet from the outcropping and 2900 feet from the initial touchdown point. There were no injuries to the occupants, but the aircraft was severely damaged. There was no post-impact fire.
Passenger evacuation was ordered once the pilot had assessed the situation. Passengers attempted to open the main cabin door but were unable to do so. After several failed attempts to evacuate, the passengers succeeded in exiting via the left cockpit door with the aid of cockpit lighting. Cabin lighting had not been turned on.
The accident was investigated by the Transportation Safety Board of Canada (TSB). They listed the following findings as to causes and contributing factors;
Not using all en route information led the pilot to underestimate the severity and duration of the icing conditions that would be encountered.
Inadequate awareness of aircraft limitations in icing conditions and incomplete weight and balance calculations led to the aircraft being dispatched in an overweight state for the forecast icing conditions. The aircraft centre of gravity was not within limits, and this led to a condition that increased stall speed and reduced aircraft climb performance.
The pilot’s expectation that the flight was being undertaken at altitudes where it should have been possible to avoid icing or to move quickly to an altitude without icing conditions led to his decision to continue operation of the aircraft in icing conditions that exceeded the aircraft’s performance capabilities.
The severity of the icing conditions encountered and the duration of the exposure resulted in reductions in aerodynamic performance, making it impossible to prevent the descent of the aircraft.
The inability to arrest the descent of the aircraft resulted in the forced landing on the surface of Great Slave Lake and the collision with terrain.
The pilot self-dispatch system employed by Air Tindi did not have quality assurance oversight or adequate support systems. This contributed to the aircraft being dispatched in conditions not suitable for safe flight.
The investigation report, which served as the source for this blog, can be accessed by clicking on the .pdf file below;
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