The Bering Air Textron Aviation Cessna 208B that crashed in February, killing 10 people, had exceeded the maximum take-off weight (MTOW) allowed in icing conditions and had slowed significantly in the moments before going down.
Investigators examining the wreckage also found ice accumulated on some of the aircraft’s leading-edge surfaces, according to a preliminary report released by the US National Transportation Safety Board (NTSB).
“Minor ice accumulation was observed on the leading-edge surfaces aft of the [ice-protection] membranes. Significant ice accumulation was observed on the base of the beacon/strobe light located at the top of the vertical stabiliser,” says the NTSB’s report, updated on 21 March.
The Cessna 208B crashes on 6 February when flying from Unalakleet on Alaska’s west coast, across Norton Sound in the Bering Sea, to Nome. Those cities are about 145 miles (233km) apart.
The NTSB says the aircraft had been equipped with a “TKS ice protection system” composed of “porous titanium panels” installed on the leading edges of its wings and wing struts, and along its horizontal and vertical stabilisers.
It had also been modified with a package called the “Aircraft Payload Extender III”, which increased its MTOW from 8,807lb (3,995kg) to 9,062lb.
However, when flying in icing conditions, the 208B’s MTOW was not to exceed the baseline 8,807lb figure, according to the aircraft’s pilot operating handbook, says the NTSB.
“The preliminary weight calculations for the accident flight indicated that the gross take-off weight was about 9,776lbs,” the report says. “This was about 969lbs over the maximum take-off gross weight for flight into known or forecast icing conditions under the TKS system supplement.”
The agency is unsure if the TKS system had been filled with sufficient ice-protection fluid. Doing so is the pilot’s responsibility, and regulations do not require such records be kept, says the NTSB.
The aircraft, operating Bering Air flight 445, had taken off from Unalakleet at 14:37 local time. It climbed to 8,000ft for the cruise across Norton Sound to Nome.
At 15:12, however, a controller told the pilot that Nome’s runway would be closed for 10-15min to be de-iced, prompting the pilot to slow the Cessna during decent.
The aircraft’s airspeed slipped to 99kt, and then, when at about 3,100ft, to 70kt, the NTSB says.
The final report from the aircraft’s ADS-B system came in seconds later.
Responders located the aircraft’s wreckage about 34 miles southwest of Nome. The crash killed all nine passengers and the single pilot on board.
At the time of the accident, winds at Nome were calm. Light snow was falling from an overcast sky and the temperature was -10°C (14°F), says the NTSB.
Bering Air did not respond to a request for comment.
