FireWhirls invisible danger for aerial firefighting

Compiled from original article appearing in Canadian Underwriter

Four Fire Boss AT-802A Air Tractor aircraft were in the process of releasing water onto a forest fire near Cold Lake, Canada – normal routine firefighting. The TSB (Transportation Safety Board of Canada) hinted that the pilot of aircraft wouldn’t have notice a “fire whirl” in his path, which produced severe turbulence that downed one of the four aircraft.

4 Wing Cold Lake consists of an air weapons range, as well as a couple of Royal Canadian Air Force CF-18 tactical fighter squadrons bestriding the Saskatchewan-Alberta border. The Alberta branch of wildfire management sent out squads from the staging base of Beaver Lake once the forest fire was reported. Shortly afterward, four Conair Group operating Fire Boss aircraft dropped water twice on the top border of the fire upon scooping water a couple of miles west of the city. Water was dumped from up to 200 feet over ground level. Once the water was released, the aircraft ascended back to approximately 1,000 feet AGL.

TSB reported that the fire often changed direction as it grew. One Fire Boss administered the third dump around 4:30 p.m., experiencing heavy turbulence under a minute later. Within 1.2 seconds, the aircraft’s DAAM (Data Acquisition Alarm Monitor) registered a shift from 4.8Gs to 2.1Gs. After the pilot’s head hit the canopy, the aircraft rolled right and swerved left. Its speed lowered to zero, but seconds after the turbulence hit, TSB reported that the pilot regained consciousness. 10 seconds later, the Air Boss went through turbulence of its own. Within a second, its DAAM reported a fluctuation (4.2Gs to -3.2Gs).

After the T692 ascended, it descended back down to about 400-500 feet over ground level. The plane then rolled left, followed by entering downward altitude. The T692 hit the ground with its right wing lowered. The pilot received fatal injuries.

TSB discovered that the aircraft was moving at a ground speed of 129 knots before entering a two-celled vortex. The water bomber’s DAAM data that endured the turbulence – as well as the crashed plane – show that the two-celled vortex formed when a fire whirl started.
The pilot wasn’t able to notice the fire whirl in time, since it hadn’t absorbed sufficient unconstrained debris, according to the TSB. The pilot hadn’t experienced difficulties on prior drops.

Details from Bret Butler and Jason Forthofer, mechanical engineers from the Department of Agriculture in the U.S., isolated four factors that develop fire whirls. They include erratic atmospheres, a large source of heat, a powerful rotation/vortices source and a low-to-medium ambient wind. Butler and Forthofer’s report was shown at the third Fuels and Fire Behavior Conference.

Once the Fire Boss ascended into the fire whirl, the pilot probably plunged into the overhanging covering at a negative acceleration of -3.2G, according to the TSB. Leaving the core could have once again placed the airplane’s nose in the updraft as the tail pointed in the opposite direction, resulting in the halted airplane to enter a nascent spiral.

The TSB cited entering the fire whirl as one contributing factor and cause. The other involved the low-level altitude, which wasn’t sufficient to recover from the spiral.

In March of 2016, a training session was added by the TSB that focuses on the environmental dangers of a forest fire and identifying its circumstances.

Conair also set up its AT-802 aircraft with 5-point harnesses. At present time, TSB claims that there aren’t any supervisory prerequisites in formal training for dangerous weather surrounding a wildfire.

Fire whirls are similar to dust devils on a minor level. On a large one, they are likened to tornadoes produced from a thunderstorm, according to the TSB.



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