FALL OUT FOUND AT 

 BONNERS FERRY AIRPORT 

 9/1/67 



Figure 15. - Debris 

 and possible 

 firebrand material 

 found at Bonners 

 Ferry airport, 

 September 1, 1967. 



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of this debris was widespread over the area 

 northeast of the fire. Much of it was scorched 

 or blackened but the particles had not burned 

 to completion. Neither had they successfully 

 ignited their surroundings. Reports of burning 

 material falling near residences in the 

 Kootenai Valley were received but none of 

 this material was positively identified as burn- 

 ing when it landed. 



What kind, size, and shape of burning 

 material could most likely have been carried 

 such distances? Work done in Spain by Tarifa 

 (8) was examined. He investigated the flight 

 paths of spheres, cylinders, and rectangular 

 plates and their burning characteristics to 

 determine the lifetime of firebrands and the 

 probable range. Various species of wood were 

 used and some study was made of pine cones 

 and charcoal elements. 



In studying his results, we found that 

 wood elements would have to be large and 

 carried quite high in the convection column 

 to have a 10-mile range. The burning process, 

 according to Tarifa, is glowing combustion, 

 not flaming. Flaming firebrands existed only 

 at low windspeeds. Charcoal brands have 



much longer burning times than any of the 

 woods or natural firebrands. Pine cones burn 

 much like wood once the scales have burned 

 off. 



For our study we had an estimate of the 

 wind and the distance the brands had to 

 travel. We calculated the drag coefficient, ter- 

 minal velocity, and mean velocity of fall for 

 four types of brands: (a) a charcoal sphere, 1 

 inch in diameter; (b) a charcoal cylinder, 1,75 

 inches long and 0.6 inch in diameter; (c) a flat 

 charcoal plate, 1.1 inches square and 0.43 

 inch thick; and (d) a pine cone about 3.75 

 inches long and 2.2 inches in diameter. We 

 used the techniques presented by Tarifa to 

 calculate the maximum flight time possible 

 for each brand. This we considered as the 

 time the brand would remain burning; thus 

 the particle would have to reach the ground 

 within this time period to ignite other mate- 

 rial. From the mean velocity of fall and time 

 of flight, we could calculate the height at 

 which a particle of the given size must be 

 cast — that is, the ejection altitude. In turn, 

 we could then calculate the vertical velocity 

 in the convection column required to carry 



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