It takes cool nerves and more than a little experience to fly a giant, 650,000 pound 747 jumbo jet over jagged mountain ridges, then descend at 170 miles an hour through turbulent smoke, down to within 200 feet of the treetops, following the rising and falling terrain, and deposit up to 18,000 gallons of retardant in a line precisely ahead of an advancing wildfire.
“Our goal,” said pilot Marcos Valdez, who flew the specially modified 747 Supertanker here recently, “is to buy time for the firefighters working on the line”.
Taking off from McClellan Air Force Base near Sacramento, Valdez and four other pilots who work for Global Supertanker Services LLC under contract to Cal Fire made the round trip to Sonoma County in the 747 Supertanker in about 40 minutes. In five days they flew 25 sorties to the Atlas and Tubbs fires, and made nearly 45 drops.
By laying a pink barrier of retardant directly on the brush and trees in front of the flames, the supertanker can slow the fire’s advance. After the load is released, the pilot pushes the throttles forward to quickly add power to the four engines, climbs hard away, heads back and reloads, and does it again.
Their Global Supertanker, the Spirit of John Muir, was just one of a fleet of small and large aircraft working to slow and suppress the complex of fires in Sonoma and Napa counties in October. Performing the air drops requires a carefully coordinated choreography. In the air, the overall control of where to send aircraft, deciding where the fire is heading, keeping ground crews safely away from the drop zones, and avoiding collisions with criss-crossing lines of traffic, falls to the Cal Fire Air Attack Coordinator.
Sonoma County residents and evacuees watching the aircraft and their drops were relieved and heartened to see them in action, after dense smoke and wind conditions in the first two days of the fire kept the air team grounded. But most observers weren’t aware the vivid pink retardant doesn’t actually put out the fire.
The retardant is a special slurry, brightly colored so it can be seen from the air, manufactured under the brand name Phos-Chek. Mostly water, it contains some additives, which work as thickeners to make it stick to trees and leaves (crews report it has the consistency of snot), and a chemical salt, ammonium phosphate. It’s the salt compound, not the water, that does the work.
According to George Matousek, a representative with ICL Performance Products, Phos-Chek’s manufacturer, the retardant is intentionally dropped ahead of the fire. The ammonium phosphate then reacts with the heat of the advancing flames, and first dehydrates, then chars the wood and grass, so it’s no longer fuel for the fire. The heat reaction turns the outside of the plants to carbon — like the graphite in a pencil — which doesn’t burn.
“It essentially takes fuel away from the fire,” Matousek said, slowing it down and reducing its intensity.
That can give firefighters a chance to build firebreaks and attack the fire directly. The retardant also is used where the terrain is too rugged or inaccessible.
Is it safe? Yes, according to the US Forest Service and EPA, who’ve tested and studied the compound, the retardant is nontoxic. The fire-retardant properties of ammonium phosphate were first discovered in 1821 by French chemist Joseph Louis Gay-Lussac. Because the compound contains nitrogen and phosphorus, two of the main ingredients in household fertilizers, Matousek explained, the guidelines require that it not be used within 300 feet of a waterway. In waterways or lakes, it could produce a sudden overgrowth of plants and algae that can deplete oxygen, harming wildlife.