A Sikorsky Helitanker uses a siphon to fill its tank from a phosphate retardant tank while fighting the Derby Fire in Southern Montana. McCleod, Montana September 5, 2006. A helotanker is a helicopter used in firefighting, especially against forest fires, which drops water or other fire retardant on the fire. Over the Labor Day weekend, a large fire burning in southern Montana forced hundreds of people to evacuate from their homes. The Derby Fire had scorched about 178,400 acres between the towns of Big Timber and Absarokee as of September 5, 2006, according to the daily report from the National Interagency Fire Center.