CHEYENNE, Wyo. (AP) — Only one of seven large air tanker planes that were contracted last month by the U.S. Forest Service to fight wildfires is doing so now that fire season has heated up.

The other six state-of-the-art planes still need to be certified to fly. The Forest Service says that process could take up to two more months.

The so-called "next-generation" turboprop and jet planes meet minimum standards for speed and slurry capacity. The only such plane flying right now belongs to 10 Tanker Air Carrier, a company in the process of moving its headquarters from Victorville, Calif., to Casper, Wyoming.

Ten Tanker's modified DC-10 passenger plane can drop up to 11,700 gallons of slurry at a time.

The plane has been fighting recent wildfires in California and New Mexico.