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Retardant dropped from planes typically is used to bolster a line cut by firefighters on the edge of a fire, and water dropped from helicopters is usually used to cool hotspots within a fire.

The current fleet is made up of Lockheed P-2Vs, anti-submarine patrol planes dating to the 1950s that have been modified with jets to supplement the piston engines. More than half are due to retire in 10 years.

The number of large aircraft has steadily dwindled since 2004, when the forest service grounded 33 air tankers after a number of high-profile crashes. In March, senators from Oregon, New Mexico, Alaska and California asked the Government Accountability Office to evaluate whether the service had done a good job of analyzing the types and numbers of aircraft needed, the cheapest way to get them, new technologies and where the planes will be based.

Associated Press writer Matt Volz in Helena, Mont., and Jeff Barnard in Grants Pass, Ore., contributed to this report.