BOISE, Idaho — A giant aircraft that can fly high above oceans on intercontinental flights instead jets in low and slow over a flaming forest, trailing a long plume that settles on the ground and creates a wildfire-stopping barrier.The operators of the Boeing 747 converted from a passenger jet into a firefighting air tanker say it has proven itself battling forest fires in countries outside the U.S. The modifications allow it to drop more than 19,000 gallons (72,000 liters) of a flame-squelching combination of ammonium phosphate and sulfate mixed with water that comes billowing out in a red-colored line."We just happen to be the biggest, fastest firetruck in the air," said Jim Wheeler, CEO of Global SuperTanker Services.But the company says the U.S. Forest Service is seeking to keep the plane grounded by offering a contract limiting firefighting aircraft to 5,000 gallons (18,900 liters) of fire suppressant and won't say why. The company says the federal agency is putting homes and lives at risk just as the current wildfire season surges past the 10-year average for land area burned in a decade that includes some of the most destructive and deadly wildfire seasons on record.Late last month, the company filed a protest with the Forest Service contesting the size limit that appears to conflict with the Forest Service's 2012 air tanker modernization strategy report. That document identifies large-capacity tankers as an important part of the firefighting effort as the agency tries to pay for fighting fires without using money intended for such things as improving recreation opportunities for forest visitors.Forest Service spokesman Mike Ferris said in an email the agency couldn't comment specifically about the possible 747 contract because of the company's protest.Watchdog and firefighter advocacy groups said the agency might be trying to cut firefighting costs that have been using up big chunks of its budget.