Meanwhile, plans for two other large tankers have been sidelined by a dispute over a contract that called for paying up to $496 million over nine years to a Montana company for exclusive use of two aircraft. A decision has not been made whether to scrap the deal or try to fix it.
In another glitch, federal legislation gave the Forest Service the authority to obtain up to five extra air tankers, but didn't provide the funds to lease them.
The agency can also draw on eight military tankers, if needed. Eight others can be dispatched from Alaska and Canada, but those are not always available.
The agency is moving earlier this year to get firefighters and equipment into threatened areas.
"We are going to be looking at the weather and conditions ... and move things as we need to, to try to head things off before they get ahead of us," said agency spokesman Mike Ferris.
The Forest Service does not own the large tankers but strikes agreements with aviation companies that buy used aircraft, modify them for firefighting duty and then offer them for government lease. The agency also leases helicopters and smaller aircraft to douse fires.
The up-and-down pursuit of a faster, more reliable tanker fleet has played out against a backdrop of increasingly destructive blazes.
From 2000 to 2008, at least 10 states had fires of record-breaking size. In 2011, a wildfire scorched 538,000 acres in Arizona and New Mexico, an area so large it would cover much of the state of Rhode Island.
Costs for daily air tanker availability doubled from $15 million in 2007 to $33 million in 2010. Meanwhile, the cost of fighting wildfires has soared, up from 13 percent of the agency's budget a decade ago to over 40 percent. That's forced the agency to strip funds from other programs to keep up, officials say.
When homes or lives are lost, the Forest Service often faces questions about a lack of tankers, or how it uses them.
Shortly before 19 members of a firefighting Hotshot crew were killed in Arizona last year, records showed officials summoned six air tankers, but none arrived because of the limited number of tankers in the fleet and dangerous weather conditions. Fire officials said even if the big planes were available, winds were so strong they couldn't have been used to save the firefighters. Among recommendations after the deaths, investigators said more instructions were needed for the "effective use" of the largest tankers.
As part of the commemoration of the deaths this year, Forest Service firefighters will hold discussions on risks and hazards "to avoid (a) similar outcome," Ferris said in an email.
Government studies generally agree tankers play an important role in suppressing wildfires, particularly in "initial attack" — the early stages when failure to knock down a blaze quickly can lead to an inferno.
Harbour and other experts stress that the biggest job of extinguishing fires remains with ground crews, since embers and brush can continue to burn after a water or retardant hit. Instances of aircraft extinguishing fire are rare.
In another move by Congress, the Forest Service would receive seven, large HC-130 aircraft from the Coast Guard for firefighting. But it could be at least 2017 before they are all modified to carry retardant.
For now, the planes remain with the Coast Guard, even though the law aimed to have them transferred months ago.
"Is there part of me that hopes, starting July 4, it starts raining?" Harbour asked. "You bet."
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