On Wednesday afternoon, as the already-fearsome Black Forest fire roared into the most destructive wildfire ever to strike Colorado, emergency officials asked for help from the sky.

But, at least initially, according to a report from the Rocky Mountain Area Coordination Center, the request for two large air tankers came back UTF — federal government code for “unable to fill.” The denial appears to have been only temporary, and officials fighting the Black Forest fire stress they were given all the resources they need.

But it also underscores a growing challenge for fire commanders trying to slow dangerous fires before they turn truly wicked: There aren’t enough big firefighting airplanes to go around.

Right now, the federal government has exclusive contracts for only 10 large air tankers nationwide, said Jennifer Jones, a spokeswoman for the U.S. Forest Service, which handles federal contracts for all large air tankers. Another plane is reserved on a “call-when-needed” basis. Seven more planes are scheduled to come online by the end of the summer. All of the planes are owned and operated by private companies.

The limited supply of large air tankers — there were more than 40 such planes in the federal government’s arsenal a decade ago, Jones said — means each plane is typically always working. And that means management decisions for the big tankers have become a dilemma for federal fire officials. Every new fire a large air tanker is assigned to means the tanker must be pulled off another fire, perhaps several states away.

Last year, nearly half of all requests for large air tankers were unable to be filled, the blog Fire Aviation has reported.

“There was some delay because air crews were up and engaged all over the state,” El Paso County Sheriff Terry Maketa said of initial orders for air tankers. “Air support was flying all over the state, and they weren’t just sitting there waiting for us to call them out. It’s not like you put in a call, and they’re suddenly in the air and on their way here.”

The massive DC-10 air tanker that arrived late Wednesday to fight the Black Forest fire offers a good example.

Rick Hatton, the president of 10 Tanker Air Carrier, the company that owns the plane, said it has been used in California and New Mexico so far this year and was working a fire in Arizona at the start of last week. On Tuesday it received a call to fly to Colorado to fight the Royal Gorge fire, which, like the Black Forest fire, broke out that afternoon. By Wednesday afternoon, the plane had been diverted farther north to the Black Forest fire, as that fire eclipsed the Royal Gorge fire in its threat.

The DC-10 holds 11,000 gallons of slurry — nearly four times what any other plane in the current fleet can carry — but it is also the Forest Service’s priciest plane. Jones said the plane’s daily availability rate is $26,750, plus an hourly flight rate of $4,550.

Hatton said the plane’s crew is always ready to deploy somewhere new.

“We basically sleep under the wing, so to speak,” he said.

As the DC-10 made slurry drops in Colorado, federal fire managers were balancing requests for help on fires in New Mexico, California, Nevada, Idaho and elsewhere. On Friday, the National Interagency Fire Center reported 13 large fires across the country that had burned more than 100,000 acres combined.

“It was hard to stretch the number of resources we had from Albuquerque to Boise,” said John Gould, the Bureau of Land Management’s chief of aviation and an official at NIFC.

Fortunately, fire managers say, large air tankers aren’t the only — or the most important — tool they have in combatting wildfires. Federal agencies also have contracts with dozens of small, single-engine air tankers and hundreds of water-hauling helicopters. State and local governments also have their own contracts for helicopters or small air tankers.

The Forest Service also has eight Modular Airborne Firefighting System units, which can be loaded into the back of military aircraft. Two such units were used at the Black Forest fire.

“I have never seen national resources deployed so quickly,” Maketa said Friday morning.

Large air tankers also have limitations. They are less effective during high winds. And, even at their best, they are merely tools to help firefighters — by ringing a fire in slurry to slow its progress, not actually extinguish its flames.

Jim Fletcher, the manager at the Rocky Mountain Area Coordination Center, which moves firefighting resources around a five-state region, said crews almost always trump airplanes when battling fires.

“It’s the firefighters that put the fires out, not aircraft,” Fletcher said. “It really boils down to the boots on the ground.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold

Staff writer Jordan Steffen contributed to this report.