The U.S. Forest Service on Monday announced a much-anticipated plan to field the 'next generation ' of large air tankers - a move that could modernize and nearly double the nation's dwindling fleet of large slurry-dropping planes.

Serious questions remain, however, on when those planes would take to the sky.

Hours after the agency announced plans to award $158 million in contracts for seven tankers over five years, at least one business threatened to appeal the move.

The threat mimicked a similar contract dispute last year, which kept a seven-plane fleet from flying during one of the nation's busiest fire seasons. As a result, the nation relied heavily in 2012 on eight Defense Department C-130 aircraft and another fleet of tankers on loan from a Canadian/Alaskan partnership.

U.S. Sen. Mark Udall, D-Colo., called for tanker companies to honor the agency's decision. In a statement, he urged them to 'refrain from unnecessarily tying up these tanker contracts in red tape. '

Dan Snyder, chief operating officer with Neptune Aviation Services, though, vowed to file a contract protest within the next 10 days 'based upon pricing. '

'First of all, we understand that there is a need to protect lives and property, ' Snyder said. 'But we also have to support an organization. '

The contract announcement came amid mounting concerns over the size and age of the Forest Service's current fleet, which has dwindled to less than one-fifth of its former size a decade ago.

The current eight-plane fleet ? called the 'legacy ' fleet - includes planes built during the Korean War. Eleven 'Legacy ' planes were available last year, though two crashed early in the summer. Large and very large air tankers are typically any plane that can carry at least 1,800 gallons of retardant.

The Forest Service originally meant to address that tanker shortfall before the 2012 firefighting season by awarding contracts for seven planes for its new 'next generation ' fleet.

Those 'next generation ' planes must be turbine-powered and must cruise at speeds of at least 345 mph while carrying a full load - at least 3,000 gallons.

Two companies - 10 Tanker Air Carrier LLC and Coulson Aviation (USA) Inc. - protested the Forest Service's decision. As a result, those seven tankers never flew, and businesses were asked to submit new proposals.

10 Tanker Air Carrier, though, was awarded the contract on Monday, and Coulson Aircrane (USA) Inc. - which has the same owner as Coulson Aviation - also received a contract, said Tom Harbour, Forest Service's director of fire and aviation managment.

'We're quite pleased... ' said Rick Hatton, president of 10 Tanker Air Carrier. 'We feel as though a future with more DC-10s is good for the country and for the states that have these kinds of fires. '

Meanwhile, Neptune was the lone company to be awarded a contract last year and to be left off this year's list.

Snyder said he asked the Forest Service to pay the same amount in 2013 as in last year's proposal.

Neptune already fields seven of the Forest Service's eight 'legacy ' tankers.

Those 'legacy ' contracts, however, did not include any options to extend the contract from five years to 10 - as was the case with Monday's contracts.

Contract length is key, he said, because it takes millions of dollars to retrofit planes to fight fires.

If no protest is filed, the seven moderinzed tankers must be flight-ready within 60 days of their contract being finalized - a process that could take a few days, Harbour said.

If a protest is filed, the Government Accountability Office must make a ruling within 100 days.