Facing a season of potentially dangerous wildfires and a dwindling number of large air tankers to help fight them, the U.S. Forest Service today took steps to add four more planes to its fleet.

The additions will bring the federal fleet of large air tankers up to 13, still far below the number that critics - and the forest service itself - say are needed to fight fires adequately from above.

A series of high-profile crashes in 2002 and 2004 led to stricter safety standards that gradually eliminated dozens of older air tankers from the fleet, dropping the number of available air tankers from 44 in 2006 to only 11 this season.

That number fell to nine Sunday, after two pilots were killed when their tanker crashed while dropping fire retardant in Utah.   Another tanker was damaged when its landing gear failed and the crew was forced to make a belly landing in Nevada.  Nobody was injured in that incident.

The causes are under investigation, but the incidents highlight concerns over the age and safety of the firefighting tanker fleet. Both planes were the same model Lockheed P-2V - airplanes originally designed for the U.S. Navy in the 1940s.

"The average age of the fleet is over fifty years," Tom Harbour, director of aviation and firefighting management for the Forest Service, told ABC News. "They're old."

Critics say the U.S. Forest Service - which contracts with private aviation companies to fly the tankers - has moved too slowly to modernize the fleet. The agency has taken bids for the next generation of tankers it says will be faster, safer, and more efficient. Those contracts will be awarded on June 25.

U.S. Senator Ron Wyden, a Democrat whose home state of Oregon has had its share of fires, is urging his colleagues to waive the waiting period so the contracts can be awarded earlier.

"The sooner the Forest Service can award these contracts, the sooner the companies that receive the awards can begin to prepare to deliver those next generation air tankers and get them out fighting fires," Wyden said Tuesday during a speech on the Senate floor.

One of the newly-available tankers announced today will come from Canada, and another from the state of Alaska. Two more tankers will come from California's state firefighting agency, CalFire. The forest service is also calling up five large helicopters capable of dropping 700 gallons of water or fire retardant.