By Tim Gaynor
(Reuters) - A combination of human error and a severe "microburst" during a thunderstorm caused a military air tanker to crash while fighting a South Dakota wildfire in July, killing four airmen, according to an accident report released on Wednesday.
The U.S. Air Force C-130 Hercules crashed on July 1 while battling a blaze near Edgemont, South Dakota, killing four North Carolina Air National Guard airmen. Two others were seriously injured.
An Air Force Air Mobility Command accident investigation report released on Wednesday found that an "inadequate assessment" by the cockpit crew of "operational conditions resulted in the aircraft flying into a microburst and impacting the ground."
The report describes a microburst as a severe, localized wind gust, blasting down from a thunderstorm, typically covering an area less than 2.5 miles in diameter and lasting less than 5 minutes.
Those killed were identified as Lieutenant Colonel Paul Mikeal and Major Joseph McCormick, both pilots; Major Ryan David, a navigator; and Senior Master Sergeant Robert Cannon, a flight engineer.
The plane was one of eight U.S. Air Force aircraft that can be quickly converted into firefighting tankers for a Defense Department-U.S. Forest Service program when private and commercial fleets cannot meet the need.
The investigation also found factors that substantially contributed to the mishap included the "failure of the Lead Plane and Air Attack aircrews to communicate critical operational information; as well as conflicting operational guidance concerning thunderstorm avoidance."
The wildfire, which had consumed a few thousand acres (hectares) at the time of the crash, was one of thousands across the United States during one of the most devastating fire seasons on record.
According to the National Interagency Fire Center, more than 53,000 wildfires have been reported this year to date, which together have consumed more than 9 million acres (3.65 million hectares).
(Editing by Jim Loney)