The aircraft collided with the terrain after descending to 600 ft AGL and encountering a weather phenomenon which prompted an airspeed decay and uncommanded loss of altitude. Two other aircraft in the flight of 3 experienced the same phenomenon to a lesser degree apparently because of their higher altitude AGL. The pilot of one of the accompanying aircraft landed on a nearby road after seeing the accident and in proceeding to the scene saw a 'huge dust devil.' He also said the wind would shift from 300 degrees to about 120 degrees then shift to 60 degrees with a buffet before returning to the prevailing 300 degrees. The two closest weather stations 45 or more miles away are separated from Duckater by mountain ranges. The pilot of the accident aircraft was stabilized in flight with 10 degrees of flaps at 85 knots when he noted a drop in airspeed to 70 knots and then 60 knots and a vertical speed of 500-700 feet per minute descent. When the airspeed continued to drop the pilot added full power and retracted the flaps. The aircraft continued to descend at 1000-1200 feet per minute until impact.