The aircraft had descended through a cloud layer enroute for the scheduled landing at IAD and according to the crew had picked up some trace rime ice on its wing surfaces. Although pitot and SAS (stall avoidance system) heat were used, the captain elected not to use engine inlet anti-ice. Just inside the outer marker, the left engine lost power followed shortly by the right engine. The captain was able to set the aircraft down in an open field where the gear collapsed during the landing roll. The engines were successfully run-up at the Garrett facility. The flight manual contains a warning and a note stating that engine heat and continuous ignition must be used after leaving icing conditions until the pilot is confident that any significant residual ice will not be ingested into the engines. Ice was found lying beneath the right wing which matched the curvature of the leading edge of the wing. A round conical shaped piece of ice was found along the wreckage path which matched the nose of the propeller spinner.