On May 26, 2011, at 1423 central daylight time, a Bell 206L-1, helicopter, N1815, was substantially damaged when it impacted terrain near Walton, Kansas. The helicopter was registered to GM Leasing Co., LLC, and operated by Air MD, LLC. Visual meteorological conditions prevailed and a company flight plan had been filed for the 14 Code of Federal Regulations Part 91 maintenance flight. The pilot and flight mechanic sustained minor injuries. The flight had originated from Newton City/County Airport (EWK), Newton, Kansas, at 1412 for a local flight. The pilot had entered an autorotation to check the flat-pitch rotor speed which was satisfactory. During recovery at about 500 feet above ground level (agl) the pilot rolled the throttle on but the engine did not respond. The autorotational descent continued and the helicopter was about 300 agl when the pilot heard a low rotor warning and observed the rotor rpm was about 80 percent. The pilot lowered the nose and executed a power-off running landing to a wheat field. As the helicopter touched down a main rotor blade struck the tail boom and the tail boom separated aft of the tail boom attach point. Both main rotor blades impacted terrain and the helicopter came to rest on its right side. The pilot and flight mechanic both reported that they were uncertain whether the engine had stopped running during the autorotation recovery or if it was only operating at idle rpm. Neither one of them remembered looking at the engine rpm gauge during the event nor did the pilot notice any annunciators other than the low rotor rpm annunciator. An examination of the helicopter found sufficient fuel on-board. Maintenance records did not show that any recent maintenance had been performed on the engine. No preimpact anomalies were discovered that would have prevented normal operation. The engine was removed and examined separately. The engine was placed in a test cell and two test runs were conducted. During the first test run the engine failed to meet specifications for take-off power by 2.4 percent and the engine anti ice valve was discovered stuck in the ON position. During the second test run the engine anti ice valve was removed and the engine met specification power for all test points. The reason for the loss of engine power could not be determined.