HISTORY OF FLIGHT On May 27, 1997 at 1110 mountain standard time (mst), a Bell 206L3, N35CH, was substantially damaged during an autorotation maneuver. The pilot was executing the maneuver during a Federal Aviation Administration (FAA) FAR Part 135 flight check when the accident occurred. The pilot had just completed two autorotation maneuvers, and this autorotation maneuver was to be a 180 degree reversal autorotation to a predetermined spot landing with a power termination. The pilot executed the maneuver, but during the level off received a low RPM warning horn. The pilot attempted to increase engine power using the throttle on the collective, but was not able to attain maximum engine RPM. The helicopter impacted hard with the taxiway, causing structural skin damage to the tail boom. The airline transport pilot and two FAA inspectors were uninjured in the accident. The 14 CFR Part 91 flight was operating in visual meteorological conditions, and no flight plan had been filed. TESTS AND RESEARCH The investigator in charge (IIC) examined the helicopter on May 29, 1997. Prior to the IIC arriving, the helicopter's tail boom and interior had been removed from the helicopter. With the interior removed from the helicopter the collective and throttle controls were moved throughout their full range of travel. With the collective control near its maximum rotor blade angle position the throttle linkage would bind with a nut when it reached the mid throttle position. If the collective was cycled with the throttle in the idle position no binding would occur. The helicopter had been modified to incorporate an external torque meter which was placed on the helicopter in view of the pilot. This torque meter was installed to aid the pilot while conducting external lifting operations. The major repair and alteration paper work (FAA Form 337), indicated an installation date of May 23, 1997. It was a nut from the installation of this torque meter which interfered with the throttle controls when the controls were cycled following the accident. ADDITIONAL INFORMATION The pilot in command of the helicopter in a written statement wrote that during the preflight cockpit check she noticed a "... slight lump in the throttle...", and reported that she pointed this out to the FAA examiner. She wrote that both pilots believed it to have been caused by the installation of the dual controls for this flight. The IIC asked the FAA examiner on June 4, 1997 if he remembered the pilot making this comment, and he remembered the pilot making some comment to him about the throttle before the flight, but could not remember the pilot's exact words. Two helicopter instructor pilots were interviewed by the IIC following the accident. Both pilots reported that any time an autorotation was executed they always assume that no engine power will be available during the recovery portion of the maneuver. Federal Aviation Regulation FAR 61.47 states that unless previously agreed upon the FAA examiner is not the pilot in command during a flight check.