CHI04FA197 · Robinson R22 Alpha

The helicopter impacted terrain while attempting to land at a National Guard Base following a reported vibration in the helicopter. The pilot contacted air traffic control (ATC) stating he was 9 miles south of the base and requesting a clearance to transition through their airspace. The pilot was cleared through the airspace. When the helicopter was about 1 mile south of the base, the pilot reported to ATC that he was "... picking up a vibration." The controller asked the pilot to repeat the transmission and the pilot stated, "Yes sir the helicopter is starting to shake more violently sir." The controller informed the pilot that he could land at the base if he declared an emergency. Approximately 3 1/2 minutes later when the helicopter was 2 miles north of the base at 1,800 feet mean sea level (msl), the pilot reported that he was coming back to land. ATC cleared the pilot to land requesting if at all possible that the pilot land on the south side of the airport. The air traffic controllers viewed the helicopter through binoculars. They reported that they did not notice anything unusual with the helicopter and it appeared normal. One controller stated the helicopter crossed the northern end of the airport at 1,600 feet msl [1,020 above ground level (agl)]. Witnesses reported hearing the engine as the helicopter approached the airport. They stated that once the helicopter was approaching the south side of the airport, the nose pitched up then down. Witnesses described the descent angle as being between 45 and 70 degrees nose down. One controller estimated the descent began at an altitude of 1,400 to 1,500 feet msl [820 to 920 feet agl]. The helicopter impacted the grass leaving a wreckage trail approximately 299 feet long. Examination of the airframe, flight control systems, and engine did not reveal any failure/malfunction which would have resulted in the accident. A sound spectrum examination was performed on a re-recording of the ATC communications to document engine, rotor, or vibration sounds on the tape. This examination revealed that the rotor system background sounds were consistent with the main rotor rating at about 101% rpm. These sounds were consistent throughout the recording including the transmissions after the pilot reported a vibration. In addition, the tape was examined to determine if any airframe or rotor system vibrations could be identified. There were no additional sounds or vibrations identified during any of the transmissions.

Verbatim NTSB analysis and probable cause from the NTSB dataset