What happened
While approaching a National Guard Base, the pilot of a helicopter reported experiencing vibrations. The pilot initially contacted air traffic control (ATC) from 9 miles south of the base to request clearance to transition through the airspace. As the aircraft reached approximately 1 mile south of the base, the pilot notified ATC that the helicopter was starting to shake more violently. In response, the controller informed the pilot that an emergency declaration would allow for landing at the base.
Approximately 3.5 minutes later, while at 1,800 feet mean sea level (msl) and 2 miles north of the base, the pilot reported an intention to return for a landing. ATC cleared the aircraft to land, requesting the pilot use the south side of the airport if possible. Air traffic controllers observed the aircraft through binoculars but reported no visible abnormalities. One controller noted the helicopter crossed the northern end of the airport at 1,600 feet msl.
Witnesses near the airfield heard the engine as the aircraft approached. They observed the nose pitch up and then down, describing a steep descent angle between 45 and 70 degrees. The descent was estimated to have begun at an altitude of 1,400 to 1,500 feet msl. The helicopter impacted the grass, leaving a wreckage trail approximately 299 feet long. There were no fatalities reported.
The investigation
Investigators examined the airframe, engine, and flight control systems, but found no failures or malfunctions that would have caused the accident. A sound spectrum examination was conducted on recordings of ATC communications to identify any unusual engine, rotor, or vibration sounds. This analysis showed that the main rotor speed remained consistent at approximately 101% RPM throughout the recording, including after the pilot reported vibrations. No additional airframe or rotor system vibrations were identifiable via the audio recordings.