What happened
A police helicopter and ground units were engaged in a pursuit of a suspect traveling on a dirt bike. The pursuit moved eastbound into a park, where the pilot attempted to maneuver the aircraft ahead of the suspect.
According to the pilot, he descended and turned south before deciding to regain altitude. During this climb, the pilot reported hearing or feeling a vibration or thud, after which the aircraft impacted the ground in an inverted position. An observer on board provided a similar account, stating that in an unsuccessful attempt to get ahead of the suspect, the pilot descended to approximately treetop height, leveled the helicopter, and began a climb. The observer noted a vibration rumbling through the aircraft followed by ground impact.
The aircraft came to rest on its left side. The occupants were not injured.
The investigation
Examination of the accident site showed that a 4-foot section of the tailboom had separated from the aircraft and was located approximately 125 feet away.
Investigators examined the main rotor, tail rotor, drive system, flight controls, fuselage, and engine, but found no evidence of mechanical malfunction. The engine was tested and operated successfully at 3150 rpm for approximately 20 minutes at 25 inches of manifold pressure. An examination of the fracture surface on the tailboom revealed evidence of overstress, with no evidence of pre-existing cracking found.