The single-engine homebuilt helicopter sustained substantial damage when it impacted trees and terrain at approximately 1500 Central time following a loss of control during cruise flight. The Airline Transport pilot sustained serious injuries; the unrated passenger sustained minor injuries. Visual conditions prevailed. According to the passenger, during cruise flight at approximately 1500 feet agl, the pilot and passenger heard a “pop” from the rear of the helicopter. Immediately, the helicopter pitched nose-down and descended, impacting trees and terrain.
