The pilot and the flight test engineer had just completed a production test flight of the helicopter and were preparing to land when the lower section of the left side of the engine cowling folded back and struck all 4 main rotor blades and made contact with all 4 main rotor blades at some point prior to the rotor stopping, resulting in substantial damage. According to the operator/manufacturer, none of the fasteners on the left side of the cowling remained attached and 3 of the fasteners were missing.Prior to the flight, the pilot observed that the fasteners attaching the generator duct to the right side of the same cowling appeared long and reported it to maintenance. No anomalies were observed with the fasteners attaching the cowling tothe airframe. Maintenance then removed the one-piece engine cowling and a mechanic replaced the generator duct fasteners before reinstalling the cowling with the help of another mechanic. The mechanic said he specifically fastened each of the fasteners on the cowling himself and did a 100% check of each fastener after the cowling was installed. Once the cowling was reinstalled a quality inspector visually inspected the engine cowling fasteners. He did not observe any anomalies and signed off that the work had been completed. Prior to the accident flight, the flight test engineer performed another preflight inspection of the helicopter and did not observe any obvious issues with any of the cowling fasteners. The helicopter then departed and flew for about an hour before the cowling folded back. Postaccident examination of the cowling by the operator/manufacturer revealed no evidence of any preaccident discrepancies with the fasteners/cowling, or the design and conformity of the helicopter structure.