On August 19, 2018, about 1320 Hawaiian standard time, a Eurocopter EC-130 B4 helicopter, N11QP, sustained substantial damage when it was involved in an accident near Hilo, Hawaii. The pilot was not injured, and the passenger sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 maintenance flight. The pilot stated that the purpose of the flight, which originated from Hilo International Airport (ITO), Hilo, Hawaii, was for the track and balance of the main rotor blades. He conducted a previous flight in the helicopter that day but returned to ITO early due to issues with the track and balance equipment. Before the accident flight, the pilot conducted a walk-around inspection of the helicopter, during which he visually checked to ensure all the doors, cowlings, and equipment were closed and secured. The pilot and the passenger, a maintenance apprentice who was seated in the right front seat, departed ITO about 1310. The pilot stated that, after completion of the maintenance checks and while en route back to ITO, he lowered the collective to decelerate and began a slow descent. He then heard the left-rear sliding door suddenly slide open. The pilot looked back and saw the door off the helicopter in midair. He stated that he turned his head forward, then heard a loud noise and felt a jolt. The helicopter began to vibrate but remained controllable. The pilot initiated a precautionary landing onto a grass field. He indicated that the door appeared to have struck two main rotor blades and that he thought the door fell into the ocean. The maintenance apprentice stated that on the accident flight, while returning to the airport and after leveling off from a descent, the door suddenly opened and separated from the helicopter. He stated that on the takeoff for the previous flight, a maintenance technician had ensured the doors were closed correctly, but on the takeoff for the accident flight, the maintenance technician did not. The accident helicopter has a sliding rear door on the left side; when opened, it is supported by a plastic latch located on the fuselage of the helicopter at the end of the sliding door track. The helicopter sustained substantial damage to the main rotor blades; two of the three main rotor blades exhibited dents and scratches across their undersides, generally initiating at or near the leading edge and propagating aft along the chord of the blade. Examination of the operator's photographs and reports from the director of maintenance revealed that the helicopter’s left sliding door tracks and door latch assemblies were normal and unremarkable. The plastic latch on the fuselage was damaged, and a portion had separated from the fuselage. The sliding door was not recovered, thereby preventing examination of the door and its locking components. A review of the helicopter's maintenance records revealed no previous door-related issues. On December 10, 2018, Airbus Helicopters published Alert Service Bulletin (ASB) ASB EC130-05A031 for operators of EC-130 B4 helicopters with sliding doors, making it mandatory to check for correct operation of the sliding door locking system by measuring the load required to unlock the sliding door. The ASB indicated the check should be conducted with a spring scale and described an in-flight opening and loss of a sliding door as the reason for the bulletin.