The private pilot and the passenger were on a local flight in the airplane; the pilot was seated in the rear seat and the passenger was in the front seat. Several witnesses, located west of the accident location, observed the airplane flying low, heading north, just west of a river. After overflying several people on a pond, the airplane continued to the north briefly and then started a right turn. The nose of the airplane dropped, and the airplane descended rapidly below the tree line. The passenger stated that the airplane started to turn and then went straight down. The airplane impacted the river and came to rest in the water. The passenger, who was seated in the front seat, did not recall exiting the airplane but did recall being underwater, surfacing, and swimming to the bank of the river. The pilot, who was seated in the rear seat, was ejected from the airplane and found face down in the water when first responders arrived.Examination of the airplane indicated that it impacted the river in about a 45° nose-down attitude. No evidence was found of any preimpact mechanical malfunctions or failures with the airframe, flight controls, engine, or related systems that would have precluded normal operation. Witness statements, the passenger's statement, and the damage to the airplane are consistent with an aerodynamic stall followed by an uncontrolled descent and impact with the river. An examination of the lap belts and shoulder harnesses in the airplane revealed that the front and rear lap belts were buckled; the front and rear shoulder harness straps were not attached to the buckles; and one side of each lap belt was not attached to its mounting bracket. For the rear seat, the left mounting bracket for the lap belt had failed in overload likely during the impact sequence. For the front seat, the left-side webbing of the lap belt was found not attached to the mounting bracket, and the right-side webbing of the lap belt was found knotted to the mounting bracket, which was an improper method for attaching the belt to the bracket. The pilot had fastened the lap belt for the passenger before the flight; had the webbing not been secured at that time, it is likely that the passenger and the pilot would have noted that fact. Therefore, it is likely that the left-side webbing was also knotted to the wire bracket and came undone during the impact sequence.Based on the passenger's statement that he was not wearing a shoulder harness, the inflight video taken Page 2 of 10 CEN17FA196by the passenger, and the postaccident finding that the front and rear seat shoulder harness straps were not attached to the buckled lap belts, it is apparent that neither the pilot nor the passenger were using the shoulder harness portions of their restraint system at the time of the accident. The damage to the airplane, the location of the pilot following the accident, and the injuries to both the pilot and the passenger are consistent with the pilot being ejected forward, into the back of the front seat, and then likely out the front windscreen of the airplane. The passenger's facial injuries were consistent with the passenger impacting the airplane's instrument panel.Investigators were unable to determine what method, if any, was used to approve the installation of the shoulder harness. It is unknown if the seatbelts and shoulder harnesses installed were part of a properly certified restrain system. Without the method of certification or approval of the restraint system, investigators were further unable to determine what role the failure to use the restraint system played in the pilot's ejection, mitigation of the pilot's injuries, and protection from injuries for the front seat occupant. However, both occupants likely would have benefited from the use of a properly certificated and installed four-point restraint system in this accident.