After recently completed maintenance, and the pilot and pilot-rated passenger decided to take the airplane for a test flight. Both occupants reported that a preflight inspection was completed and that no anomalies were noted during the preflight. Shortly after takeoff, the pilot and passenger noticed that a large part of the right engine nacelle cover had departed from the airplane. They requested to return to land at the airport and made a subsequent uneventful landing. The pilot reported that there were no adverse control issues observed during the return for landing. A subsequent inspection of the airplane revealed that the nacelle panel had impacted the right side of the horizontal stabilizer, resulting in substantial damage. Postaccident examination of the right engine nacelle revealed that most of the inspection panel had separated from the nacelle and was not located; however, a small portion of the outboard section of the panel remained attached to the engine nacelle. The remaining portion of the panel displayed significant wear around the camlock and screw retention holes; at least one of the retention screw holes was significantly larger than the retention screw head. While the missing portion of the of the inspection panel was not available for examination, it is likely that the screw holes on this portion were similarly worn. This wear should have been apparent to maintenance personnel when inspecting the airplane (but likely would have been obscured by the engine cowling and not visible during a pilot’s preflight inspection). Based on this information, it is likely, that when exposed to high airflow during departure, a portion of the panel pulled out of the retention fasteners and was pulled out into the airstream, resulting in most of the panel being torn from the airframe.