Mooney M20J accident near Leonardtown, Maryland, July 29, 2013

The NTSB's narrative final · quoted from the NTSB record

The flight instructor and certificated pilot, who was the pilot flying, were practicing instrument maneuvers about 1,500 feet mean sea level when the engine stopped responding to throttle control inputs and producing enough power to maintain level flight. The flight instructor unsuccessfully attempted to restore engine power, and the pilots chose to conduct an off-airport landing to a field, which resulted in substantial damage to the fuselage and wings. Following the accident, one of the airplane's owners inspected the airplane before the wreckage recovery. He removed the engine cowling and found a bolt and nut resting on the bottom of the cowling. He also noted that the throttle arm linkage was not connected to the throttle body arm. He reported that the throttle linkage was found in the full-forward position and that the throttle body arm was found in the low-power setting. He asked an individual to move the throttle control in the cockpit and noted that the throttle linkage moved but that the throttle body arm did not move. A postaccident test run of the engine was performed following the replacement of the bolt that connected the throttle cable to the throttle control arm, and no anomalies were noted that would have precluded normal operation. Review of maintenance logbook records revealed that the most recent annual inspection was completed on April 15, 2013, at a recorded tachometer reading of 5,276.8 hours. The mechanic who performed the inspection reported that all engine control cables were "checked" and lubricated and that the throttle linkage bolt, nut, and cotter key were in place. The most recent engine maintenance was performed 6 days and 1 flight hour before the accident by the same mechanic. During the maintenance, the starter was removed, overhauled, and reinstalled on the engine, and no maintenance was conducted in the area of the throttle linkage and throttle body arm. It is likely that, during maintenance at a previous undetermined time, the cotter pin was removed from the throttle linkage bolt and nut and not reinstalled. The nut then gradually loosened and fell off, which allowed the bolt to become loose. The missing cotter pin should have been detected during the most recent annual inspection or engine maintenance.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.