The pilot was practicing instrument maneuvers about 1,500 feet mean sea level in a Mooney M20J near Leonardtown, Maryland, with a CFI on board, when the engine stopped responding to throttle control inputs and producing enough power to maintain level flight.
The CFI 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 Mooney 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. When the throttle control in the cockpit was moved, he noted the throttle linkage moved but that the throttle body arm did not move.
Review of maintenance logbook records revealed that the most recent annual inspection was completed on April 15, 2013. 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 six days and one 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 NTSB determined the probable cause as the improper installation and inspection of the throttle linkage by maintenance personnel, which resulted in the throttle linkage coming loose and a subsequent partial loss of engine power.
NTSB Identification: ERA13LA342
This July 2013 accident report is are provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.