NTSB Factual Report

On July 29, 2013, about 1935 eastern daylight time, a Mooney M20J, N4570H, sustained substantial damage during a forced landing following a partial loss of engine power near Leonardtown, Maryland. The airline transport pilot (ATP) rated pilot and ATP rated instructor sustained minor injuries. The airplane was operated under the provisions of Title 14 Code of Federal Regulations Part 91 and no flight plan had been filed for the local instructional flight. Visual meteorological conditions prevailed for the flight that originated from St. Mary's County Regional Airport (2W6), Leonardtown, Maryland, about 1850.

According to the pilots, they were practicing instrument maneuvers about 1500 feet above mean sea level when the engine stopped responding to throttle control inputs and was not producing enough power to maintain level flight. They turned the airplane toward 2W6 and began searching for a suitable landing area. The flight instructor unsuccessfully attempted to restore engine power by turning the fuel boost pump on, switching the fuel selector to the alternate tank, and adjusting the throttle position. The pilots elected to land the airplane in a field. The airplane impacted the ground, a fence line, and then came to rest in a corn field.

A postaccident examination of the airplane, by a Federal Aviation Administration (FAA) inspector, revealed that both wings and fuselage sustained substantial damage during the accident sequence. However, due to the location of the accident, the inspector was unable to remove the cowlings to further exam the engine.

According to the pilot flying and FAA records, he held an ATP certificate for airplane multiengine land, a commercial certificate for airplane single-engine land, and a flight instructor certificate for airplane single-engine, multiengine, and instrument airplane. His most recent second-class medical certificate was issued on January 14, 2013. He reported a total flight time of 6849 hours, of which, 316 were in the same make and model as the accident airplane.

According to the flight instructor and FAA records, he held an ATP certificate for airplane multiengine land, a commercial certificate for airplane single-engine land, and a flight instructor certificate for airplane single-engine, multiengine, and instrument airplane. His most recent second-class medical certificate was issued on May 31, 2013. He reported a total flight time of 3597 hours, of which, 82 were in the same make and model as the accident airplane.

According to FAA records, the airplane was issued an airworthiness certificate on July 23, 1976. It was equipped with a Lycoming IO-360-A3B6D engine, serial number L-15188-51A. Review of airplane maintenance logbook records showed an annual inspection was completed April 15, 2013, at a recorded tachometer reading of 5276.8 hours. At that time, the mechanic reported that all engine control cables were "checked" and lubricated. The tachometer was observed at the accident site and indicated 5302.9 hours. The most recent recorded maintenance was performed on July 23, 2013, when the starter was removed, overhauled, and reinstalled on the engine, at a tachometer time of 5301.9.

According to one of the airplane owners who inspected the airplane prior to wreckage recovery, he arrived on scene to obtain the registration and airworthiness certificates. While there, he removed the engine cowling in order to look at the engine. He discovered a bolt and nut resting on the bottom of the engine cowling and also noted that the throttle arm linkage was not connected to the throttle body arm. He noted the throttle linkage to be in the full forward position and the throttle body arm to be in the low power setting. He further reported that he had asked an individual that was with him to move the throttle in the cockpit and noted the throttle linkage moved but the throttle body arm did not move.

According to an interview with the mechanic who performed the maintenance on the airplane, the bolt and cotter key were in place on the throttle linkage and throttle body arm at the time of the most recent annual inspection. In addition, the maintenance work to overhaul and reinstall the starter on the engine required work on the opposite side of the engine from where the throttle body is located and "the throttle linkage was not removed during the starter change." When asked if he had an idea of how the throttle linkage became disconnected, he stated that he did not know and that he was "curious how after all the physical activity of the aircraft landing gear up, hitting a fence line and spinning around, that the missing [bolt] could be laying on the lower cowling directly below the connection point."

A postaccident test run of the engine was performed under the supervision of NTSB personnel. The airplane was not defueled, and only the fuel on board at the time of the accident was used for the engine run. Prior to the test, an undamaged propeller was installed on the engine, a battery from another airplane was attached to the starter, and a bolt was installed to connect the throttle cable to the throttle control arm. The engine started on the second attempt, the first attempt failed due to an undercharged battery, and accelerated to 1700 rpm without hesitation when power was applied. A magneto check was performed with no anomalies noted. Engine power was reduced to idle, and the engine decelerated and ran smoothly. No anomalies or mechanical malfunctions were noted during the engine run that would have precluded normal operation.

NTSB Probable Cause

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.