WPR19LA092

The flight instructor and student pilot were performing maneuvers when the engine lost total power. The instructor pitched the nose of the airplane down and asked the student pilot to restart the engine. The student pilot was unable to rotate the engine using the starter, and the flight instructor verified that the master switch was in the “ON” position. The instructor tried to start the engine himself and was unsuccessful. The flight instructor performed a forced landing in a field. Examination revealed that the idle mixture adjustment screw and spring assembly had separated from the carburetor and were found in the lower cowling area. After the airplane was relocated to a nearby road the engine rotated via the starter. Examination of the idle mixture adjustment screw and spring assembly revealed no anomalies. The reason that the screw backed out of the carburetor could not be determined. The starter solenoid was removed and disassembled. The starter solenoid was examined and revealed a frayed and separated wire from the coil to a terminal end. The reasons for this fraying and separation could not be determined. Although the starter solenoid was faulty, it is unlikely that the engine would have restarted due to the separated idle mixture screw and spring assembly.

On March 1, 2019, about 1345 mountain standard time, a Cessna 172N airplane, N6486J, was substantially when it was involved in an accident near Melba, Idaho. The flight instructor and student pilot were not injured. The airplane operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The instructor reported that they had performed about 12 power-off and power-on stalls, while maintaining an altitude between 4,500 ft and 2,000 ft during the maneuvers. While performing the last power-off stall, with the nose of the airplane pitched up, the engine lost total power. During that maneuver, the throttle lever was in the closed position, and the carburetor heat was in the “ON” position. The instructor pitched the nose of the airplane down and asked the student to restart the engine. The student pilot was unable to rotate the engine using the starter, and the instructor verified that the master switch was in the “ON” position. The instructor tried to start the engine himself and was also unsuccessful. The instructor performed a forced landing in a field about 12 miles southwest of the departure airport. During the landing roll, the right wing impacted the ground and the right wing spar sustained substantial damage. The initial examination of the airplane at the accident site was performed by the operator's mechanic. The examination revealed that the idle mixture adjustment screw and spring assembly had separated from the carburetor and was found in the lower cowling area. While on site, the flight instructor told the mechanic about the inability to rotate the engine with the starter after the power loss. The airplane was moved to a nearby road and the mechanic was able to rotate the engine via the starter. The airplane was recovered for further examination. The examination of the airplane by a Federal Aviation Administration inspector revealed no other anomalies, and the airplane was released back to the owner. During a conversation with the National Transportation Safety Board investigator-in-charge after the accident, the owner stated that he could not rotate the engine using the starter following the accident. The ignition switch and associated wiring were subsequently examined, and no anomalies were noted. The starter rotated when the starter cable was removed from the starter solenoid and connected to the battery. The starter solenoid was removed and disassembled. The examination of the starter solenoid revealed a frayed and separated wire from the coil to a terminal end. The owner replaced the starter solenoid with a new solenoid and installed the idle mixture adjustment screw and spring assembly; the engine operated normally. He concluded by stating that, with the idle mixture adjustment screw assembly removed from the carburetor, the engine would not operate below 1,000 rpm. Maintenance records indicated that, on October 18, 2016, 1,281.6 flight hours before the accident, the carburetor was removed and the throttle shaftassembly, accelerator pump assembly, throttle shaft bushings, gaskets and cotter keys were replaced with new. The carburetor was reinstalled, and the engine was test run with no anomalies noted. On January 19, 2016, a faulty starter solenoid was replaced with a new starter solenoid and the part number was correct for this airplane. The last annual inspection was completed on January 29, 2019, about 24 flight hours before the accident.