Accident Overview
On May 26, 2018, at about 0935 Pacific daylight time, a Cessna 152 airplane, registration N152GB, sustained substantial damage during a forced landing near Hillsboro, Oregon. The flight instructor and student pilot were not injured. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.
Sequence of Events
According to the instructor, during the sixth stop-and-go landing of the flight, the student initiated a go-around. Shortly thereafter, the engine began making a "plugging sound." The instructor assumed control, adjusted the throttle, and informed the tower controller that they were experiencing a loss of engine power. As the airplane reached an altitude of about 200 ft above ground level near the departure end of the runway, the instructor executed a forced landing. The airplane touched down in the grass area beyond the departure end of the runway and nosed over.
Examination Findings
An FAA inspector examined the airplane and noted substantial damage to both wings. The aircraft was recovered for further examination. During the investigation, the carburetor was replaced and an external fuel source was connected to facilitate an engine run. When the left magneto was selected, the engine lost power. No other mechanical anomalies were observed. The magnetos were removed for detailed examination.
Magneto Analysis
Examination of the left magneto revealed that when the drive shaft was rotated, spark was produced on each post in a random order. Disassembly showed the distributor gear was intact, but the copper electrode finger was displaced from the plastic gear assembly. All other internal components were unremarkable. The right magneto produced spark in the correct firing order and its internal components were unremarkable.
Applicable Service Bulletins
Slick Service Bulletin SB2-80C, issued in February 1980 and last revised in April 1991, specifies that all 4300-series magnetos should be inspected externally every 100 hours and internally every 500 hours. Slick Bulletin SB1-15A, originally published in July 2015 and revised in November 2018, describes potential decreased service life of 4-cylinder magneto distributor gear assemblies, including the 4301 magneto. It notes that some returned products had distributor gears exhibiting loosening of the electrode finger and that typical symptoms include unusual RPM drop during magneto check, difficulty starting, and/or rough running engines. Compliance with SB1-15A includes replacement of distributor gears equipped with a copper electrode.
Magneto Service History
The operator reported that the left magneto was installed on the engine in June 2016. At that time, the engine had 1,602.2 hours since overhaul. At the accident time, the left magneto had accumulated a total of 2,632.7 hours and 402.7 hours since its most recent inspection. The operator stated that internal inspections of magnetos were conducted every 750 hours.