Accident Overview
On June 24, 2023, about 1615 central daylight time, a Cessna 421 airplane, registration N123SM, sustained substantial damage during a forced landing near Rockport, Texas. The pilot and three passengers were not injured. The flight was operated as a personal flight under Title 14 Code of Federal Regulations Part 91.
Flight Sequence
The pilot had flown from West Houston Airport (IWS) to Corpus Christi International Airport (CRP) to pick up the airplane owner and friends. After loading 130 gallons of fuel at CRP, the airplane departed about 1545 and headed northeast. At 6,000 ft msl, the left engine tachometer generator went offline, though the engine operated normally. About 2 minutes later, the left engine lost power and the airplane banked left. The pilot attempted to stabilize with aileron and rudder, but the airplane entered a spin. He reduced both throttles to idle, applied right rudder, and regained control. After leveling off, the right engine had stopped with a non-windmilling propeller, while the left engine produced no power but its propeller windmilled. The pilot activated the left engine boost pump, searched for a landing area, and executed a forced landing to a field.
Post-Accident Examination
A test run of the right engine showed it could produce rated power. Teardown of the left engine revealed the starter adapter gear had failed, stripping teeth from accessory gears, including the camshaft drive gear. The starter adapter was subject to FAA Airworthiness Directive 2007-05-15, which aimed "to prevent failure of the starter adapter assembly and or crankshaft gear, resulting in failure of the engine and possible forced landing." The AD mandated compliance with Continental Motors Mandatory Service Bulletin (SB) MSB94-4H, which includes inspections and replacement procedures.
Maintenance Records
Examination of maintenance records showed that Part 2 of the SB (visual backlash check of the viscous damper) was complied with on June 2, 2023, during the annual inspection on both engines. The airplane had flown 7 flights and 10 hours since that inspection. On the right engine, Part 3 (removal and inspection of starter adapter) was last complied with on June 10, 2020, through replacement. On the left engine, the starter adapter was removed, inspected, and reinstalled on June 20, 2020, about 100 hours before the accident. No maintenance record for 2022 was found. The mechanic who performed the most recent annual inspection reported that Part 2 readings were within specified limits and noted the procedure required patience but was not difficult. A backlash check on the intact right engine after the accident did not detect excessive wear; damage precluded the check on the left engine.