Cessna 421 N123SM accident near Rockport, Texas

Probable cause

The failure of the left engine starter adapter gear, which resulted in catastrophic damage to other engine components, and the subsequent loss of engine power.

NTSB analysis

The pilot reported that, while climbing through 6,000 ft above mean sea level (msl), the left engine tachometer generator went offline but the engine was still operating normally. He stated that about 2 minutes later, the left engine lost power and the airplane banked left. He attempted to stabilize the airplane with aileron and rudder control, but the airplane entered a spin. The pilot was able to regain control, but after the spin the right engine’s propeller was stopped and the left engine’s propeller was windmilling. The pilot executed a forced landing to a field and the airplane sustained substantial damage to the horizontal stabilizer.

During a postaccident engine test run, the right engine operated normally. Examination of the left engine revealed the starter adapter gear had failed, which resulted in damage to other engine components that rendered the engine inoperable.

The starter adapter was subject to an FAA Airworthiness Directive (AD), and an associated manufacturer mandatory service bulletin (SB). Maintenance records indicated that the AD and SB had been complied with less than 1 month and 10 operating hours before the accident. The mechanic that performed the SB inspection reported that he had performed this test numerous times in the past and did not find the procedures difficult but required patience. He recalled that the backlash on both starter adapter assemblies were well within the limits specified in the SB.

Text from the NTSB. Open the NTSB docket for CEN23LA267 for the full record.