NTSB Factual Report

On December 8, 2015, at 0850 Pacific standard time, a Cessna LC41-550FG airplane, N932AC, experienced a total loss of engine power and forced landed on open terrain on Naval Weapons Station Seal Beach, Seal Beach, California. The airplane sustained minor damage and the commercial pilot was not injured. The airplane was registered to Sotto International, Inc., and operated by the pilot as a Title 14 Code of Federal Regulations Part 91, personal flight. Visual meteorological conditions prevailed for the flight. The flight originated from San Diego, California, about 0830, and was destined for Long Beach, California.

The pilot reported to the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), that the engine started "missing" [running rough] just after departure but he corrected the condition by leaning the mixture. While in cruise the engine started "missing" again but smoothed out after he turned towards a nearby airport, at which point he decided to continue to his destination. When approximately 4 miles away from his destination the engine started to miss then quit. The pilot made a forced landing in an open area on the Naval Weapons Station Seal Beach facility. The pilot stated that he did not perform a magneto check at any time after the engine began running rough.

The airplane was ground transported to a fixed base operator located at the Long Beach Airport. The airplane and engine were examined by an airframe and power plant mechanic who reported that there was no external damage to the engine or propeller, and that the right magneto had gear teeth missing. The magneto timing of the left magneto was confirmed correct. The NTSB IIC took possession of the two magnetos. The SD memory card that normally records engine performance data was not installed in the primary flight display at the time of the incident.

The magnetos were examined at Continental Motors, Mobile, Alabama, on April 12, 2016, under the supervision of the NTSB IIC. Both magnetos were tested on a magneto test bench followed by a teardown examination. The left magneto, serial number D07OA109, bench test demonstrated spark at all distributor leads throughout the normal operating range. When the magneto was disassembled brass colored dust covered the interior of the distributor. The inspection determined that the gear washer and nylon washer had been installed in reverse order resulting in rapid bushing wear excessive radial play in the distributor drive gear. The excessive radial play of the gear had the potential to cause the internal timing to migrate. The right magneto, serial number D08BA119, bench test demonstrated sparks on the numbers 6 and 3 leads only. When the magneto was disassembled it was determined that the distributor gear had sequentially shed 12 teeth.

A review of the airframe and engine maintenance records revealed that the most recent annual inspection was on July 15, 2015, at an airframe total time of 942 hours. Both magnetos were removed for a 500-hour inspection on June 8, 2012, at airframe total time of 566.6 hours. Both magnetos were overhauled and then placed back on to the engine. On March 1, 2014, at airframe total time 705 hours, an engine logbook entry stated "REMOVED AND REPLACED THE RH MAG FOR OVERHAUL. SEE AERO ACC 8130 DATED 1/2/2014." However, the Aero Accessories 8130 for the right-hand magneto, serial number D08BA11, was dated 21 March 2014. An entry in the airplane's Propeller Maintenance Logbook, dated 12/16/2014, stated "Removed left magneto and replaced with overhauled magneto. All work done IAW Cessna Maintenance Manual. See enclosed form 8130-3." No FAA Form 8130-3 for the left-hand magneto, serial number D07OA109, was located in the maintenance records.

NTSB Materials Laboratory Examination

The right-hand magneto distributor gear was sent to the NTSB Materials Laboratory for examination. The distributor gear was examined visually and under an optical stereo microscope. The teeth showed accumulations of dark-colored grease on both the contact and non-contact faces. Metallic orange flakes were also observed within the grease. Twelve teeth were fractured, and the remaining teeth were intact with no cracks detected.

Tooth fractures had been initiated at both the contact and the noncontact sides of the teeth. A close view of the fracture surfaces features generally showed a relatively smooth area at the fracture initiation side of the tooth covering approximately 1/4 to 1/2 of the fracture area. Curving lines representing the fracture front were observed at the boundary of the smooth area. Raised hackle features and multiple curving crack front lines were present between the smooth area and the fracture termination. The lab was able to replicate the fracture characteristics by impacting an intact gear tooth with a chisel and hammer.

Additional Information

Australian Civil Aviation Safety Authority Airworthiness Bulletin (AWB) 17-005, Issue 3, dated October 20, 2014, lists numerous potential causes for nylon distributor gear failures, including propeller strikes, kickback during starting before fire events, and any other event that can cause shock on the gear train driving the distributor gear.

Columbia 400 Pilot's Operating Handbook, Section 3-24 (extract)

"Engine Roughness – The most common cause of a rough running engine is an improper mixture setting. Adjust the mixture in reference to the power setting and altitude in use. Do not immediately go to a full rich setting since the roughness may be caused by too rich of a mixture. Do not immediately go to a full rich setting since the roughness may be caused by too rich of a mixture. If adjusting the mixture does not correct the problem, reduce throttle until roughness becomes minimal, and perform a magneto check."

"Check operations on the individual left and right magnetos. If the engine operates smoothly when operating on an individual magneto, adjust power as necessary and continue. However, do not operate the engine in this manner any longer than necessary. Land as soon as possible for determination and repair of the problem. If individual magneto operations do not improve performance, set the magneto switch to R/L, and land as soon as possible for engine repairs."

NTSB Probable Cause

Improper maintenance performed on the left magneto which led to a loss of engine power. Contributing to the incident was the pilot's failure to check the magnetos in flight in compliance with manufacturers guidance regarding rough running engine. The pilot's decision to continue the flight after experiencing the rough running engine contributed to the necessity for a forced landing.