Introduction
On April 30, 2015, a Cessna 152, registration N49158, experienced an engine malfunction and made a forced landing near Pilot Point, Texas. The instructional flight, operated by U.S. Aviation Group, LLC., under Part 91 visual flight rules, had departed Denton Enterprise Airport (KDTO) about 1100 central daylight time. Both the flight instructor and student pilot were uninjured.
Sequence of Events
The student pilot had just completed steep turns when the engine began running roughly, but the roughness subsided. After another steep turn, the roughness returned. The flight instructor executed various troubleshooting procedures, including a magneto check, but was unable to restore smooth operation. Engine rpm fluctuated between 1,200 and 2,000, and the instructor could not maintain altitude. He selected a nearby field for landing, which resulted in minor damage to the nose landing gear and engine mount.
Examination Findings
Post-incident examination of the engine revealed that the number 2 and number 4 lower spark plugs were lead-fouled and inoperative; number 1 and number 3 lower spark plugs were also lead-fouled but still operational. Further inspection identified that the distributor gear finger (part number 510406) on the left magneto (Slick Magneto, serial number 14040289/model 4381) had separated. The same component on the right magneto (serial number 14040835/model 4381) was found loose. Both magnetos had accumulated 478.8 hours of total time.
Fleet-Wide Discovery
Following the incident, U.S. Aviation Group, LLC. inspected their remaining fleet and discovered 19 additional magnetos with loose distributor gear fingers. The affected models included 4381 (the same as the incident magnetos), as well as models 4370, 4371, 4372, and 4374. Total times on these magnetos ranged from 48 hours to 2,168.5 hours. All had been installed new at engine overhaul or during maintenance. The company's Director of Maintenance noted that the method of attaching the finger to the gear differed from earlier designs.
Manufacturer Response
Champion Aerospace, the manufacturer under the Slick brand, reported that they had seen 16 magnetos exhibiting similar failure modes: 9 with distributor finger separation and 7 with loose fingers. They stated there had been no design change. However, controls for insertion of the finger into the gear and an additional inspection of this procedure had since been implemented. On July 2, 2015, Slick by Champion Aerospace issued Service Bulletin 1-15, requiring removal and replacement of the distributor gear assembly on certain Slick magnetos at the earliest convenient maintenance time, not to exceed the next 50 hours. The magnetos involved in this incident were among the models covered by the bulletin.