Incident Overview
On 23 May 2018, at 10:05 UTC, a student pilot departed Babice airfield in a Cessna 152 for a cross-country flight to Nowe Miasto at an altitude of 1,700 ft. After completing the enroute portion, the pilot began the descent for landing. During the descent to circuit altitude (1,500 ft), after activating carburetor heat and reducing engine RPM to 1,500, the engine began to run unevenly, with RPM fluctuating between 1,100 and 1,500.
Pilot Actions
The pilot immediately advanced the throttle to full power and checked the mixture control, which was in the rich position. Despite these actions, the RPM continued to fluctuate between 1,200 and 2,000. The pilot informed the flight operations manager, who granted landing priority and instructed the pilot to execute an approach to Runway 08 as if the engine were not operating.
Landing and Inspection
The aircraft landed without further incident. After landing, a post-flight engine inspection revealed that the igniters (spark plug assemblies) were excessively contaminated with grease.
Probable Cause
The incident was caused by intermittent interruptions in the operation of the igniters due to grease, applied in excessive quantities during routine maintenance, that had melted under high temperature and migrated onto the contact breaker points.
Findings
No injuries were reported. The pilot was the sole occupant and uninjured. No recommendations were formulated by the investigating body.