Event Description
On 5 March 2013, a Cessna C-152 was taking off from Bielsko-Biała Aleksandrowice Airport (EPBA). After lift-off and transitioning to climb, the crew—a student pilot and an instructor—noted a drop in engine power accompanied by strong vibrations. The instructor determined that aborting the takeoff and landing straight ahead would not allow the aircraft to stop safely within the airport boundaries, so they decided to continue. The aircraft completed a small circuit and landed safely. After taxiing, an engine run-up and compression check were performed. Cylinders 1, 2, and 4 showed normal pressure readings, while cylinder 3 registered zero pressure. The ignition system was found to be functioning correctly.
Maintenance Background
Prior to the incident, the engine had undergone maintenance by Organization A1, including 100-hour and 400-hour inspections, as well as tasks required by Service Bulletin SB388C. This bulletin involved checking valve stem-to-guide clearances. During these checks, cylinders 3 and 4 were found to have excessive clearances beyond recommended limits. Consequently, both cylinders were sent to Organization B1 for repair. After repair, Organization A reinstalled the cylinders on the engine, performed adjustments and an engine run-up, all parameters being normal. The engine then operated for approximately 50 minutes before the incident.
Investigation Findings
Following the incident, cylinder 3 was removed and sent to Organization B for verification. Inspection confirmed that the exhaust valve was stuck in the open position. The guide bore and valve stem diameters were within specifications, but scoring was observed on the lower part of the guide. The scoring was caused by a fragment of carbon deposit that had become lodged between the guide and the valve stem during engine operation. The investigation was unable to determine the origin or timing of when the carbon fragment entered the gap.
Probable Cause
The investigation concluded that the incident resulted from a carbon deposit fragment entering the space between the guide and stem of cylinder 3's exhaust valve during engine operation, causing the valve to jam open. The specific source and moment of entry could not be established.