Introduction
On 27 September 2011, at approximately 16:44 UTC, a Cessna 172 H (registration EC-CXP) operated by Aeroclub de Sevilla experienced an engine malfunction during a local instructional flight from Seville Airport (LEZL). The aircraft was occupied by a 33-year-old instructor, a 27-year-old student pilot, and one passenger. No injuries were reported, and the aircraft sustained damage limited to the engine.
Flight and Incident
The aircraft departed Seville Airport at 16:39 UTC from runway 09. While climbing to about 1,000 ft over point Sierra, the engine began misfiring and losing revolutions per minute (RPM). The instructor decided to return to the airport and informed the control tower. Unable to reach the runway, the instructor chose to land at the old San Pablo military base, located southwest of the airport. The aircraft landed without further incident.
Aftermath and Inspection
Post-landing inspection revealed no damage to the airframe, but the engine emitted loud noises at low RPM. Disassembly of the engine showed the rocker arm for the number 2 cylinder exhaust valve broken into two pieces. The remaining cylinders' rocker arms were undamaged. No other mechanical anomalies were detected, and oil residue indicated proper lubrication. The rocker arms installed on the engine carried part number 35636/RR.
Maintenance History
The engine, a Teledyne Continental Motors TCM/RR O-300-D, had accumulated 490 hours since its last overhaul in 2004. On 18 August 2011, cylinders 2 and 4 were repaired due to low compression caused by seized exhaust valves. During that repair, the rocker arms were visually inspected but not replaced, as the maintenance manual did not require it. The aircraft had flown approximately nine hours since that repair.
Manufacturer Information
According to the engine manufacturer, the rocker arm part number 35636 was declared obsolete in 1971, replaced by part number 639446. The O-300-D engine was not listed among those originally using part 35636; the closest model was the O-300-A.