What happened
On September 27, 2011, a CESSNA 172 H “Reims”, registration EC-CXP, was conducting a dual-control instructional flight departing from Seville Airport (LEZL). The aircraft was occupied by an instructor, a student pilot, and one passenger. Approximately four minutes after takeoff, while at an altitude of roughly 1,000 feet, the engine began misfiring and experiencing a loss of RPM.
The instructor notified the control tower of the engine issues and initially intended to return to the airport. However, as the aircraft continued to lose altitude, the crew determined they could not reach the runway and opted to perform an off-field landing at the old San Pablo military base, located southwest of the airport. The landing was completed without serious injury to the occupants, though the engine sustained damage.
The investigation
Following the incident, the engine was disassembled for inspection. Investigators discovered that the rocker arm for the number 2 cylinder exhaust valve had broken into two pieces. This specific cylinder had previously undergone repairs approximately six weeks prior due to low compression and a seized exhaust valve; however, the rocker arms and bushings were not replaced during that maintenance event.
Technical analysis of the broken component revealed a fatigue fracture originating on the outer surface of the cup side. Laboratory testing of the materials showed no significant structural or compositional anomalies that would have inherently caused the failure. Furthermore, investigators noted that the rocker arms installed in the engine bore the part number 35636/RR, which the manufacturer had declared obsolete in 1971 and replaced with a different part number. The manufacturer's documentation did not explicitly list the O-300-D engine as a compatible user of this specific obsolete part.
Findings
- The CESSNA 172 H experienced an engine malfunction due to the fracture of the number 2 cylinder exhaust valve rocker arm.
- The fracture prevented the valve from opening correctly, obstructing exhaust gas escape and causing the engine to misfire.
- The engine was using obsolete rocker arms that had been superseded by the manufacturer decades earlier.
- While the engine had undergone recent repairs for low compression in cylinders 2 and 4, the rocker arms and bushings were not replaced during that service.
- The investigation could not definitively determine if the failure was linked to the use of obsolete parts or the lack of bushing replacement during the previous overhaul.