What happened
On November 12, 2010, a Cirrus SR22, registration N918SR, departed from Milan Bresso airport for a private flight to Rimini. The aircraft was operated by a single pilot. Approximately 34 minutes into the flight, the engine began experiencing a loss of power.
While flying at a constant altitude, the pilot initially signaled an intention to divert to Carpi airport due to technical difficulties. However, by 09:38 UTC, the pilot declared an emergency and initiated a forced landing. The pilot identified a clear area in the municipality of Luzzara, free of houses and obstacles, and executed the landing in a ploughed field. During the impact, the aircraft's nose gear and left main gear struck the ground, causing the aircraft to bounce and eventually come to rest on its right wing. The impact resulted in substantial damage to the aircraft, including the separation of the right wing from the fuselage, but the pilot escaped without injury.
The investigation
The ANSV investigation utilized digital navigation data from the aircraft's Avidyne Multi-Function Displays (MFD), which recorded engine parameters and flight paths. Analysis of the engine data revealed a sequence of thermal anomalies. At approximately 09:13 UTC, a reduction in fuel flow was observed alongside rising exhaust gas temperatures (EGT) across all six cylinders. Shortly after, a localized and extreme temperature spike occurred in cylinder number 5, with EGT reaching 904°C and significant increases in cylinder head temperature (CHT).
Following this spike, the engine experienced high oil temperatures, triggering the oil warning light. The investigation team also examined the engine components at a certified facility. Physical inspection of the engine revealed that the top spark plug of cylinder number 5 had suffered insulator damage. Furthermore, metallic debris from a broken piston ring in cylinder number 5 was found distributed through the intake tracts of other cylinders, causing minor damage to their piston heads.
Findings
- The primary cause of the engine power loss was a localized pre-ignition event occurring at the edge of the cylinder number 5 head, near the piston pin.
- This pre-ignition caused intense localized heat, leading to the partial melting of the piston head and the subsequent failure of the upper piston ring.
- The failure of the piston ring released metallic fragments into the intake system, which were then drawn into other cylinders.
- While the exact trigger for the pre-ignition in cylinder 5 could not be determined with absolute certainty, the engine had been modified with a turbocharging system (STC) that increased thermal and mechanical stresses.
- A subsequent mandatory service bulletin from the turbocharger manufacturer noted that deteriorating ceramic insulators in certain spark plug models could induce similar pre-ignition phenomena.