What happened
On March 9, 2003, a private SOCATA Rallye 100ST, registration EC-ICI, departed from Cuatro Vientos Airport for a planned three-hour VFR flight. Approximately two hours and 20 minutes into the flight, while cruising at 4,000 feet near the Castejón VOR, the engine experienced a sudden drop in power, with RPM falling from 2,400 to 2,200. The pilot followed standard emergency procedures, including enriching the mixture and adjusting the fuel pump, which temporarily restored power. However, the engine suffered a second power loss, followed shortly by a third, more severe failure that dropped RPM to 1,600.
Unable to maintain altitude, the pilot searched for a suitable landing site within the difficult terrain of the Guadalajara region. The pilot identified a section of the CM-2007 road near Pastrana as the only viable option. After verifying the road was clear of vehicles and establishing emergency contact with another aircraft via the 121.5 MHz frequency, the pilot attempted the landing. During the final approach, the engine ceased functioning entirely. While the aircraft successfully reached the roadway, the right wingtip struck a traffic sign, causing the aircraft to veer right, exit the pavement, and come to a rest against a concrete curb. Both occupants survived the accident without injuries.
The investigation
Investigators examined the wreckage and the engine components. The aircraft had undergone a 100-hour maintenance inspection only two days prior to the accident, which had included successful cylinder compression tests. Field inspections revealed damage to the engine cowling, propeller, and nose gear, as well as a broken right wingtip.
Upon disassembly of the engine, investigators found that the spark plug for cylinder number 3 had been struck by debris. The cylinder contained fragments of the piston, valve, and piston rings. Laboratory analysis of the exhaust valve fragments revealed radial cracks caused by thermal fatigue. Furthermore, the valve's retaining semi-cones showed significant wear due to friction, which had eventually allowed the valve stem to dislodge.
Findings
- The primary cause of the engine failure was the breakage of the exhaust valve head in cylinder number 3 due to thermal fatigue.
- The failure sequence began when fragments of the valve head broke off and fell into the combustion chamber.
- As the piston moved upward, it struck a valve fragment, causing a violent displacement that dislodged the valve'retaining semi-cones.
- The loss of the valve stem led to the valve falling into the combustion chamber, where it interfered with the piston and cylinder head, resulting in the total engine failure.