What happened
On September 2, 2004, at approximately 11:45 local time, a Cessna 172N, registration EC-HAT, departed from Cuatro Vientos Airport for a local instructional flight. The flight was being conducted as a dual-instruction operation with an instructor and a student on board.
After approximately one hour of flight maneuvers, the crew performed a simulated engine failure near Villadelprudence, Madrid. While the aircraft initially recovered to 2,500 rpm and gained sufficient altitude and speed, the engine power subsequently dropped to 2,000 rpm once the aircraft reached 600 feet above ground level and flaps were retracted to 10 degrees. This drop in power was accompanied by severe vibrations and engine surging. The pilot attempted to rectify the issue by adjusting the mixture settings, first leaning and then enriching it, but the engine failed to respond. Unable to maintain altitude, the pilot executed an emergency landing in a field at Finca del Alamín, Toledo. Both crew members survived the landing without injury, and the aircraft sustained no damage.
The investigation
The investigation focused on a detailed inspection of the Lycoming O-320-H2AD engine. During the teardown, several critical findings were identified in cylinder number 2:
- The pushrod and its protective sleeve were found to be bent.
- Significant scoring was present on the valve stem.
- Carbon deposits were noted at the bottom of the cylinder, and a distinct circular mark was visible on the piston.
Technical analysis determined that the circular mark on the piston was caused by the exhaust valve. The investigation established that the valve had become stuck in a fixed position, and the impact of the piston against the valve caused the release of excess material.
Findings
- The primary cause of the engine power loss was the seizing of the exhaust valve due to high temperatures.
- The valve became stuck in an open position, which prevented the cylinder from maintaining compression during the compression stroke, leading to an ineffective combustion cycle and insufficient power.
- The presence of contaminants and inadequate lubrication likely contributed to the valve seizing via a welding effect under high thermal loads.
- The impact of the piston hitting the stuck valve was powerful enough to transmit force through the assembly, resulting in the bending of the pushrod.