What happened
On July 31, 2002, at approximately 14:58 UTC, a Schweizer S 26 and C helicopter, registration I-LIAO, was conducting an aerial reconnaissance mission for wildfire detection near the Boccadifalco airport in Luparello, Palermo. Shortly after takeoff, while maintaining an altitude of approximately 50 to 60 meters, the pilot heard an unidentified noise followed by a left yaw. Fearing a total engine failure, the pilot initiated an autorotative descent. During the landing, the aircraft maintained an excessive pitch attitude, causing the tail rotor to strike the ground.
The investigation
The investigation focused on the mechanical condition of the engine following the incident. Technical inspections of the engine revealed that cylinder number 2 lacked compression, with a noticeable leak occurring through the exhaust. A borescope examination identified an impact mark on the piston head. Upon further disassembly of the engine, investigators discovered that a buildup of carbon deposits on the exhaust valve stem had caused the valve to stick. This malfunction led to the valve striking the piston head, resulting in a deformation that left the valve stuck in the open position, which caused a significant loss of engine power.
Findings
- The primary cause of the incident was a loss of engine power during takeoff.
- Carbon deposits on the exhaust valve stem caused the valve to seize.
- The resulting impact between the valve and the piston head deformed the valve, leaving it open and preventing proper compression.
- The pilot's execution of the autorotative landing was improper, characterized by an excessive pitch attitude that led to the tail rotor strike.
- The pilot sustained no injuries during the event.