Incident Details
On 12 September 2006, at approximately 16:00 UTC, a Robinson R44B helicopter, registration CS-HDM, operated by Helibravo, was en route from Coimbra Aerodrome to Maia Aerodrome (Vilar da Luz) on an aerial photography mission. The aircraft was carrying a pilot and one passenger (photographer) and was flying at about 1000 feet above mean sea level near Anadia, Portugal. Suddenly, a loud bang was heard, accompanied by a tail shake. The low rotor rpm warning horn sounded as the engine rpm dropped. The pilot immediately reduced collective pitch, maintaining rotor rpm with partial engine power, and executed an autorotation landing in an unprepared but obstacle-free field near Carvalhal. The occupants exited the helicopter unaided, with no injuries. The aircraft sustained no damage during the landing.
Investigation
A visual inspection of the engine revealed an anomaly in the pushrod tube of the exhaust valve of cylinder No. 2, which was found bent, suggesting a probable fracture of the valve stem. The helicopter was transported to the company's maintenance base for further examination. Subsequent disassembly and boroscopic analysis confirmed that the exhaust valve was stuck due to the accumulation of hardened carbon deposits (from combustion) on its seat. The cylinder was removed, and all cylinders were sent for rectification, which included cleaning, valve lapping, and replacement of bushings in two rocker arms. The engine had a total of 957:45 hours since new, with 286:35 hours since overhaul.
Probable Cause
The investigation determined that the incident was caused by sticking of the exhaust valve of cylinder No. 2 due to the accumulation of lead salts between the valve stem and guide. This condition had been previously addressed in Textron Lycoming Service Instruction No. 1425A (January 1988), which noted that such sticking can occur, especially in high ambient temperatures, due to lead salt deposits from combustion accumulating in valve guides.