What happened
On September 12, 2006, a Robinson R44B helicopter, registration CS-HDM, was conducting an aerial photography mission from Coimbra to Maia. The flight was carrying a pilot and one passenger. While cruising at approximately 1,000 feet AMSL near Anadia, the crew heard a loud bang followed by a sudden tail movement. Simultaneously, the engine power dropped, and the low rotor RPM warning light activated.
Responding immediately, the pilot adjusted the collective to stabilize the rotor RPM and executed an autorotative landing in an unprepared agricultural field near Carvalhal. The landing site, a vineyard with some small pine trees, allowed for a safe approach. The pilot successfully cleared larger cypress trees in the vicinity due to the remaining engine power during the descent. The occupants exited the aircraft uninjured, and no damage was sustained by third parties.
The investigation
The GPIAAF investigation focused on the engine's performance and the physical state of the components. A visual inspection of the engine revealed a deformed sleeve around the exhaust valve stem of cylinder number 2, indicating a probable fracture of the stem.
Upon disassembly and borescope analysis, investigators confirmed that the valve had become stuck due to the accumulation of hardened carbon deposits in the valve seat. Further examination of the engine revealed lead salt deposits between the valve stem and the guide. The investigation also noted that the aircraft had experienced relatively low usage in recent months, with long periods of inactivity.
Findings
- The engine suffered a partial power loss caused by the sticking of the cylinder number 2 exhaust valve.
- The valve seizure was caused by the accumulation of lead salts between the valve stem and the guide, as well as in the valve seat.
- These deposits originated from the combustion of 100LL aviation gasoline, which contains a high percentage of lead.
- The accumulation of these residues was exacerbated by the aircraft's low utilization rate and long periods of inactivity, which allowed the deposits to harden.
- Environmental factors, such as high ambient temperatures and limited cooling during low-speed flight, contributed to the formation of these deposits, a known issue in Lycoming engines as documented in various Service Instructions.