What happened
On March 31, 2022, a Robinson R44 helicopter, registration CS-HFU, was performing a private flight from Évora to Moura, Portugal. Approximately 10 minutes into the flight, while cruising at 1,500 feet, the pilot was alerted by audible and visual warnings indicating low rotor RPM. The pilot immediately lowered the collective to reduce engine power demand and recover the RPM.
While the rotor RPM initially returned to 100%, the pilot observed engine RPM fluctuations between 100% and 104%. Due to these oscillations, the pilot notified flight information services of the intention to return to Évora. During the return flight, the pilot attempted to use the throttle to manage the engine RPM but was again alerted to low rotor RPM. When attempting to regain power to maintain altitude and flight path, the rotor RPM dropped once more.
Unable to maintain altitude, the pilot executed a precautionary landing at the intersection of two rural roads. During the final phase of the controlled descent, the rotor RPM dropped below established limits again. The pilot performed a "rolled" landing with minimal horizontal speed, bringing the aircraft to a stop within a few meters. Upon exiting the aircraft, the pilot discovered significant oil leakage from the engine cowlings and on the ground. There were no injuries to the pilot, but the engine sustained substantial damage.
The investigation
The GPIAAF conducted a detailed analysis of the engine following the incident. The investigation focused on the Lycoming IO-540 six-cylinder engine. The examination revealed that the exhaust valve camshaft in cylinder #2 had failed due to the valve becoming stuck.
Findings
- The failure was caused by the exhaust valve becoming stuck due to the accumulation and solidification of lubricating oil between the valve stem and the seat. This process caused surface erosion, leading to mechanical blockage.
- In helicopter engines, which operate at a near-constant engine speed (2,665 rpm for 100% NR in this model), components like valves are exposed to high levels of thermal stress.
- The investigation noted that buildup of combustion deposits on the valve stem can act as an abrasive, accelerating wear in the valve guide and leading to poor sealing and eventual failure.
Safety action
Following the incident, the operator implemented several mitigation measures:
- Increased frequency of inspections for Lycoming MSB 388C and SB 301B to every 100 flight hours instead of the standard 300 hours.
- Implementation of more frequent engine oil sampling and analysis, potentially as often as every 25 to 50 flight hours.
- Briefing all R44 fleet pilots on the contributing factors regarding valve sticking as identified in manufacturer documentation (SI 1425A and SL 197A).
- Evaluating options for engine monitoring modifications for older aircraft in the fleet where manufacturer-approved retrofits are currently unavailable.