What happened
On May 25, 2018, a Robinson R44 II, registration HB-ZJW, was conducting a mountain flight training mission (MOU) departing from Sion Airport. The crew, consisting of an instructor and a student pilot, intended to perform several landings at various mountain locations, including the Staldenhorn site in the Canton of Bern.
After completing several landings at altitudes below 2,000 meters, the helicopter landed at the Staldenhorn landing site. Shortly after touchdown, while the engine was at reduced power, the crew heard a sharp noise. This was immediately followed by an increase in manifold pressure, a drop in engine RPM, and smoke emerging from the engine compartment. The engine subsequently ceased operation. Following the incident, engine oil was observed leaking from the aircraft, and the helicopter was deemed unserviceable.
The investigation
The investigation focused on the Lycoming IO-540-AE1A5 engine, which had 804 hours since its last overhaul. The engine was transported to a specialized center for a comprehensive teardown, and specific components were sent to a laboratory for macroscopic analysis.
Upon disassembly, investigators identified that the intake valve of cylinder number one had broken. The valve head had fractured into three separate pieces, and the stem had become detached from the head. \n## Findings
- The primary cause of the engine power loss was the rupture of the intake valve in cylinder number one.
- Detailed examination suggested that the engine may have been subject to engine detonation, although the specific origin of these detonations could not be determined.
- These detonation events likely led to the premature wear of the intake valve in cylinder number one.
- While cylinder number five was noted as the hottest cylinder, cylinder number one benefited from better ambient air convection cooling; however, the structural failure of the valve led to the engine failure.