What happened
On July 26, 2003, a Robinson R44 Raven, registration HB-ZCU, was conducting a private VFR flight near Selzach, Switzerland, carrying a pilot and two passengers. While cruising at approximately 4,500 ft AMSL and 80 knots, the aircraft experienced sudden low-frequency vibrations and fluctuating engine RPM. As the engine power failed, the pilot initiated an autorotation to perform an emergency landing.
During the final phase of the autorotation, the aircraft touched down in a field with a high sink rate and minimal forward speed. During this maneuver, the tail skid made contact with the ground, causing a main rotor blade to strike the tail boom. Despite the impact, all three occupants escaped the Robinson R44 Raven without injury. The aircraft sustained significant damage to the rotor blades, engine cowling, and tail boom, while minor damage to the surrounding terrain was reported.
The investigation
Investigators examined the engine components and the flight sequence to determine why the power loss occurred. The investigation focused on the mechanical state of the Lycoming engine and the maintenance history of the aircraft. The team analyzed the engine's valve clearances and the impact of engine oil additives on component longevity. Additionally, the investigation reviewed the pilot's emergency procedures and the physical evidence at the crash site, including the condition of the tail boom and the engine's internal components.
Findings
- The engine suffered a significant loss of power because an exhaust valve in cylinder number 3 became completely blocked.
- This blockage caused the valve pushrod to bend and break.
- Carbon, lead, and bromine deposits on the valve stems and guides contributed to the reduced clearances that led to the valve seizure.
- The use of multi-viscosity oils, which contain synthetic additives, may have increased the tendency for coking (carbon buildup) at high operating temperatures.
- The pilot correctly identified the engine failure and executed a proper autorotation procedure.
- The collision between the main rotor blade and the tail boom occurred either due to the tail skid hitting the ground or the high sink rate during the hard landing.