What happened
On June 29, 2015, at 08:06 UTC, a Robinson R22 Beta II helicopter, registration OK-ZYZ, was performing a flight from Kolín (LKKO) to Hradec Králové (LKHK) when a mechanical failure occurred near Hrádek u Nechanic. During the flight, the pilot heard a dull, muffled impact originating from the engine compartment. Simultaneously, the clutch warning light illuminated, and the engine RPM exceeded the rotor RPM.
The pilot immediately responded by entering autorotation and reducing engine power. Despite two subsequent attempts to briefly increase engine power to restore normal drive, the engine RPM continued to overshoot the rotor RPM. The pilot successfully completed the autorotation and performed a controlled landing on a field path. There were no injuries to the two-person crew, and no significant damage to the aircraft was observed upon landing.
The investigation
An investigation by the ÚZPLN focused on the drive system and the condition of the belts. The aircraft had undergone maintenance between February and June 2015, during which the drive belts and engine mounts had been replaced. At the time of the incident, the aircraft had flown 17 hours and 12 minutes since that maintenance.
Technical inspections of the drive system revealed that the rear belt had suffered a longitudinal split, with a portion of the belt having slipped off both pulleys. The upper connecting layer of the rear belt had separated and was largely missing, with fragments found near the engine. The front belt had shifted one groove backward on the driven pulley but remained intact.
Measurements of the pulley alignment, engine height, and transmission angle were conducted according to the Robinson Maintenance Manual and were all found to be within required tolerances. A detailed examination of the damaged belts was performed by the manufacturer in the presence of an FAA inspector. The manufacturer's analysis indicated that the damage was not due to a manufacturing defect but was characteristic of belt overload.
Findings
- The primary cause of the incident was the overloading of the rear drive belt.
- This overload resulted from the partial slipping of the rear belt from the grooves of the driving pulley during engine start-up.
- The manufacturer noted that if a belt slips in this manner, it may continue to function for 10 to 30 minutes before total failure occurs.
- The pilot's reaction to the engine failure was correct and in accordance with the flight manual, leading to a safe landing.