What happened
On June 7, 2006, a Robinson R22 Beta 2, registration F-GUPC, was conducting a flight training session near the Le Touquet Paris Plage aerodrome. The flight began with fifteen minutes of ground effect maneuvering, followed by circuit patterns. During the second circuit, while flying on a downwind leg, the student pilot heard an unusual noise and felt a sudden jerk on the yaw axis. Immediately following this, the "LOW RPM" audio warning sounded and the "CLUTCH" light illuminated, signaling a loss of engine power transmission. The instructor took control, increased rotor RPM, executed a left turn, and performed an emergency autorotation, landing the aircraft in a garden within a forest.
The investigation
The investigation focused on the mechanical state of the clutch system, which consists of two double belts. Investigators discovered that one half of the rear belt was missing, and the remaining belt segments had slipped out of their grooves and folded over themselves. While the belts themselves did not show signs of pre-existing wear, the missing belt segment had left traces on the engine and airframe components before being ejected.
Analysis of the fan shaft nut position indicated that the engine had experienced an overspeed condition, which is consistent with a sudden loss of load from the rotor. Furthermore, damage to the fan flange and the washer mounted on the fan drive shaft revealed that this assembly had been improperly tightened. The aircraft had undergone a major inspection in 2005, during which the tail boom and clutch belts were replaced. The investigation determined that the fan drive assembly was likely not tightened correctly during this maintenance period.
Findings
- The primary cause of the power loss was the failure of a belt segment to remain seated in its groove during engine start-up.
- The missing belt segment was eventually severed by the rear edge of the pulley after approximately twenty minutes of flight.
- The failure of the remaining belts was caused by the severed segment becoming lodged between a pulley and the other belts, causing them to flip out of their grooves and interrupt power transmission.
- Improper maintenance during a 2005 overhaul, specifically the inadequate tightening of the fan drive assembly, created vibrations that likely contributed to the belt slipping out of its groove during start-up.
- The pilot did not detect any belt tension anomalies during the start-up sequence.