What happened
During flight, the pilot of VH-SXC, a Robinson helicopter, identified a malfunction within the drive train. The pilot observed a simultaneous increase in engine speed and a decrease in rotor speed. Following the pilot's manual, the pilot responded to the clutch warning light by pulling the clutch circuit breaker to isolate power. Amidst a burning rubber smell and audible banging, the pilot navigated the aircraft toward Long Island. The pilot successfully executed an emergency landing on a suitable location, resulting in no injuries and minimal damage to the aircraft.
The investigation
Investigators examined the maintenance history and the drive belt stretching procedure used on the aircraft. While repairs and belt replacements had followed manufacturer instructions and engineering orders, the investigation focused on the method used to address tight belts. Although the procedure involved engaging the clutch actuator to stretch the belts, the Robinson Helicopter Company later identified that this specific method could contribute to excessive slack in the belts during start-up. Furthermore, while the manufacturer intended for the stretching process to be a single application, feedback indicated it was being performed multiple times, which likely caused the belts to reposition incorrectly during operation.
Findings
- The drive belt stretching procedure in use at the time likely caused the belts to be overstretched.
- This overstretching allowed the drive belt set to become misaligned during system tensioning.
- Two forward drive belts moved from their sheaves and became trapped against rotating parts, causing them to abrade.
- The resulting load shift onto the remaining two rear belts disrupted the rotor system drive, necessitating the landing.