What happened
On 3 May 2002, a Schweizer 269C helicopter, registration EI-LRS, was performing a private flight near Lisacul, Co. Roscommon. The pilot intended to fly from his home base to Knock Airport to refuel. After a successful pre-flight inspection and engine start, the pilot lifted off into a hover at approximately 5 feet.
While maintaining this hover, the pilot engaged the engine speed governor. Immediately following this action, the engine RPM surged rapidly, and the aircraft began to spin violently to the left. The sudden loss of transmitted power caused the helicopter to rotate roughly 150 degrees before the skids struck the ground, causing the aircraft to topple. The impact resulted in an economic write-off of the aircraft, with significant damage to the main rotor blades, gearbox casing, and tail boom, though there were no injuries to the pilot.
The investigation
An investigation by the AAIU examined the wreckage and the maintenance history of the aircraft. Investigators focused on the clutch system, which uses V-belts to connect the engine to the main gearbox. The investigation established that the clutch cable had failed.
Metallurgical analysis of the cable revealed that the failure occurred at the swaged ferrule. The examination found that the outer strands of the cable had been twisted, which caused the internal core strand to carry a disproportionate amount of the load. This uneven distribution of stress led to an overload failure of the core strand. Additionally, investigators noted evidence that pliers had been used on the turnbuckle during adjustment, rather than the provided tool hole, which may have contributed to the cable twisting.
Findings
- The primary cause of the accident was the failure of the clutch cable due to unequal strand loading.
- The cable had been twisted during the tensioning process, likely during the recent replacement of the V-belts.
- The twisting of the cable caused the core strand to bear excessive stress, leading to a fatigue-related or overload fracture.
- The sudden separation of the cable caused the engine to disconnect from the rotor load, leading to an immediate engine overspeed and a loss of rotor RPM.
- The pilot's decision to engage the governor during the hover may have contributed to the failure by creating a sudden surge in load.
Safety action
- The manufacturer should update the Maintenance Manual to caution against cable twisting during tensioning.
- Maintenance instructions should be revised to mandate the use of the turnbuckle hole to prevent rotation during adjustment.
- The manufacturer should review the procedure for adjusting the clutch cable to determine if adjustments can be made without tension.
- The manufacturer should promote the installation of an upgraded clutch cable design via service bulletins.
- Flight Manual procedures regarding the use of the engine speed governor during take-off and hover should be clarified to ensure safe operating practices.