What happened
On 26 May 1998, a Robinson R22 Beta, registration G-BOAM, was conducting a training flight at Redhill Airfield. The student pilot was preparing to perform a simulated engine-off autorotation from 1,000 feet. As the instructor moved the throttle into the detent to begin the descent, a loud bang originated from the rear of the aircraft, followed by a rapid yawing motion to the left and right.
Taking immediate control, the instructor fully closed the throttle and completed an engine-off landing. An initial external inspection by the crew revealed no obvious damage, and a subsequent engineering check of the drive train also showed no faults. Although a single feather was found near the approach path, suggesting a possible bird strike, no other evidence supported this theory. During a later inspection at the dispersal, the crew noted that the clutch pulley occasionally hung up during rotation, prompting the removal of the assembly for investigation.
The investigation
Upon dismantling the sprag clutch assembly, investigators discovered that the component had suffered significant internal damage. One sprag had completely failed, with its ends broken and lost, while another sprag remained in the cage but was missing one end.
Detailed metallurgical analysis by the AAIB determined that the failures were caused by a medium cycle tension fatigue mechanism. The examination revealed that the sprags had been subjected to heavy frettage and wear, and the load transmission faces showed signs of spalling. This damage indicated that the sprags had been tilting relative to the rotational axis during operation. Furthermore, the outer diameter of the driveshaft showed evidence of eccentric running within the assembly.
Investigators also examined a sprag clutch from another Robinson R22, registration G-BUIW, which had been involved in a fatal accident earlier that year. The metallurgical findings from that component were nearly identical to those found in G-BOAM, showing a failure of a sprag ear due to high cycle fatigue.
Findings
- The primary cause of the disturbance was the failure of the sprag clutch assembly due to fatigue.
- The damage was driven by heavy frettage and wear on the sprags and housing.
- A manufacturing process change implemented by the clutch supplier affected specific batches of components used in both R22 and R44 helicopters.
Safety action
Following the identification of the faulty manufacturing process, the helicopter manufacturer issued Service Bulletins SB-85 (for the R22) and SB-32 (for the R44). These bulletins mandated the replacement of all affected sprag clutch assemblies within 150 flight hours or by 31 July 1999, whichever came first.