What happened
On 9 January 1999, a Robinson R22 Beta, registration G-TTHC, was conducting a private flight from Bristol Airport (Lulsgate). Shortly after takeoff, at an altitude of roughly 400 feet, the pilot detected a strong burning odor, which was initially suspected to be an electrical fire. In response, the pilot deactivated the alternator, notified air traffic control, and entered a right-hand orbit to prepare for an emergency landing on a taxiway.
During the transition into a hover, the aircraft experienced a simultaneous increase in both engine and rotor speed beyond the yellow arc limits, accompanied by a descent. Despite these fluctuations, the pilot managed to complete a successful run-on landing. There were no fatalities and no injuries to the two occupants. Following the landing, the engine was shut down and the rotor brake applied. Although the Airport Fire Service attended the scene, no actual fire was detected.
The investigation
Investigators examined the aircraft and the specific component involved in the incident. The investigation focused on the upper sheave bearing (Assembly No A184-3), which is located on the main gearbox/tail rotor drive shaft. This bearing supports the large pulley driven by V-belts from the engine. The mechanism uses a linear actuator to tension these belts, a process controlled by load-sensing switches.
While the actuator was found to be fully functional, the bearing itself had suffered extreme overheating. Because the temperatures reached during the failure were so high, the grease, balls, and synthetic rubber seals were destroyed, making direct microscopic analysis of those elements impossible. However, the absence of long-term wear on the bearing's races suggested that the overheating was a sudden event caused by a recent loss of lubrication.