What happened
On 13 September 2000, a Robinson R22 Beta, registration G-BYHE, was involved in an accident at Wycombe Air Park during a training flight. The pilot, who was working towards a Private Pilot's Licence, was conducting a pre-solo check flight under the supervision of a Chief Flying Instructor (CFI). The weather conditions were fine with light winds and no low cloud.
After completing initial circuits and being declared competent for solo flight, the instructor exited the aircraft. While the helicopter was on the ground, the pilot began raising the collective pitch control lever. As the manifold air pressure reached 20 inches, the aircraft experienced a violent juddering. Shortly thereafter, the aft section of the tail boom detached, leaving the tail rotor resting on the ground. The instructor, observing the failure from a short distance, immediately entered the aircraft and shut down the engine to prevent an attempted takeoff.
The investigation
The investigation focused on the structural failure of the tail boom and the cause of the engine overspeed. The tail boom was found to have separated into three distinct portions, with two total separations of the boom structure occurring. The investigation examined the tail rotor drive shaft, which exhibited a torsional failure at the forward coupling.
Technical analysis of the shaft's vibration characteristics revealed that the damage was consistent with the shaft whirling in a specific harmonic mode occurring at 132% RPM. This high-speed vibration caused the internal frames of the boom to fragment. Furthermore, the engine was inspected, and evidence of valve bounce and cylinder bore damage was found, both of which are indicators of an engine overspeed. The governor system was also tested; while the system functioned correctly, the friction brake torque on the actuator was found to be higher than the manufacturer's specification, meaning more force was required to move the throttle.
Findings
- The structural failure of the Robinson R22 Beta was caused by the tail rotor drive shaft entering a high-frequency whirl mode at 132% RPM.
- The engine overspeed was most likely caused by incorrect management of the throttle by the pilot, specifically by opening the throttle and preventing it from backing off while raising the collective.
- The pilot's focus on monitoring the manifold air pressure gauge, combined with the use of an Active Noise Reduction (ANR) headset, likely prevented the audible increase in engine RPM from being detected.
- The governor actuator's friction brake was set to a higher torque than recommended, making the throttle harder to manipulate than normal.