History of the Flight
On 13 September 2000, at 1423 UTC, a Robinson R22 Beta (registration G-BYHE) was engaged in a training flight at Wycombe Air Park (Booker), Buckinghamshire. The student pilot, who held a Commercial Pilot's Licence for aeroplanes, was preparing for a solo check under the supervision of the Chief Flying Instructor (CFI). Weather conditions were fine with light wind and no low cloud.
The student completed a pre-flight inspection and started the engine using the checklist. After receiving clearance from ATC, he hover taxied to the helicopter operating area 'November' on the north side of the airfield. Two dual circuits were flown to the CFI's satisfaction, who then declared the student competent for a solo circuit. The CFI delivered a short briefing, exited the helicopter, secured his seat belt, and walked away in the 10 o'clock position from the nose. Before leaving, he noted that engine RPM (ERPM) was set at 104% and the governor was engaged with its warning light extinguished.
After walking approximately 10 metres, the CFI heard the engine note increase and turned to see the aft section of the tail boom detached and hanging down, with the tail rotor resting on the ground. The student remained seated with the engine running. The CFI motioned for him to stay on the ground, then entered the helicopter and shut down the engine. No injuries were sustained by either occupant.
Examination of the Helicopter
The forward section of the helicopter showed no abnormal features. The tail boom, however, was found in three portions. The first portion, consisting of three bays, remained attached to the helicopter. The second portion comprised the next two bays, and a further failure occurred at the penultimate frame station, leaving the final bay attached to the tail rotor gearbox. Two total separations of the boom structure had occurred, with the drive shaft as the only component uniting the failed sections.
The drive shaft was complete but the forward coupling exhibited a torsional failure. The drive through the tail rotor gearbox was intact and free to turn. The damper bearing support bracket had been pulled from its mounting on the frame and broken away from the strap, likely due to striking the interior of the forward boom section during break-up. The bearing cage was damaged from being twisted out of alignment, but the bearing surfaces were undamaged.
Examination of the rivet failures revealed no fractured rivets remaining in the holes, unlike typical overload failure. There was elongation of the holes and polishing of the mating faces of the skin lap joints. No predominant failure loading direction could be determined. The frames coincident with the two failed joints were reduced to small fragments, while other frames were less damaged.
Tail Rotor Drive Shaft Whirl Modes
The manufacturer supplied information indicating that the tail rotor drive shaft design featured whirl modes at 15%, 40%, 60%, and 132% RPM. The shaft fundamental (15%) is normally damped by the damper bearing. The mode at 40% is a fundamental whirling of the section between the damper bearing and rear coupling. The second order whirl of the whole shaft length occurs at 60% RPM and should be damped. However, since the drive is normally engaged above these speeds, the shaft accelerates and decelerates rapidly, leaving insufficient time for significant vibration amplitude to develop.
At 132% RPM, a further whirl mode is encountered where the damper bearing has no effect as its position coincides with a node. The shaft deflects into three anti-nodes; the two rearmost fall close to the planes of two frames, while the intervening node falls almost in the plane of another frame. The two frames near the rearmost anti-nodes were highly fragmented and coincident with the failed riveted joints. The frame/joint near the node between them was lightly damaged. The structural damage was therefore consistent with the effects of the shaft whirling in the harmonic mode at 132% RPM. No other explanation for the damage distribution could be postulated.
Further confirmation of excessive rotor speed came from the overspeed inspection performed per the maintenance manual. This called for examination of the main blade root bearings: severe 'notchiness' was found on both units. The engine was stripped and showed evidence of valve bounce on the piston crowns and damage to the cylinder bores, both indicators of overspeed.
Governor System
The three main components of the engine governor were inspected and tested by the manufacturer with an AAIB Inspector present. The system uses an electrical signal from the right magneto to maintain RPM within a datum band (102.5-105.5% ERPM). Individual tests revealed only one area outside tolerance: the friction brake torque loading of the governor actuator. The friction brake is designed to prevent inadvertent throttle movement and is pre-loaded to 11-12 inch pounds at manufacture. The actuator from G-BYHE was set to 15.5 inch pounds, requiring slightly higher force to open the throttle or resist its closing as the collective pitch lever was raised. The actuator electric motor required 30% of available power to start movement (normally 20%), though once moving, 20% sufficed.