What Happened
On 26 February 2022, at approximately 1030 local time, a student pilot and an instructor were conducting a low-level hover training exercise at Gold Coast Airport, Queensland, in a Robinson Helicopter R22, registered VH-THM, operated by Professional Helicopter Services. After about 30 seconds of stable hover just above the ground, a 'bang' was heard through the airframe. The helicopter then pitched nose up, developed a large vibration, and yawed to the right. The instructor took control immediately and landed. No injuries occurred. Subsequent inspection revealed that the tail rotor gearbox and empennage assembly had separated from the tailcone (boom).
What the ATSB Found
The tail rotor tip cap detached due to moisture-induced failure of the adhesive bond. Tap testing of the tail rotor blades during the most recent scheduled inspection, intended to detect adhesive failure, did not identify any disbonds that were very likely present at the time. The tailcone aft casting then fractured due to the detachment of the tail rotor tip cap, which resulted in a tail rotor imbalance and severe abnormal loading on the casting.
Failure Sequence
The proximity of the tail rotor blades to the horizontal stabiliser meant that contact was only possible if the empennage assembly separated. The remaining damage to the tail rotor drive system was caused by the tail rotor striking the stabiliser. Separation of the empennage assembly occurred due to fracture of the tailcone casting. Fractographic analysis of the casting showed that the fracture resulted from a high applied stress intensity, with no significant defects that would have caused fracture during normal operation. The casting was most likely subjected to abnormal loading conditions around the time of the occurrence. Apart from the tip cap detachment, no evidence of an operational event or issue with the helicopter tail or tail rotor drive system that might have led to abnormal loading was found. Consistent with Robinson service bulletin advice, in-flight detachment of the tip cap would have caused severe vibration, resulting in immediate, high-frequency, high-amplitude loading and rapid fracture of the low-ductility casting. The failures occurred while the helicopter was in a low hover. The instructor’s quick actions were effective in preventing a more serious outcome.
Tail Rotor Tip Cap Adhesive Failure
The condition of the residual adhesive at the tip cap was consistent with adhesive failure over approximately 85% of the original bonded area. Given the normal forces acting at the time, reduced bond effectiveness was the only reasonable explanation for the tip cap separation. The presence of fluid patches, likely water, in the bond of blade 5803 suggested that the adhesive failure was associated with moisture. Both blades exhibited adhesive failure, but a marked difference in residual cohesive failure area indicated more rapid degradation in blade 5803. The blades, installed as a set, had experienced the same operational life and environmental conditions, consistent with similar erosion and pitting of the leading edge. A leading-edge skin crack, existing prior to tip cap separation, provided a moisture pathway to the adhesive bond in addition to attack around the bond line circumference. This likely contributed to the differing extent of disbonding between the cracked blade and the opposing blade. However, a reported tip cap separation involving an R44 had no evidence of blade skin cracking, indicating that cracking was not a prerequisite for adhesive failure.
Maintenance Inspections
Defence against tip cap detachment involves visual inspections of external blade condition and tap testing of the bond area. The extent of adhesive failure at the detached blade tip indicated that a significant disbond likely existed at the time of the most recent maintenance inspection, 21 flight hours prior to the occurrence. No recorded defects or indications of disbond from tap testing were noted per R22 service letter SL-93. Tap testing relies on operator judgement to identify acoustic differences; unbonded substrates maintaining good contact with the adhesive, or bonds with extensive adhesive failure, may not produce a clear acoustic difference. The reason disbond indications were not detected by tap testing in this occurrence was not explored in detail, but the limited examples highlight variability and limitations of tap testing.
Actions Taken
Robinson released updated service bulletins and service letters in response to tail rotor blades corroding to an unserviceable condition and reports of blade tips coming loose due to corrosion. Revision A of R22 service letter SL-93, released in June 2022, required blade replacement with any evidence of bond line corrosion, included inspection criteria, and mandated a minimum 10x magnification inspection of the bond line to increase detection likelihood. Other recommended tail rotor blade maintenance practices were also updated. Robinson identified certain part and serial number blades as susceptible to corrosion, requiring additional visual inspection before the first flight of the day until replacement with less susceptible blades. The United States Federal Aviation Administration released an airworthiness directive regarding this matter.
Safety Message
In-service detachment of a tail rotor tip cap due to adhesive failure is uncommon but presents a significant hazard. The resultant imbalance could lead to loss of tail rotor drive or tail assembly. Pilots and maintainers should recognize that tap testing alone may be insufficient to detect adhesive disbonds; detailed visual inspection for signs of corrosion around the bond line assists in identifying blade degradation. Pilots should be familiar with immediate corrective actions for loss of tail rotor effectiveness, per the pilot operating handbook, to reduce the likelihood of loss of helicopter control.