SAFETY SUMMARY What happened At 1115 Eastern Daylight-saving Time on 4 February 2011, a Robinson Helicopter Company R44 Astro helicopter (R44), registered VH-HFH, commenced circuit operations at Cessnock Aerodrome, New South Wales. On board the helicopter were a flight instructor, a pilot and a passenger. Following a landing as part of a simulated failure of the hydraulic boost system for helicopter’s flight controls, the instructor elected to reposition the helicopter to the apron. As the helicopter became airborne, it became uncontrollable and collided with the runway and caught fire. The pilot exited the helicopter; however, the instructor and passenger were fatally injured. What the ATSB found The Australian Transport Safety Bureau (ATSB) identified that a flight control fastener had detached, rendering the aircraft uncontrollable. The helicopter manufacturer had not recorded any previous instances of separation of this fastener. A number of separated components could not be located, preventing the identification of the specific reason for the separation. A number of human factors contributed to the accident, including that the ‘feel’ of the flight control fault mimicked a hydraulic system failure. Finally, the ATSB identified that fatal injuries sustained by the instructor and passenger were due to the post-impact fire and that a large number of R44s had not been modified to include upgraded bladder-type fuel tanks that reduce the risk of post-impact fuel leak and subsequent fires. What has been done as a result In response to the identification of a number of failures of the same type of selflocking nuts in aircraft, the helicopter manufacturer and Civil Aviation Safety Authority have highlighted the issue to operational and maintenance personnel. The helicopter manufacturer also reduced the compliance time on a current service bulletin requiring that allaluminium fuel tanks fitted to older R44 helicopters be replaced with more impactresistant bladdertype fuel tanks. A second bulletin aimed at removing a possible impactrelated ignition source was also issued. Safety message This accident reinforces the importance of thorough inspections by maintenance personnel and pilots. It is also a powerful reminder not to take off identifying a possible problem with an aircraft. In addition, the accident highlights the risk of carrying unnecessary personnel during practice emergencies, and reinforces the
assembly/disassembly cycles. Failure of one of the fasteners occurred via shearing of the bolt following the application of 276 in.lbs of torque. The second fastener failed at 240 in.lbs due to selflocking nut thread damage that prevented subsequent disassembly of the nut and bolt. In both instances, the trislot tool bit could only apply up to about 192 in.lbs of torque before distorting the bolt head sufficiently to drive the bit out of the recess. Disassembly unauthorised tools A final test involved the disassembly of the fastener using masked multi and lockingtype grips. That testing identified that only the lockingtype grips could secure the bolt head sufficiently in order to allow removal of the self-locking nut. Both types of grips left distinctive contact marks on the bolt head (Figure 18). No comparable marks were identified on the detached bolt from the accident helicopter. Figure 18: Typical contact marks left by masked multigriptype hand tools Additional information Carriage of passengers The requirements prohibiting the carriage of passengers on certain flights are detailed in Aviation Regulations 1988 (CAR) r. 249. CAR 249 (1) (b) stated: ...The pilot in command of an aircraft that carries a passenger must not engage in any of the following types of flying: ...practice of emergency procedures in the aircraft... Hydrogen embrittlement During the course of this investigation, three cracked self-locking nuts (Figure 19) from R22 helicopters, of the same specification as that fitted to the detached fastener, were subjected to detailed examination. This examination identified that they had cracked due to hydrogeninduced delayed cracking. Figure 19: Cracked selflocking nut following paint removal When highstrength steel, which has been exposed to hydrogen is sufficiently stressed, it can fail prematurely in a sudden, brittle manner. In the case of the examined selflocking nuts, the source of hydrogen was likely to have been from the cadmium plating process that was specified during manufacture for corrosion resistance. Under conditions of sustained stress, such as that associated with an assembled fastener, plus any residual tensile stresses from manufacturing, the presence of hydrogen can result in brittle cracking, typically less than 1 week from the time of application of the sustained stress. The helicopter manufacturer advised that two of the selflocking nuts that were examined were from a different lot number to that of the locknut that was fitted to
the detached servo fastener. The origin of the third cracked selflocking nut could not be established. Liquidmetal embrittlement Liquidmetal embrittlement is a sudden, brittle failure that occurs when a normally ductile metal is coated with a thin film of liquid metal while sufficiently stressed. An examination of the fracture surface of one of the self-locking nuts from VHHFH identified a pattern that was characteristic of liquid metal embrittlement. In a crosssection of another of the nuts, cadmium was detected remote from the originally-plated surface, indicating that it had flowed in the liquid form. The source of the liquid metal was the cadmium plating that normally plated the nuts, but had melted during the postimpact fire.