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 (CAR) r. 249. CAR 249 (1) (b) stated: Aviation Regulations 1988 ...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
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
ao2011016_final.docx