What happened
During the takeoff phase, the number two wheel separated from the left main landing gear assembly. Prior to the event, a company mechanic had performed a walk-around inspection and noted that both the number one and number two tires on the left main landing gear assembly required replacement. The mechanic reported that the number two tire had been removed and replaced, noting nothing unusual during the procedure.
The investigation
Investigators examined the recovered wheel and found the axle nut/spacer assembly still attached. The axle nut lock-bolts were found installed in the axle nut/spacer assembly, with safety wire properly connecting the lock-bolts to the axle nut. There was no damage observed on the lock-bolts. Additionally, the interfacing threads between the axle nut/spacer assembly and the axle showed no signs of being stripped or worn. At the request of the NTSB, a demonstration of wheel installation and a review of the relevant installation procedures were conducted. It was noted that for the lock-bolts to properly engage the axle keyslot, the axle nut/spacer assembly must be correctly torqued and the wheel bearings must be fully seated.