Piper PA-28-181 Cherokee Archer II, G-BNGT
Prior to take off the aircraft suffered a nose landing gear collapse due to the failure of the axle. The axle rod assembly contains a through-bolt, a spacer and an axle plug on each end, which are fastened with a washer and nut. Metallurgical examination showed that the end of the plug had been plastically deformed prior to being separated from the remains of the plug. Multiple progressive, intergranular corrosion paths were present in the failure, which is typical of a slow stress corrosion mechanism in extruded aluminium alloys. The general direction of progression was normal to the radial tension stresses induced by the plastic deformation, which resulted from overtightening of the nut on the wheel through bolt. The aircraft maintenance manual contains information for the assembly and installation of the nose wheel and states ‘tighten nuts until no side play is felt (allow wheel to rotate freely)’. Several other axle plugs returned to AAIB also showed some distortion similar to the accident axle plug, and one showed similar radial and circumferential cracking. The maintenance organisation policy is now to change the axle plugs whenever distortion is observed during any disassembly of the axle.