Incident Overview
On 17 December 2005, a Piper PA-28-181 Cherokee Archer II, registration G-BNGT, experienced a nose landing gear failure while taxiing at Edinburgh Airport. The aircraft was on a private flight with two crew and one passenger onboard; no injuries were reported.
Sequence of Events
The aircraft had taxied to the holding point Uniform 1, where it completed a 180-degree turn into wind to perform pre-takeoff power checks. No steering issues were noted during taxi or the turn. After the checks, while turning again to line up at the holding point, the nose wheel detached from the nose landing gear assembly. The propeller tips struck the taxiway surface, but the engine continued running until the pilot shut it down.
Damage
The incident resulted in damage to the nose landing gear assembly and propeller tips.
Nose Landing Gear Assembly
The aircraft has a fixed tricycle landing gear. The nose landing gear consists of a strut and fork, with the wheel and tyre secured by an axle rod assembly. This assembly includes a through-bolt, a spacer, and an axle plug on each end, fastened with a washer and nut. The maintenance manual instructs to tighten the nuts until no side play is felt while allowing the wheel to rotate freely.
Metallurgical Examination
The failed axle plug was examined by the AAIB. The end of the plug had plastically deformed before separating; final failure was mainly in shear. Paint on parts of the fracture faces indicated that the separation was progressive and cracks existed at the time of last painting. Multiple progressive intergranular corrosion paths were present, typical of slow stress corrosion in extruded aluminium alloys. Stress corrosion requires simultaneous tensile stress and a corrosive environment. The progression direction was normal to radial tension stresses from plastic deformation, likely caused by over-tightening the nut.
Previous Occurrences
Similar axle plugs are used on other Piper aircraft and some Cessna models, though Cessna uses steel plugs which are less susceptible to this failure. An identical incident occurred to a PA-28 Cherokee in October 1981, documented in GASIL 1/82, which also noted two other aircraft with similarly cracked plugs. The maintenance organisation changed its policy to replace axle plugs whenever distortion is observed during disassembly.
Probable Cause
The failure resulted from stress corrosion in the axle plug, initiated by over-tightening of the nut on the wheel through-bolt.