What happened
On 17 December 2005, a Piper PA-28-181 Cherokee Archer II, registration G-BNGT, was participating in a private flight at Edinburgh Airport. After completing pre-takeoff power checks at the holding point, the pilot performed a 180-degree turn into the wind to line up for departure. During this maneuver, the nose wheel detached from the nose landing gear assembly. As the wheel separated, the propeller tips struck the taxiway surface. Although the engine remained running, the pilot promptly shut it down. There were no injuries to the two crew members or the single passenger on board, though the aircraft sustained damage to the propeller tips and the nose landing gear assembly.
The investigation
The AAIB investigation focused on the structural integrity of the nose landing gear assembly, specifically the axle rod assembly. This component utilizes a through-bolt, a spacer, and an axle plug on each end, secured with a washer and nut. Investigators examined the failed axle plug and performed a metallurgical analysis of components provided by the maintenance organization.
Further inspection of the maintenance organization's inventory revealed several other axle plugs exhibiting similar distortions. One of these plugs showed radial and circumferential cracking, though it had not yet failed. The investigation also identified a historical precedent, noting a similar incident involving a Piper Cherokee in 1981.
Findings
- The failure was caused by the failure of an axle plug on the nose landing gear axle rod assembly.
- Metallurgical examination determined that the end of the plug had undergone plastic deformation, likely caused by over-tightening the nut on the through-bolt.
- The primary cause of the component's separation was stress corrosion, a mechanism where tensile stress and a corrosive environment work together to degrade the metal. The cracks were identified as intergranular corrosion paths typical of extruded aluminium alloys.
- The presence of paint on the fracture faces indicated that the cracking process was progressive and had been occurring since the component was last painted.
- While the plug design is used on other aircraft, the aluminium construction of this specific plug makes it more vulnerable to this type of failure compared to steel versions used on certain Cessna models.