What happened
On 22 March 2007, a DHC-6 Twin Otter with registration G-BZFP was taxiing at Glasgow Airport following a commercial passenger flight. While performing a left turn onto Taxiway Alpha, the crew felt a slight impact, described as being similar to a vehicle tyre running over a road marker. This was followed by the detachment of the lower section of the nose landing gear, including the wheel, from the oleo strut.
The separation caused the aircraft to come to an immediate halt, resting on the projecting remains of the nose leg. The underside of the aircraft's nose made contact with the ground, causing damage to the fuselage skin and the nose leg itself. There were no injuries to the two crew members or the seven passengers on board. Following the incident, the crew requested the Airport Fire Service, and passengers were evacuated via the rear airstairs.
The investigation
An investigation into the incident and a subsequent technical examination of the landing gear revealed that the fork fitting, wheel, and axle had detached from the piston tube. This separation was caused by the progressive disengagement of the screw threads of a locknut used to secure the wheel strut. Corrosion had compromised the threads, allowing the nut to wrench free.
Further examination of the components identified several maintenance-related issues:
- A non-standard rubber disc had been bonded over the locknut to prevent salt-water spray from entering the cavity, a practice stemming from the aircraft's regular operations at beach landing sites.
- The semi-circular cap fitting, which clamps the lower part of the gear housing, had been installed upside down. This incorrect installation caused the upper torque link arm to strike the cap, resulting in deep indentations.
- The lower torque link arm was found to be stiff due to the over-tightening of its pivot bolt during a previous overhaul.
Findings
- The primary cause of the gear detachment was the breakdown of thread profiles on the locknut due to corrosion.
- The use of an unauthorized rubber sealing disc contributed to the concealment of the locknut's condition.
- The incorrect orientation of the cap fitting led to structural interference and damage to the torque link arm.
- Maintenance practices regarding the assembly of the nose gear were susceptible to error due to a lack of specific guidance in the manufacturer's manual regarding the cap fitting orientation.
Safety action
Following the investigation, the operator implemented several changes to its maintenance procedures, including:
- Adding a requirement for the disassembly and inspection of the shock strut piston tube and locknut assembly during annual inspections.
- Implementing a wet assembly process for the locknut using an approved primer to mitigate corrosion.
- Reducing the inspection interval for the torque links from 2,400 hours to 200 hours.
- Prohibiting the installation of any parts, such as the rubber disc, that are not specified in the official technical documentation.