What happened
On 13 February 1999, a Shorts 3-30 Variant 100, registration G-SSWT, was conducting a private training flight at London-Luton Airport. During the approach to Runway 08, the crew notified Air Traffic Control of an unsafe indication regarding the right landing gear. Despite attempts to secure a 'down-and-locked' confirmation, the crew performed a low pass over the airfield. While the right gear appeared extended during the fly-past, controllers could not verify that it was locked. Following a touchdown, the aircraft continued its landing roll, at which point the right main landing gear collapsed, causing minor damage to the right main landing gear sponson. There were no fatalities and no injuries to the two crew members on board.
The investigation
An investigation by the AAIB and a third-party maintenance organisation examined the right landing gear assembly. The inspection revealed that the rear outer bearing bush, an aluminium alloy component with a fibrous liner, had suffered significant corrosion. The resulting aluminium corrosion products had expanded, effectively 'jacking' the liner and causing it to bind on the rear pintle. This expansion caused the pintle to seize during the final stage of the landing gear extension cycle.
Investigators noted that while this specific bush was visible for inspection, other similar bushes in the landing gear system are nearly impossible to inspect for corrosion without removing the fibrous material, which would destroy the component. Furthermore, during the investigation, two other aircraft of the same type undergoing maintenance were found to have heavily corroded bearing bushes with detached liners.
Maintenance records showed that G-SSWT had experienced several recent issues with the right landing gear indicators throughout January 1999. Although various switches and actuators had been replaced, the underlying mechanical seizure caused by the corrosion remained undetected.
Findings
- The collapse was caused by the seizure of the landing gear pintle due to the expansion of aluminium corrosion products within the bearing bush.
- The aluminium corrosion products increased in volume by approximately 50%, creating enough pressure to bind the mechanism.
- The maintenance programme did not require the measurement of gear extension and retraction times, meaning the slowing of the cycle went unrecorded.
- The existing maintenance procedures made it difficult to detect corrosion in certain internal bearing bushes without destroying the component.
- The aircraft was a low-utilisation airframe, and the maintenance programme lacked clear calendar-based inspection intervals for these specific components.