What happened
On 20 September 2013, a Beechcraft Duchess 76, registration EI-BUN, was performing a series of training circuits at Weston Airport. The flight, involving a student pilot and an instructor, was intended for the revalidation of a multi-engine instrument rating. After completing five successful circuits, the aircraft attempted a full-stop landing on Runway 25.
During the landing roll, the student pilot heard a loud "pop," followed immediately by the collapse of the left-hand main landing gear. This failure caused the aircraft to veer sharply to the left, departing the runway surface and coming to rest in the adjacent grass. While the aircraft sustained substantial damage—including damage to the propeller blades, wing skin, and flap assembly—both occupants were able to exit the aircraft without injury.
The investigation
The AAIU examination of the aircraft focused on the mechanics of the landing gear assembly. Investigators discovered that a flathead pin, which secures the shock absorber pin in the suspension fork, had failed. This failure allowed the shock absorber pin to migrate out of position, triggering the gear collapse.
Metallurgical analysis of the broken pin revealed evidence of shear failure and reversed bending fatigue. Furthermore, the investigation looked into the condition of the shock absorber bushings. It was found that the bushings installed in the left-hand gear were made of an incorrect material—aluminium bronze—rather than the required corrosion-resistant steel lined with PTFE. The investigation also noted that the maintenance organization that overhauled the shock absorbers was performing work outside its approved scope.
Findings
- The primary cause of the collapse was the failure of the flathead pin due to fatigue loads.
- The use of incorrect aluminium bronze bushings increased friction between the shock absorber pin and the bushings.
- This increased friction created rotational forces that led to the fatigue and eventual shearing of the retaining pin.
- The maintenance organization responsible for the overhaul lacked the proper approval for landing gear component work.
- The existing maintenance schedule did not include specific inspections for these particular pins and bushings.