What happened
On 14 January 2016, a Cessna 402B, registration G-MAPP, was conducting a training flight at East Midlands Airport. The flight involved an instructor and two trainee pilots. During the fifth approach of the session, the instructor performed a flapless approach to a full stop. While the landing itself was initially smooth and the landing gear indicators showed the gear was down and locked, the aircraft's left wing began to sink during the landing roll.
As the aircraft slowed, the left main landing gear collapsed, causing the left propeller blades to strike the ground and the wing to scrape along the runway. The aircraft subsequently departed the paved surface and came to rest on the grass south of the runway. There were no injuries to the two crew members or the passenger on board.
The investigation
The AAIB investigation focused on the structural failure of the left main landing gear components. Metallurgical analysis of the pivot bolt and mounting lugs revealed that the failures were the result of an overload rather than pre-existing cracks. The investigation examined the downlock mechanism, specifically the downlock link and the set screw used to secure the downlock pin.
Measurements of the left downlock link showed that the set screw was positioned such that the head sat 4 mm above the casting, with approximately five threads visible. In comparison, the right-hand side of the same aircraft showed a much tighter fit. This discrepancy indicated that the set screw had not been driven far enough into the bore to properly secure the downlock pin.
Findings
- The left main landing gear side brace was likely not in the over-centre position during the landing roll.
- The incorrect fitment of the set screw prevented the downlock pin from being properly secured.
- As the pin moved forward, it caused the bellcrank and its associated lugs to deform and eventually fail under the loads of the landing.
- There was no evidence of a hard landing or pre-existing fatigue in the failed components.
- Existing maintenance manuals and service letters did not provide sufficient instructions or repetitive inspection criteria to ensure the set screw was properly installed.