What happened
On April 6, 2006, a CESSNA 421-B, registration EC-IPO, was performing a local flight training mission at Girona Airport (LEGE) for a multi-engine instructor rating. The flight, operated by a private entity, was being conducted by a student pilot under the supervision of an instructor.
During the first landing run of the flight, the aircraft began to deviate to the left while on the runway. As the aircraft veered, the left main landing gear failed, causing the aircraft to depart the runway and come to a stop on the grass. The impact caused the aircraft to rest on its left wing and the outer part of the left wingtip. There were no injuries to the two occupants, and no fire or fuel leaks occurred.
The investigation
The investigation focused on the mechanical failure of the left landing gear assembly. Physical inspection of the wreckage revealed that the left main landing gear leg had almost completely retracted into the wheel well. Investigators found that the pivot bolt had sheared, and several components of the bellcrank and truss assembly had fractured or broken. Specifically, the right lug fitting of the truss assembly was broken at the pivot bolt hole, and the lower lug of the bellcrank was fractured.
Laboratory analysis of the fractured components, including the pivot bolt, determined that the failures were not due to metallurgical defects or pre-existing cracks. Instead, the analysis concluded that the fractures resulted from overload conditions that exceeded the mechanical limits of the materials. The investigation also reviewed maintenance records, confirming that all required inspections, including those mandated by Cessna Supplemental Inspection Documents (SIDs), were up to date.
Findings
- The left landing gear leg collapsed because the locking mechanism lost continuity due to the failure of the overcenter lock.
- The primary cause of the failure was the overload and subsequent shear failure of the pivot bolt.
- This overload was triggered by landing forces being redistributed through the mechanism in a way that was outside of design criteria.
- The investigation identified that the mechanism was likely subjected to excessive loads due to either a pre-load created during the adjustment process or an oscillatory motion caused by incorrect clearance adjustments and slack in the mechanism.
- The landing itself was determined to be normal, with no evidence of a hard landing that would have independently caused the failure.