On 31 March 2013, a Cessna Aircraft Company 441 aircraft, registered VH-XBC, arrived at Coonawarra Airport, SA, following a charter flight from Adelaide, SA. The pilot reported that the initial flare and touchdown was normal, however during the landing roll, he heard a distinct ‘pop’ or ‘bang’ sound, after which the aircraft began veering to the left. Initially the pilot was able to maintain directional control and keep the aircraft on the runway, however as it slowed, it began pivoting around the left landing gear and came to rest to the left of the paved runway surface. There were no injuries to passengers or crew.
What the ATSB found
The runway excursion resulted directly from the failure of the left main landing gear trunnion, which allowed the left main wheel to displace upward and contact the wing underside – producing an asymmetric and uncontrolled braking effect. Both the ATSB and component manufacturers’ laboratory examinations attributed the trunnion failure to the development and growth of progressive fatigue cracking through the horizontal beam and central gusset section, with cracking propagating to the point where nominal landing loads produced the final overstress fracture and collapse of the assembly. Due to the extent of post-failure damage sustained by the fracture surfaces, neither laboratory examination was able to specifically identify the factors behind the initial development of the fatigue cracking.
The ATSB’s research of Australian and international air safety records did not identify any other instances of this specific mode of failure within the trunnion type, nor was the manufacturer aware of any known cracking or related problems with the trunnion in the affected areas. As such, it is likely that the event was an isolated occurrence, and consequently, the likelihood of similar occurrences in the future is low.
What's been done as a result
In response to this occurrence, the aircraft operator undertook a targeted, one-off non-destructive inspection of the trunnions from the four other aircraft in its fleet. All were found to be defect free. Additionally, the operator’s maintenance schedule has been extended to include the eddy-current inspection of the trunnion gusset/web regions concurrently with the routine 2000-cycle periodic inspections specified by the aircraft manufacturer.
While the failure sustained in this instance was an unusual and isolated case, it does highlight the necessity of general vigilance and attention to detail during general visual inspections, or during other examinations of structures and components. Such activity can present opportunities for the detection of emerging or unknown airworthiness issues that may not have otherwise been identified during prescribed maintenance programs.