What happened
During a routine daily inspection at Warsaw Chopin Airport (EPWA), maintenance technicians discovered that three bolts securing the rim of the number 3 main landing gear wheel on an Airbus A320 were missing. Upon removing the wheel for further inspection, technicians found that the brake calipers were failing to release even after the parking brake had been disengaged.
Following the discovery, the brake assembly was replaced, and a new set of rim bolts was installed. While the wheel axle showed no irregularities and no overheating was detected, investigators noted that the tire had shifted its position on the rim relative to its previous state, resulting in scraped paint on the rim surface. Further inspection revealed that the remaining bolt studs had been sheared off along with their nuts, leaving only the broken ends embedded in the rim.
The investigation
The investigation focused on the mechanical integrity of the brake unit and the sheared bolts. Pressure testing of the replacement brake unit revealed hydraulic fluid leaks from four of the 14 pistons. Significant surface corrosion was observed on the brake housing and the heat pack components. Additionally, the pistons failed to return to their neutral position consistently after pressure was reduced in both brake systems. The heat pack contact surfaces were found to be slippery due to hydraulic fluid contamination.
To determine the cause of the bolt failure, the sheared components were sent to the Silesian University of Technology for material analysis. The metallurgical examination confirmed that the bolts failed due to a combination of fatigue and sudden fracture, starting with a single bolt and progressing to the others. The analysis found no material defects in the high-strength steel used; however, it suggested that the fatigue cracks could have been initiated by external factors, such as over-torquing the bolts during installation.
Findings
- The investigation could not definitively determine a single cause for the bolt failure, but identified two primary possibilities: a dragging brake condition where the brake failed to release upon landing, causing the tire to shift on the rim and shear the bolts, or a valve malfunction that prevented pressure from decreasing in the brake line.
- The brake assembly exhibited significant corrosion and hydraulic leaks.
- The material integrity of the bolts was found to be within standard specifications, suggesting the failure was likely due to operational or maintenance-related stresses rather than manufacturing defects.