Occurrence
On May 4, 2015, during a daily inspection of an Airbus A320 at Warsaw Chopin Airport (EPWA), maintenance personnel discovered three missing bolts on the half-wheel of wheel No. 3 on the main landing gear. The wheel was removed, and inspection of the installed brake revealed that the brake pistons were not releasing despite the parking brake being disengaged. The brake was replaced, and the aircraft continued operations with a new wheel, brake, and a new set of half-wheel bolts. The wheel axle was inspected and found to be normal. The tire had rotated on the rim relative to its previous position, and paint on the half-wheel was scuffed, partially adhering to the tire.
Investigation Findings
In the locations where bolts were missing, the bolt studs remained in the half-wheel, with the bolt heads and nuts sheared off. The remaining bolts were intact; one was tested with a non-destructive test (NDT) and showed no damage. The removed brake was subjected to a pressure test, which revealed hydraulic fluid leaks from 4 of the 14 pistons. Significant surface corrosion was found on the brake housing and heat pack components. The brake pistons operated incorrectly, failing to always retract after pressure reduction, affecting both brake systems. All heat pack contact surfaces were slippery due to hydraulic fluid wetting; externally, no fluid traces were present except where pistons leaked. Spring tests showed no anomalies. The brake was thoroughly cleaned.
The sheared bolts were sent to the Silesian University of Technology for material examination. The conclusions were:
- The bolt damage occurred due to fatigue and subsequent overstress fracture, starting with bolt No. 2, then the others.
- The material was typical for high-strength bolts.
- No material defects were found that could have caused the damage.
- The initiation of fatigue cracks may have resulted from non-material factors, such as excessive tightening torque.
Probable Cause
The investigation could not determine a single cause for the bolt damage. The most likely causes were:
- A dragging brake: momentary wheel lockup on landing due to brake not releasing, causing tire rotation on the rim and shearing three bolts.
- A valve failure preventing pressure reduction in the brake system at that wheel.
Preventive Actions
The operator was instructed to monitor aircraft wheels. No irregularities were observed during subsequent operations. The safety investigation concluded with no safety recommendations.