What happened
On 13 November 2006, an Airbus A321-231, registration G-MIDC, was operating a commercial passenger flight from Dublin Airport to London Heathrow. During the flight, there were no reported unusual noises or vibrations, and the aircraft's fault monitoring systems recorded no defects. However, during a post-flight walk-around inspection at London Heathrow, the flight crew discovered that the acoustic panels inside the left engine by-pass duct had been damaged. Upon further inspection, it was found that a thrust reverser blocker door was missing from the engine.
The investigation
The AAIB examined the recovered blocker door and its mounting hardware. The investigation revealed that the door actuation arm had failed due to an overload. Specifically, the lugs on the forward edge of the door, which house the bearings, had failed. Laboratory analysis of the fracture surfaces showed evidence of inter-laminar and inter-granular corrosion cracking at the lug holes, indicating stress corrosion.
Further examination of the bearings showed that the upper bearing had seized within its race, with evidence of corrosion staining present. While residue suggested that the bearings had been primed before installation, the investigation noted that the thickness of this primer might have been insufficient to prevent corrosion. The investigation also found that the lower lug had been distorted by a torsional load applied before the final failure.
Findings
- The loss of the blocker door was caused by the failure of the door mounting lugs due to corrosion cracking and the seizure of a blocker door hinge bearing.
- The seizure of the upper bearing introduced additional torsional loads, which accelerated the propagation of existing cracks.
- Galvanic corrosion likely initiated the cracking due to the contact between the stainless steel bearing and the aluminium alloy door structure.
- The investigation noted that the installation process of the bearings may have introduced stresses that contributed to the cracking, as the door had been free of cracks during its last major maintenance in 2002.
- The aircraft maintenance program does not require routine lubrication of these specific blocker door bearings.