Incident
Shortly after a normal touchdown on Runway 27L at London Heathrow Airport, the flight crew of an Airbus A320-211, registration D-AIPX, felt light vibrations originating from the left side of the aircraft. Approximately four seconds later, two caution messages—'GEAR NOT DOWN' and 'SYSTEM DISAGREE'—appeared on the Electronic Centralised Aircraft Monitoring (ECAM) displays. The aircraft remained controllable and vacated the runway, stopping on a taxiway for inspection by the Airfield Fire Service and the operator's maintenance organisation. It was found that most of the tread had become detached from the left main landing gear inboard tyre; however, the tyre carcass had remained inflated. After insertion of the left main landing gear safety pin, the aircraft was towed to a parking stand where the passengers disembarked normally.
Aircraft and Tyre Examination
The aircraft was an Airbus A320-211 built in 1991, powered by two CFM56-5-A1 turbofan engines. The flight crew consisted of seven crew members and 112 passengers; no injuries were reported. Damage was limited to the left main landing gear inboard tyre and the trailing edge flaps.
Two Landing Gear Control and Interface Units (LGCIUs) control landing gear extension/retraction and door operation, supplying information to ECAM and sending ground/flight signals to other systems. Proximity sensors on the landing gear struts were damaged by flailing tyre debris, causing the ECAM messages. The tyre had been fitted on 18 June 2002 and had accumulated 131 landings—less than half the expected tyre life. It had been retreaded once; the operator's policy allowed a maximum of one retread, though the contractor permitted two.
Findings
The tyre and recovered tread pieces were examined at the operator's facility in Frankfurt and later at the tyre retread contractor's facility in the Netherlands. Most of the thrown tread was recovered and positioned on the carcass. Examination showed no evidence of foreign object damage, but revealed signs of 'working separation' with associated tear lines. On the carcass, the separation area corresponded to a piece of spiral belt material from which the tyre tread is constructed. It was concluded that the separation initiated as a result of reduced adhesion levels over this region, which developed during service, probably forming a slight bulge, until forces during wheel spin-up on landing caused the tread to separate.
Tyres are inspected after retread using shearography, a non-destructive inspection (NDI) technique using holographic laser imaging to reveal flaws. Areas of poor adhesion appear in the images, but the process depends on inspector interpretation. Review of shearograms from the retread identified an anomaly in the region where separation initiated. It thus appeared that the tyre may have been released to service due to a lapse in the NDI process. The tyre manufacturer interviewed the NDI inspector and initiated an audit of previous releases to assess risk to tyres in service, including a review of shearography records.