What happened
On 30 November 2009, a Korean Air A330-300, registration HL7551, was climbing through 20,000 feet en route from Incheon International Airport to Istanbul when the crew received a "L WSHLD HEAT" warning. Shortly after passing the NOPIK waypoint at 25,000 feet, a loud cracking sound was heard, and the inner glass of the captain's side windshield began to fracture severely, obstructing the pilot's forward vision.
As the crew initiated an emergency descent to 10,000 feet, the structural integrity of the windshield further degraded, resulting in fine pieces of glass dropping into the cockpit. The captain transferred control to the first officer, and the aircraft returned to Incheon. Due to the aircraft's weight exceeding maximum landing limits, the crew performed overweight landing procedures, and the first officer completed an autoland on runway 33L. There were no injuries among the 168 passengers and 15 crew members.
The investigation
The ARAIB examined the aircraft's windshield components, specifically the pre-modification model (part number SPSA340-2-4-1A). The investigation focused on the windshield heating system, the condition of the silicone pressure seals, and the electrical insulation of the braid wires. The investigators also reviewed the aircraft's maintenance history and the physical damage to the three-ply glass structure.
Findings
- The primary cause of the incident was overheating near the lower braid wire of the windshield heating system, which led to cracks in two structural plies of the glass.
- This overheating was triggered by electrical arcing on the braid wire, caused by a double insulation failure.
- The silicone pressure seal had worn down due to the long-term use of the windshield, contributing to the failure mechanism.