NTSB Factual Report

On September 17, 2011, about 1045 central daylight time, an Airbus Industrie A320, C-FGYL, sustained minor damage when the copilot’s window cracked during cruise at flight level 340 northwest of Chicago, Illinois. The flight crew declared an emergency, and diverted to the Chicago O'Hare International Airport (ORD), Chicago, Illinois, landing about 1113 without further incident. There were no injuries to the 2 flight crewmembers, 4 cabin crewmembers, and 134 passengers. The airplane was registered to GECAS and the flight was operated by Air Canada as flight 791, under the provisions of 14 Code of Federal Regulations Part 129 as a scheduled international passenger flight. Day visual flight rules conditions prevailed for the flight, which operated on an activated instrument flight rules flight plan. The flight originated from the Toronto Pearson International Airport, near Toronto, Ontario, Canada, about 0940, and was destined for the Los Angeles International Airport, near Los Angeles, California.

The operator’s incident report stated an 18-inch long crack developed in the first officer’s (FO) windshield, extending from the left bottom corner to the top right corner, during cruise flight. The crack subsequently progressed into multiple multi directional cracks that obscured the FO’s view through the window. The flight crew was unable to determine the severity of the cracks and how many plies were affected. The flight crew complied with the quick reference handbook guidance and diverted to ORD. The flight crew performed an overweight landing and taxied the airplane to the gate.

The incident windshield, part number NP-165311-8 with serial number 07169H9453; CA122417, was made from laminated glass. The layers, from the outboard surface to the inboard surface; included a Herculite II glass layer coated with a Nesatron anti-ice system and its associated conductive bus bar, a urethane interlayer, a vinyl interlayer, a urethane interlayer, a Herculite II glass layer, a vinyl interlayer, and a Herculite II glass layer.

A NTSB aircraft structures and maintenance specialist oversaw a postincident examination of the windshield at PPG Industries, Inc., near Huntsville, Alabama, on March 9, 2012. The examination revealed that the fracture origin was located at the center of the windshield. An area of arcing was located along the electrical bus bar at the lower edge of the windshield, near the forward corner. This location coincided with the area in which the power and sensing element wires were routed around the structural glass plies. This area of arcing was surrounded by a cloudy and degraded interlayer, which was consistent with the presence of moisture. The moisture seal was worn and the moisture seal upper edge had been repaired. Appearance of the moisture seal’s forward and lower edges was consistent with a factory condition.

The latest windshield revisions produced by PPG Aerospace, with part numbers NP-165331-1/-2, are now available and include enhancements that reduce moisture ingress and its subsequent effects on the electrical system.

According the Accredited Representative from the Bureau d’Enquêtes et d’Analyses (BEA), Airbus had issued a Flight Operation Telex (FOT) on May 25, 2011, to inform operators about the issuance of a revised Cockpit Windshield/ Window Cracked operational procedure in case of a cockpit window cracking. The new procedure asked the crew to check if the inner ply is affected by the crack by using either a pen or finger nail. If the inner ply is not cracked, based on the fail safe design, the flight crew can continue the flight, without other restriction.

NTSB Probable Cause

The infiltration of moisture into the windshield heating layer that induced arcing in the heating system that subsequently cracked the windshield.