On February 9, 2011, about 1330 central standard time, a Hawker Beechcraft Corp. C90, N2057C, sustained minor damage during its climb to flight level (FL) 250 near Evansville, Indiana, when a windshield cracked/shattered. Visual meteorological conditions prevailed at the time of the incident. The pilot descended and landed without further incident at the Evansville Regional Airport (EVV), near Evansville, Indiana. The 14 Code of Federal Regulations Part 91 instructional flight was operating on an activated instrument flight rules flight plan. The certified flight instructor (CFI), commercial pilot receiving dual instruction, and two passengers were uninjured. The flight originated from the University of Illinois-Willard Airport, near Champaign/Urbana, Illinois, about 1200, and was destined for the Orlando Sanford International Airport, near Orlando, Florida. The CFI reported that the co-pilot’s windshield outer pane shattered and a loud bang was heard during the climb to FL 250. The flight was about FL 234 when the windshield shattered. The CFI noted that the flight was in clouds and that the temperature aloft at the altitude the flight was climbing through was -23 degrees C. Additionally, he stated that there was no visible structural icing present and the electric window heat was on. The CFI declared an emergency and asked the air traffic controller for the closest airport with a 5,000 foot long runway. The flight was given an immediate clearance to descend and vectors to EVV for the emergency landing where a visual approach and landing was made. The CFI held a Federal Aviation Administration (FAA) airline transport pilot certificate with an airplane multiengine land rating. He held commercial pilot privileges with airplane single-engine, rotorcraft helicopter, and instrument helicopter ratings. He held a certified flight instructor certificate with single-engine, multiengine, and instrument airplane ratings. The CFI reported that he had accumulated 23,790 hours of total flight time and 3,017 hours in the same make and model as the incident airplane. The CFI reported that the pilot receiving dual instruction held a FAA commercial pilot certificate with airplane single-engine, multiengine, and instrument ratings. N2057C, a 1979 Hawker Beechcraft Corp. C90, King Air, was an all-metal, low-wing, twin-engine, turboprop monoplane, with serial number LJ-827. The airplane’s cabin can be pressurized. According to the pilot’s report, the airplane seated two crew and five passengers. The airplane’s airworthiness was maintained under a manufacturer’s inspection program and it had accumulated 8,654.6 hours of total time at its last continuous airworthiness inspection on January 5, 2011. The co-pilot’s windshield attach bolts were retorqued in July of 2010 during the airplane’s phase III inspection. The co-pilot’s windshield reportedly accumulated about 8,200 cycles. The airplane was equipped with a laminated glass windshield with part number 50-420069-36 and serial number 99217H4994. The windshield was manufactured of the following materials: from the outboard surface to the inboard surface; a 0.140 inch thick Herculite I thermally strengthened glass layer; a 0.020 inch PPG Aircon vinyl heating mat layer for deice, combined with a 0.235 inch vinyl layer; and 0.375 inch Herculite I thermally strengthened glass layer. The windshield was a bolted edge design that utilized aluminum retainers to secure the windshield in the fuselage opening. The anti-ice function was accomplished using PPG’s Aircon heating mat with small wires sewn into the vinyl mat to provide clear vision during inclement conditions. The heated area of the windshield was a horizontal strip about 11 inches tall beginning about 3 inches above the lower edge of the inner glass pane. A polysulfide moisture (hump) seal around the window perimeter protected the interlayer materials from moisture ingression. An excerpt from the airplane’s flight manual, in part, stated: CRACKED WINDSHIELD 1. If it is positively determined that the crack is on the outer panel, no action is required. CAUTION Windshield wipers may be damaged if used on cracked outer panel. Heating elements may be inoperative in area of crack. 2. If it is determined that the crack is on the inner panel, descend or reset the pressurization controller to achieve 3 psi or less differential pressure within ten minutes. Visibility through the windshield may be significantly impaired. The damaged right main co-pilot’s windshield, P/N 50-420069-36 CHG. C, serial number 99217H4994, was removed from the airplane and shipped to PPG Aerospace in Huntsville, Alabama, for detailed examination. Examination of the window revealed that the outer ply had not fractured. The window's inner ply had fractured. The examination of the inner ply fracture pattern pointed to an initiation point located about 5 inches aft of the forward edge and 2 inches above the lower edge of the windshield assembly. Microscopic examination of the initiation area revealed an expanding triangular or V-shaped area emanating from a single point consistent with a peel chip. The glass shards were removed from the initiation area and the removal confirmed that the V shaped area of the outer surface of the inner pane of glass remained adhered to the vinyl layer while the other areas of glass separated from the vinyl layer easily. According to the windshield manufacturer, peel chips usually occur outside the heated areas of the windshield and will only occur at the vinyl to glass interface areas where there is an excessive local adhesion of the glass to the vinyl. The fracture will exhibit progression lines, as the chip grows larger, wider, and/or deeper. Once the fracture depth penetrates the compressive layer of the glass pane, spontaneous fracture of the pane will occur. The peel chip type fracture of the inner ply of glass has historically been an issue on the King Air windshield. In response, the windshield manufacturer made a product improvement in July 2001. The new windshield improvement incorporated in part number 50-420069-37/-38 places a urethane layer between the vinyl interlayer and the inner glass ply. The purpose of the urethane layer is to relieve stresses on the inner glass ply and prevent peel chip fractures. To date, there has not been a peel chip type fracture on the 50-420069-37/-38 windshields. The pilot safety recommendation stated the visibility through the co-pilot’s windshield was “nil.” and that the landing “can only be accomplished from the side that has not failed.”