N46PTPiper PA 46-350P · 1990

On April 25, 2010, about 1640 central daylight time, a Piper PA 46-350P, N46PT, incurred minor damage when the pilot side window shattered and departed the airplane, while in cruise flight near Owensboro, Kentucky. The certificated commercial pilot and the two passengers were not injured. Instrument meteorological conditions prevailed, and the flight was operating on an instrument flight rules flight plan. The personal flight, which departed Southern Illinois Airport (MDH), Carbondale, Illinois, was destined for Smith Reynolds Airport (INT), Winston Salem, North Carolina. The flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.

According to the pilot, he had just leveled the airplane at 15,000 feet mean sea level, and was adjusting the engine to the cruise power setting. The airplane was pressurized, and all of the instruments were indicating normally. The pilot's side window then suddenly "blew out," depressurizing the airplane immediately. The pilot reported that his headset was "sucked out" of the airplane. The pilot utilized the headset installed on the copilot's side of the airplane and advised air traffic control of the situation. Air traffic control subsequently issued radar vectors, and cleared him for an instrument landing system approach to Owensboro-Daviess County Airport (OWB), Owensboro, Kentucky. The pilot landed the airplane without further incident.

The remaining portion of the window, which was estimated to be about 30-percent of its total surface area, was forwarded to the NTSB Materials Laboratory for further examination. According to the Materials Laboratory Factual Report, a primary overstress fracture emanated from the radius of the pilot's side window, storm window cutout, and followed a path aft and downward (relative to the window's installed position on the airplane). Other fractured pieces along the top and forward edge of the window exhibited fractures attributed to the secondary overstress cracks associated with the overall in-flight loss of the window. Detailed examination of the primary fracture near the point of initiation showed a locus of preexisting craze cracks along the lower aft radius of the storm window cutout. The cracks had a thumbnail shape and emanated from the edge radius or on the flat face of the storm window cutout edge.

On December 13, 2006, the airframe manufacturer issued Service Bulletin (SB) Number 1175, which detailed requirements for repetitive inspections and/or replacement of the pilot side window. An inspection of the lower aft corner of the storm window for cracking was required after the first 350 hours in service, and every 50 hours thereafter. If cracks were found, the window would require replacement, and continued inspection at the previously stated intervals. In May 2007, the airframe manufacturer superseded the service bulletin with a revision (SB 1175A) that allowed for the installation of a newly designed pilot side window, which eliminated the storm window and upon installation, and relieved the repetitive inspection requirement.

The primary fracture noted on the incident airplane's pilot's side window occurred outside, but adjacent to, the inspection called out in the SB. Following the incident, the airframe manufacturer released a revised SB 1175B, which expanded the inspection area and incorporated a photograph from the incident window illustrating an example of the cracking mechanics should inspect for.

Review of the incident airplane's available airframe maintenance records by a Federal Aviation Administration inspector revealed no evidence that either the inspection or replacement of the window had been performed in compliance with any revisions of the SB.