On December 5, 2016, about 1300 mountain standard time, a Piper PA-31T airplane, N909PW, made an emergency landing at Missoula International Airport (MSO), Missoula, Montana, following a fracture and separation of the left windshield from the airframe. The private pilot and passenger were not injured. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Instrument meteorological conditions prevailed, and an instrument flight rules flight plan was filed. The flight originated from Kalispell City Airport (S27), Kalispell, Montana at 1215.

The pilot reported that he was in cruise flight at flight level 230 for about 10 minutes, with an outside air temperature of - 40° Fahrenheit, when suddenly the left windshield departed the airplane. At the time of the windshield failure, the pilot heard an abrupt "swish" sound, followed by an instantaneous "loud roar" with a simultaneous blast of freezing air. The pilot and passenger donned their oxygen masks, and the pilot initiated an immediate descent. He made a distress radio call to air traffic control (ATC) declaring an emergency, and stated his intention to divert to Missoula, Montana; however, he was not able to hear a response from ATC due to the noise in the airplane. The pilot landed at MSO without further incident.

The propeller driven, twin engine, low wing, pressurized airplane equipped with a retractable tricycle landing gear system, was manufactured in 1977. It was powered by two Pratt & Whitney Canada PT6 engines, each rated at 620 horsepower. The airplane was equipped with right and left electrically heated main windshields that were manufactured with two layers of glass. Data plates indicated that both windshields were manufactured by PPG Industries in Huntsville, Alabama. The left windshield was manufactured in January 1977 and the right windshield was manufactured in August 1981. A review of maintenance records indicated that the airplane was issued a standard airworthiness certificate on August 26, 1977 and mentioned that the airplane was exported. The next entry in the logbook was dated December 8, 1981, with the total time of 1712.12 hours. The airplane was ferried back to the United States and issued a Standard Airworthiness Certificate on December 17, 1981, with the total time of 1734 hours. There were no logbooks supplied for the period when the airplane was registered overseas. No information was found in the records to indicate when or why the right windshield was replaced. The most recent inspection of the airplane was accomplished on February 1, 2016, at a total time of 6267.5 hours with no discrepancies reported.

The examination of the airplane revealed that most of the left windshield glass and vinyl departed the airframe during the event. The aluminum retainer and the vinyl beneath the fuselage windshield frame remained installed. Small areas of glass and vinyl were present around the edge of the windshield frame. Only a few small glass fragments were found in the cockpit. The right windshield remained intact and installed in the airplane. There was no evidence of impact damage to the fuselage aft of the windshield or the tail of the airplane. There were small fragments of glass embedded in the left propeller blades. No evidence of bird impact was noted anywhere on the airplane. The fuselage windshield frame, sealant, and paint around both windshields were intact. The fractured remains of the left windshield and the intact right windshield were removed and subsequently examined at PPG Aerospace Transparencies, Huntsville, Alabama.

The fractured left windshield aluminum retainer and flange area was intact. Several areas of retainer discoloration with a lighter color (white versus gray) were noted in the flange area on the inboard and outboard sides of the retainer. The largest discolored area was located along the lower flange and extended across the width of the retainer for about 6 inches on both the inboard and outboard sides of the retainer. The area was examined and a white powdery residue consistent with corrosion of the aluminum retainer was present, and the vinyl was no longer adhered to the aluminum.

The right windshield was intact with no fracture of the inboard or outboard glass layers. Areas of retainer discoloration with a lighter color (white versus gray) were noted in the flange area on the inboard and outboard sides of the retainer though it was less severe than the left windshield. These areas were scattered around the periphery of the windshield with varying sizes and there was no large single area like noted on the left windshield. There was cloudiness, interlayer cracking, and delamination noted along the top and bottom edges of the windshield consistent with moisture ingression into the laminate. The discrepancies were noted along the entire upper edge and extended about 3/8 inch from the edge of the outboard glass layer towards the center of the windshield. The same discrepancies were also noted along the forward 13 inches of the lower edge and extended about 1/2 inch from the edge of the outboard glass layer towards the center of the windshield. Delamination was also noted at all four corners and along the bus bars at the lower forward and lower aft ends of the glass area.

The Piper Cheyenne Service Manual provides guidance to operators for window inspection and repair. The manual defines three areas of the windshields: the critical area of the windshield defined as the viewing area used for taxiing, takeoff, climb, cruise and landing; the semi-critical area defined as the viewing area used for general flight vision and the non-critical areas defined as viewing areas normally not used for flight operations. Furthermore, the manual defines anomalies such as distortion, cracks, crazing, scratches, chips, haze, blemishes, mark-off, and delamination for use when inspecting the windshields. Cracks are considered critical for the glass windshields. Cracking of either the inboard or outboard glass layer is cause for immediate replacement. Delamination as evidenced by a cloudy or milky appearance is indicative of moisture or solvent penetration into the windshield laminate. Any delamination present in the critical and semi-critical areas should be replaced at the earliest opportunity. In addition, if the semi-critical section exhibits evidence of chipping of the inner glass surface, the windshield should be replaced.