Event Overview
On December 5, 2016, at approximately 1300 mountain standard time, a Piper PA-31T airplane, registration 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 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.
Flight Details
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 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.
Aircraft Information
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 manufactured with two layers of glass. Data plates indicated both windshields were manufactured by PPG Industries in Huntsville, Alabama. The left windshield was manufactured in January 1977 and the right windshield in August 1981. A review of maintenance records indicated that the airplane was issued a standard airworthiness certificate on August 26, 1977, and mentioned export. The next entry in the logbook was dated December 8, 1981, with a 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 a total time of 1734 hours. There were no logbooks supplied for the period 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 was on February 1, 2016, at 6267.5 hours with no discrepancies reported.
Examination Findings
Examination 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. There was no evidence of impact damage to the fuselage aft of the windshield or the tail. Small fragments of glass were embedded in the left propeller blades. No evidence of bird impact was noted. 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 examined at PPG Aerospace Transparencies, Huntsville, Alabama.
The fractured left windshield aluminum retainer and flange area was intact. Several areas of retainer discoloration (white versus gray) were noted in the flange area on the inboard and outboard sides. The largest discolored area was along the lower flange, extending across the width for about 6 inches on both sides. 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 glass layers. Areas of retainer discoloration were noted, though less severe, scattered around the periphery. There was cloudiness, interlayer cracking, and delamination along the top and bottom edges consistent with moisture ingress. Discrepancies were noted along the entire upper edge extending about 3/8 inch from the edge, and along the forward 13 inches of the lower edge extending about 1/2 inch from the edge. Delamination was also noted at all four corners and along bus bars.
Service Manual Guidance
The Piper Cheyenne Service Manual provides guidance for window inspection and repair, defining critical, semi-critical, and non-critical areas of the windshield. It describes anomalies such as distortion, cracks, crazing, scratches, chips, haze, blemishes, mark-off, and delamination. Cracks in either glass layer require immediate replacement. Delamination evidenced by cloudy appearance indicates moisture or solvent penetration. Any delamination in critical and semi-critical areas should be replaced at the earliest opportunity. Chipping of the inner glass surface in the semi-critical section also requires replacement.