HISTORY OF FLIGHT On September 3, 2008, at 1553 central daylight time, a Socata TB-20 Trinidad, N597TB, received substantial damage on impact with terrain and post impact fire near Cassville, Wisconsin. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 flight was not operating on a flight plan. The commercial pilot sustained serious injuries, and the private pilot sustained fatal injuries. There were no ground injuries. The flight originated from Dubuque Regional Airport (DBQ), Dubuque, Iowa, and was en route to Prairie du Chien Airport (PDC), Prairie du Chien, Wisconsin, when it diverted to Cassville Municipal Airport (C74), Cassville, Wisconsin. The fixed base operator's line worker, who refueled the accident airplane, was scheduled to work from 1430-1800 on the day of the accident. He stated that he arrived at work at 1430 and refueled the airplane at 1530. According to fuel records, the airplane was fueled about 1530. The line worker did not see the accident pilots and estimated that the airplane took off about 1/2 hour after it was refueled. Air traffic control transcripts were requested for the accident flight; however, these transcripts stated that N597TB was cleared for takeoff from DBQ at 1353:22 and there were no further radio transmissions by N597TB after acknowledging its takeoff clearance at 1353:28. The left seat pilot stated that he and the right seat pilot preflighted the airplane and planned to pick up a friend at PDC. According to the left seat pilot, the right cabin door opened in flight after an unknown amount of time had elapsed after takeoff. The right seat pilot then held the door closed, and the left seat pilot decided to divert to C74 (about 21 nautical miles northwest of DBQ). The left seat pilot stated that the approach to C74 "didn't look good" so he performed a go-around during which the right cabin door opened and the airplane rolled to the right. The left seat pilot stated that he "lost control" of the airplane and did not know whether the airplane stalled. He did not know what caused the door to open in-flight. PERSONNEL INFORMATION The left seat pilot was enrolled in the University of Dubuque's Aviation Program. He held a commercial pilot certificate with airplane single-engine land and instrument airplanes ratings. He accumulated a total flight time of 329 hours of which 53 hours were in Socata TB-20 Trinidad airplanes. The left seat pilot stated that he received flight instruction from a non University of Dubuque flight instructor pertaining to an airplane door opening in flight. During that instruction, a Cessna airplane was used and the door was opened in-flight. The right seat pilot was enrolled in University of Dubuque's Aviation Program in the commercial airplane course. He held a private pilot certificate with airplane single-engine land and instrument airplane ratings. He accumulated a total flight time of about 181 hours, of which 2.1 hours were in the TB-20. He had not completed 10-hours of training within the commercial curriculum to receive an instructor sign off that would have allowed him to be checked out in the TB-20. AIRCRAFT INFORMATION The 1997 TB-20, serial number 1817, was equipped with gull-wing type doors made of composite for airplane serial numbers 2000 - 9999 or metal for serial numbers 1- 9999. The doors articulate around a hinge on the top of the fuselage and the doors opening and closing are slowed by a compensating cylinder located on the rear part of the door. According to the manufacturer's representative, the TB-20 airplane had not been flight tested with an open cabin door. The Pilot's Information Manual, Section 2, Limitations, Use of Doors, states, "Flight with doors open and ajar is prohibited." WRECKAGE AND IMPACT INFORMATION The main wreckage was resting between two motel cottages and about 500 feet from the approach end of runway 11 (3,000 feet by 50 feet, asphalt) at C74. The airplane was in an upright position on a tail to nose heading of about 140 degrees. The airplane and the two cottages were consumed by fire. Slash marks consistent with propeller strikes were noted in the motel driveway. The landing gear was extended and the flap jack screw extension equated to a 10 degree flap extension. The fuel selector was in the right fuel tank position. The right hand door hinge assembly was attached to the remaining fuselage and door. Only one of four door hook assemblies was found. The door hook assembly was separated from the door and within the main wreckage. The door hook assembly had two of the mounting screws attached to a portion of the remaining door. Each screw had three washers in place. The door hook assembly spring was in place. Only one of four access door mechanism latching ring assembly was found. The latch ring was of metal construction and fracture surfaces within the assembly were granular in appearance consistent with overload. TESTS AND RESEARCH Federal Aviation Administration (FAA) aircraft registry records show that there are 23 airplanes registered to the University of Dubuque, 4 of which are Socata TB-20 Trinidad airplanes. The Socata TB-20 Trinidad airplanes are registered as: N597TB, S/N 1817 N1866S, S/N 1866 N1887S, S/N 1887 N1909S, S/N 1909 According to the airplane manufacturer's representative, N597TB was equipped with steel door hook assemblies during its manufacture. According to the University's Director of Maintenance for the aviation program, the other Socata TB-20 door hook assemblies were checked using a magnet to determine if they were steel assemblies. All of the airplanes were reportedly equipped with steel door hook assemblies. The Director of Maintenance stated that there were no previous reports of cabin doors opening in flight on any of the TB-20s. The University's maintenance procedures make use of "squawk" sheets that are located in the aircraft dispatch book. The aircraft dispatch book was not recovered from the main wreckage and reportedly no archive of airplane squawk sheets or maintenance discrepancies is kept by the University. The University does not voluntarily submit reports to the FAA Service Difficulty Reporting (SDR) database. Unlike commercial operators, the University is not required to retain aircraft maintenance discrepancies and submit SDRs. A query of the Federal Aviation Administration Service Difficulty Reports for TB20 airplanes on November 25, 2008, yielded 34 records with only one relating to a cabin door.