HISTORY OF FLIGHT On April 5, 2009, about 1130 Pacific daylight time, a Liberty Aerospace, Inc., Liberty XL-2, N519XL, experienced the loss of the majority of its left side cockpit door during cruise flight about 3 miles east of the Camarillo Airport, Camarillo, California. Separated door components impacted and dented the airplane's left stabilator. The airplane was operated by the Armed Forces Aero Club, Inc., San Diego, California, and it sustained minor damage during the incident. Visual meteorological conditions prevailed at the time, and an instrument flight rules flight plan had been filed. Neither the certified flight instructor (CFI), who held an airline transport pilot certificate, nor the private pilot, who was receiving instruction, was injured. The flight was performed under the provisions of 14 Code of Federal Regulations Part 91 and originated from San Diego about 1000. The CFI reported to the National Transportation Safety Board investigator that the airplane's pre-takeoff checklist was followed. The CFI opined that the student properly and securely closed the left cockpit door prior to takeoff. No discrepancies were noted with the door's operation. At no time during the instrument training flight did the student open or attempt to open the door. According to the CFI, when the incident occurred the student was receiving instrument flight instruction and was tracking inbound on the localizer to the Oxnard Airport, which is located a few miles west of Camarillo. The airplane was cruising in level flight about 4,000 feet mean sea level. The student's left hand was holding the control stick. The door "popped open" minutes after the airplane entered an area of moderate clear air turbulence. Thereafter, the door separated from the airplane, and the CFI took control of the airplane. Observing that the nearest airport was Camarillo, the CFI diverted to that airport. The CFI stated to the Safety Board investigator that despite application of full engine power, level flight could not be maintained. Fortunately, because of their close proximity to the Camarillo Airport, they were able to immediately land without further mishap. As the airplane taxied to parking, the local air traffic controller advised them that an airframe component had dropped from their airplane onto the tarmac. Subsequently, the component was recovered from the tarmac and was noted to be the lower portion of the separated door, which included the latching mechanism. The CFI opined that the component must have become attached to a portion of the airframe structure after it ripped off from the upper portion of the door. The upper portion of the door had remained attached to the airframe by its hinges. PERSONNEL INFORMATION The CFI reported that his total flight time is 14,469 hours. His total flight time in the Liberty XL-2 is 34 hours. The student's total flight time is 181 hours. The student's flight time in the Liberty XL-2 is 38 hours. AIRCRAFT INFORMATION The Safety Board investigator examined the normal category airplane, serial number 14. The airplane was manufactured and issued a standard Federal Aviation Administration (FAA) airworthiness certificate on April 18, 2006. By the accident date, the 3-year-old airplane's total time was about 253 hours. Its last annual inspection was accomplished in July 2008, at a total time of 178 hours. The airplane was designed with two doors that attach to the airframe with hinges located on their upper (top) surface. The doors are secured in the locked position by engagement of 2 locking pins. One pin extends from the body of the front portion of the door frame into a receptacle located in the fuselage structure, adjacent to the front of the door. The other pin extends from the body of the aft portion of the door frame into a receptacle located in the fuselage structure, adjacent to the rear of the door. To close the door, the occupant rotates the door handle into the forward, closed position. This action extends the locking pins from inside the door into the fuselage receptacles and secures the door. WRECKAGE AND IMPACT INFORMATION The Safety Board investigator examined both the right side cockpit door, which was not damaged, and the left side cockpit (incident) door. The right side cockpit door's latching mechanism was found functional. The fuselage structure adjacent to the door's forward and rear pin receptacles was found abraded, and evidence of chipped paint was noted in these areas. The marks were consistent with impact damage resulting from the door being partially closed with the locking pins extended. The Safety Board investigator noted that if an attempt is made to close the door with the locking pins extended, the sides of the pins contact the side of the fuselage receptacles, and the pins do not enter the receptacles to secure the door in a locked position. Similar impact marks were noted in the left side fuselage, adjacent to the incident door's forward and aft locking pins. The left (incident) door was observed broken into several pieces. By design, the airplane's doors are constructed of a composite material. The Safety Board investigator noted that when the door is open, the body of the door is not rigid, and the door's frame can be torsionally twisted by application of force on either the door's lower front or lower rear surface. TESTS AND RESEARCH At the Safety Board investigator's request, the Liberty Aerospace manufacturer's participant compiled and evaluated reports of other inadvertent openings of canopy doors. The participant reported that its review of data indicated about 6 instances wherein a canopy door had opened during flight. The participant concluded that in these instances, a door pin had likely not been seated in its receptacle. This condition likely allowed the canopy door to open in flight. While participating in the Safety Board's investigation, Liberty Aerospace evaluated the door's design and locking mechanism. The participant reported that Liberty's research validated its original design, which indicated that if both the forward and aft locking pins are properly engaged, the door and airframe can not flex and allow the disengagement of a door pin. Liberty's evaluation further indicated that if one of the two door pins on a door is not in its receptacle when the door is closed, the door could flex and open. ADDITIONAL INFORMATION A review of the airplane's FAA approved flight manual (AFM) current on the April 5, 2009, date of this incident revealed it contained a "BEFORE TAKEOFF" checklist. In the checklist it states "Canopy Doors...CLOSED AND SECURE." The manufacturer reported that, in response to information learned during this and other investigations, it revised the AFM checklists for the Liberty XL-2. The revisions were made to further enhance safety. The revisions include specific language requiring the verification of each individual door pin engagement prior to takeoff. The revised checklist states: "Canopy Doors (both FWD pins and both AFT pins) ENGAGED AND SECURE. [[NOTE]] Verify doors are closed by pushing on aft section of both doors." Also, as a further precautionary measure, the manufacturer procured a tapered door pin for installation in the aft portion of the door. The new tapered pin has an offset at its end to assist the capture of the fuselage receptacle when the door pin is moved into the closed position. The newly designed pin will be utilized in the manufacture of all future Liberty XL-2 airplanes, in addition to being made available to owners of previously manufactured airplanes through a Service Document. Service Document number SB-09-003 was issued on December 15, 2009. In part, it states the following: "To ensure that the aircraft’s doors are properly closed, latched and locked, the forward and aft pins must both be seated in their respective receptacles. If a pin is not properly seated in its receptacle, the door is not secure and could open in flight resulting in possible door and aircraft damage."