On June 22, 2002, at 1030 central daylight time, a Beech BE-300, twin-engine airplane, N224CR, was substantially damaged following the in-flight separation of a portion of the left engine cowling during the initial climb from the Tulsa International Airport (TUL), near Tulsa, Oklahoma. The airplane was owned by Wells Fargo Bank Northwest of Salt Lake City, Utah, and was being operated by Flight Concepts, Inc., Tulsa, Oklahoma, under Code of Federal Regulations Part 91. The airline transport pilot and the passenger were not injured. Visual meteorological conditions prevailed for the cross-country flight, and a flight plan was not filed. The personal flight was originating at the time of the accident. On the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the pilot reported that while climbing through 9,500 feet msl, the "left inboard engine access door opened." The pilot reduced the power to flight idle, and then requested and received a clearance to return to TUL for landing. During the turn to return to the airport, at approximately 90 degrees of the turn, the "door separated" from the airplane. The pilot continued to the airport and performed a normal landing without further incidence. In the same report, the pilot reported that the latches for the aft cowling door (Raytheon part number 130-910031-40) had "failed," allowing the door to become open, and puncturing the fuselage. The FAA inspector, who responded to the accident site, and the pilot found the lower panel of the left engine inboard cowling missing. The integrity of the pressure vessel was compromised for approximately 6-8 inches along the left side of the cabin. A review of the maintenance records by the FAA inspector, revealed that the P3 filters were removed and replaced on June 19, 2002. Company maintenance department personnel reported that both engine cowlings were opened during the maintenance, and subsequently they were secured for flight. The Pilot's Operating Handbook and FAA Approved Flight Manual, Section IV Normal Procedures, Preflight Inspection states in part: Engine Cowling, Doors, and Panels (left side)...SECURE.