On November 3, 1995, at 1500 central standard time, a Beech A90, N93RY, registered to Ryan Aviation Corporation, of Wichita, Kansas, was substantially damaged when the air-stair door opened in-flight. The airline transport pilot, copilot and passenger reported no injuries. The maintenance check 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The flight departed Wichita, Kansas, at 1445. Before takeoff the pilot-in-command (PIC) said he closed the main cabin door and did a six point check on the door. The check involved ensuring the door's four bayonet pins and two plungers were in the locked position. He said all six items were in the locked position. During cruise flight the airplane was at a maximum cabin presssure of 4.6 psi. The PIC stated he was performing a maintenance check flight on a newly installed door seal at 14,500 feet. The PIC went back to check the door for air leaks. He said he noticed air leaking and a whistling noise coming from the door. He then decided to trouble shoot by pushing the cabin door lock/unlock button, followed by moving the door handle from the "neutral" position to the locked position. The noise continued and he than began pushing on the door; at this point the door released in-flight. A Federal Aviation Administration Principal Maintenance Inspector represented the NTSB during the on-scene investigation. The investigation revealed no anomalies with the cabin door. All bayonet pins extended to the distance stated in the maintenance manual when the handle is in the locked position. A placard adjacent to the cabin door handle stated, "CAUTION DO NOT ATTEMPT TO CHECK SECURITY OF CABIN DOOR BY MOVING DOOR HANDLE UNLESS CABIN IS DEPRESSURIZED AND AIRCRAFT IS ON THE GROUND." The pilot's operating handbook emergency section concerning illumination of cabin door warning light procedures states, "WARNING the cabin door handle latching mechanism is an over-centering mechanism and must be in the over-center position to be fully latched, as indicated by the arm and plunger. If cabin is pressurized and door is not completely latched, any movement of the door handle toward the unlocked position may cause rapid and complete unlatching and opening of the door due to forces created by the two upper door hooks."