On April 10, 2008, approximately 0900 central daylight time, a Raytheon B200, N300FL, was substantially damaged during ground operations at Taylor Municipal Airport (T74), Taylor, Texas. The airplane was owned and operated by Alaro Inc.. Visual meteorological conditions prevailed at the time of the accident. The flight was being conducted under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91 without a flight plan. The commercial pilot, the sole occupant of the airplane, was not injured. However, a mechanic who was attempting to board the airplane was struck by the airplane's entry door and fatally injured. The flight had returned from a local post-maintenance flight check. According to the pilot, the vertical speed indicator (VSI) had been replaced and a flight check was conducted on the airplane. During flight, the pilot heard a loud, high-pitched, "screaming" sound emanating from behind the instrument panel. The pilot returned to the airport and radioed the mechanic regarding the loud sound. When the pilot brought the airplane to a stop, the sound diminished. With engines idling, the mechanic opened the door of the airplane to troubleshoot the malfunction. Cabin pressure existed when the mechanic attempted to open the entry door and the door "blew" outward. The door struck the mechanic's head. An inspector from the Federal Aviation Administration (FAA) examined the airplane with technical assistance from Hawker Beechcraft. The examination revealed that a vacuum line had separated from the vacuum controller at a T-fitting. The T-fitting was located in the area that the mechanic had worked in during installation of the VSI. The disconnected line disabled the entire vacuum system and the airplane's pressurization system safety valve is actuated by vacuum system pressure. According to the manufacturer's Pilot Operating Handbook (POH), the safety valve serves three purposes: to provide pressure relief in the event of malfunction of the normal outflow valve, to allow depressurization of the pressure vessel when the cabin pressure switch is moved to the "DUMP" position, and to keep the airplane unpressurized when it is on the ground when the left landing-gear safety (squat) switch is compressed. Prior to the mechanic opening the door, the pilot reported turning the environmental controls off which stopped the in-flow of cabin pressure. However, because the vacuum system could not drive the safety valve open, the airplane remained pressurized. The POH states that inadvertent door opening is protected by the requirement to depress a release button adjacent to the door handle, making door operation a two-handed motion. The door is further protected by a differential-pressure-sensitive diaphragm incorporated into the release-button mechanism. In the presence of differential pressure, it will be increasingly difficult to actuate the release-button. A review of Title 14 CFR Part 23 revealed that each external passenger or crew door must have a "means to lock and safeguard the door against inadvertent opening during flight by persons, by cargo, or as a result of mechanical failure." In addition, each door of the pressure vessel on a pressurized airplane must have a "means to lock and safeguard each external door, including cargo and service type doors, against inadvertent opening in flight, by persons, by cargo, or as a result of mechanical failure or failure of a single structural element, either during or after closure." The door assembly was examined at the Hawker Beechcraft factory by the NTSB and FAA with the assistance of Hawker Beechcraft. In order to test the integrity of the diaphragm, 16 pounds per square inch (psi) differential pressure was applied. After 1 minute, the diaphragm leaked 0.05 psi, which is within the accepted tolerance of 2 psi for retaining pressure. The diaphragm was next tested across a range of zero to 6 inches of differential pressure to calculate the force required to initially and fully actuate the release button. With 0 psi, it would take 4 pounds to initially actuate and 6.5 pounds at fully actuate the release button mechanism. With 0.5 psi, it would take 8 pounds to initially actuate and 12 pounds to fully actuate the release-button mechanism. With 6 psi, it would take 37 pounds to initially actuate and 46 pounds to fully actuate the release-button mechanism. Despite signs of wear, the accident diaphragm's provided resistance similar to a new diaphragm. Testing confirmed that the airplane's door could be opened while the airplane was still pressurized, but that action would require more force to overcome the resistance. The B200 has no means of alerting outside personnel that an airplane is sill pressurized while on the ground other than resistance against the release-button. Pilots can read cabin pressure on a gauge located on the throttle quadrant. The current POH is "C6," dated December 2004. Step 9 of the "After Landing" checklist, states "Pressurization Differential......Verify 0." The POH the pilot operated with was revision "C" dated December 2000. Revision "C" direct pilots to check the pressure differential after landing. Federal Aviation Regulations do not require Part 91 operators to update the POH.