Accident Details
On April 10, 2008, at about 0900 central daylight time, a Raytheon B200 (registration N300FL) sustained substantial damage during ground operations at Taylor Municipal Airport (T74), Taylor, Texas. The aircraft was owned by Alaro Inc. and operated under Title 14 CFR Part 91 without a flight plan. Visual meteorological conditions prevailed. The commercial pilot, the sole occupant, was not injured.
Sequence of Events
The flight had just returned from a local post-maintenance flight check following replacement of the vertical speed indicator (VSI). During flight, the pilot reported hearing a loud, high-pitched "screaming" sound from behind the instrument panel. The pilot returned to the airport and radioed the mechanic about the noise. After stopping, the sound diminished. With engines idling, the mechanic opened the entry door to troubleshoot. Upon opening, the door "blew" outward due to cabin pressure, striking the mechanic's head. The mechanic was fatally injured.
Examination Findings
FAA and Hawker Beechcraft inspectors examined the aircraft. A vacuum line was found separated from the vacuum controller at a T-fitting near the area where the mechanic had worked during VSI installation. This separation disabled the vacuum system, which in turn prevented the pressurization safety valve from opening. The pilot had turned off environmental controls before the mechanic opened the door, stopping the inflow of cabin pressure, but the safety valve could not be driven open by the vacuum system, so the aircraft remained pressurized. According to the Pilot Operating Handbook (POH), the safety valve provides pressure relief, allows depressurization via the cabin pressure dump switch, and keeps the cabin unpressurized on the ground when the left landing-gear squat switch is compressed.
Door Assembly Testing
The entry door assembly was examined at the Hawker Beechcraft factory. The differential-pressure-sensitive diaphragm in the release-button mechanism was tested. Under 16 psi differential pressure, the diaphragm leaked 0.05 psi per minute, within the 2 psi tolerance. Force required to actuate the release button increased with pressure: at 0 psi, 4 pounds initially and 6.5 pounds fully; at 0.5 psi, 8 and 12 pounds; at 6 psi, 37 and 46 pounds. Despite signs of wear, the diaphragm provided resistance similar to a new one. Testing confirmed the door could be opened while pressurized, but with greater force. The B200 has no external indication of pressurization other than release-button resistance; pilots can read cabin pressure on a gauge in the throttle quadrant.
Regulatory and Procedural Background
The POH (revision C, dated December 2000, used by the pilot) and the current revision (C6, dated December 2004) both include an after-landing checklist step to verify zero pressurization differential. Federal Aviation Regulations do not require Part 91 operators to update the POH. Additionally, Title 14 CFR Part 23 requires doors on pressurized aircraft to have means to lock and safeguard against inadvertent opening in flight.