On May 3, 2004, at 1230 eastern daylight time, a Dornier DO-328-100, N441JS, operated by PSA Airlines, as US Airways Express flight 2447, was not damaged when the main cabin door blew open after takeoff from the Lehigh Valley International Airport (ABE), Allentown, Pennsylvania. The two certificated airline transport pilots, a flight attendant, and 27 passengers were not injured. An instrument flight rules flight plan was filed for the flight which was destined for Philadelphia International Airport (PHL), Philadelphia, Pennsylvania. Instrument meteorological conditions prevailed for the passenger flight conducted under 14 CFR Part 121. According to the flightcrew, as the landing gear was retracted during takeoff, they heard a "loud boom," followed by a rush of air. The red warning panel, master warning and triple-chime alerted them to an "unsafe door condition." They confirmed through the Doors System Page, that the main cabin door had opened. The crew then declared an emergency and initiated a left turn to land on runway 6. A landing was performed without incident and the passengers were deplaned normally at the gate. Examination of the airplane by a Federal Aviation Administration (FAA) inspector, revealed no mechanical deficiencies with the door or its locking mechanisms. Additionally, the design of the "unsafe door condition" warning system consisted of an EICAS warning if the door was not closed prior to takeoff, and the contact of three microswitches in the door to cancel the warning. According to the FAA inspector, the flight attendant had no memory of the flight after the takeoff roll. She could not recall whether or not she may have inadvertently rested her arm on the door handle while seated in the jumpseat. The handle of the door was located approximately one foot from the shoulder position of the flight attendant's jumpseat. Additionally, the incident flight was the flight attendant's third flight after completing her initial training with the airline. The airplane's flight data recorder (FDR) was sent to the National Transportation Safety Board's Vehicle Recorder Laboratory for readout. According to the FDR Group Chairman's Factual Report, the downloaded data revealed the "Doors" parameter indicated "Open" approximately 1.7 seconds after the Air/Ground parameter indicated "Air," and the MFD_Sys Page Selected F/O parameter indicated "Doors" approximately 36.6 seconds after the Doors parameter indicated "Open." No additional anomalies were noted.