What happened
On May 28, 2010, a Cessna 560XL, registration N662QS, departed Westchester County Airport (HPN) in White Plains, New York, bound for Nantucket Memorial Airport (ACK). The flight was operating under instrument flight rules in visual meteorological conditions with 10 miles of visibility.
Shortly after takeoff, the captain heard an unusual sound described as the cabin door "leaking," and the first officer observed that the DOOR SEAL/CABIN DOOR annunciator light was illuminated. The crew requested and was cleared to return to runway 16. While performing the downwind leg of the traffic pattern, the main cabin door opened. The crew declared an emergency and landed the aircraft uneventfully. There were no injuries to the two crew members or the two passengers, though the aircraft sustained minor damage.
The investigation
FAA inspectors examined the aircraft's main cabin door, which was located on the left forward side of the fuselage. The investigation found that the door had been over-extended when it opened in flight and had scraped the runway during the landing. This damage prevented the door from being closed normally without significant force.
Mechanical inspections of the locking pins, pre-catch mechanism, and locking sockets revealed no evidence of mechanical failure or malfunction. The electrical monitoring systems, including the cockpit annunciator panel and the door post LED indicators, were found to be functioning correctly. The investigation also noted that the first officer, who closed the door during the passenger safety briefing, could not recall the direction in which he rotated the locking handle or if he had verified the locking indicators as required by the operator's checklist.