On December 20, 2001, about 1945 eastern standard time, a passenger was seriously injured when she fell down the stairs of a Raytheon BE-1900D, N858CA, operated by CommutAir, as flight 2529, at the General Edward Lawrence Logan International Airport, Boston, Massachusetts. The airplane was not damaged. The captain, first officer, and 10 additional passengers were not injured. Night visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed for the scheduled passenger flight that departed the Westchester County Airport, White Plains, New York, at 1848. The flight was conducted under 14 CFR Part 121. According to a written statement from the operator, the flight landed, proceeded to the ramp area, and the engines were shut down. The first officer opened the main cabin door and exited the airplane. As the first officer stood between the main cabin door airstair and the left engine propeller blades, he observed a passenger, wearing sandal type shoes with no rear strap, deplaning. The first officer also observed that the passenger carried a medium sized bag around her left shoulder, another bag being carried with her right hand, and a book in her left hand. As the passenger stepped onto the first step, the first officer assisted her by guiding the bag being carried on her left shoulder away from the handrail. The passenger continued down the stairs, and when her right foot came down onto the last step, she lost her balance, and fell to the ground. The passenger was transported to a local hospital where she was diagnosed with a broken femur bone.