TWA Flight 553, a Douglas DC-9-10 crashed near Urbana, Ohio, USA, following a mid-air collision with a Beech 55, killing all 26 occupants. TWA 553 operated on a flight from New York to Chicago via Harrisburgh, Pittsburgh, and Dayton. The aircraft operated on an IFR flight plan in visual flight conditions and had been cleared to descend from FL200 to 3000 feet. While descending through 4525 feet the DC-9 collided with a Beechcraft 55 (N6127V operated by the Tann Comp.) which was on a flight to Springfield. The descending DC-9, overtaking and converging from the left, struck the level Beechcraft from the left rear quarter. The collision angle between longitudinal axes of the two aircraft was approx. 47deg in the horizontal plane and 10deg down in the vertical plane. The collision destroyed the Beechcraft by causing it to disintegrate; portions of the Beech penetrated the forward fuselage section of the DC-9 and destroyed the integrity of the flight control system. The DC-9 entered a descending left turn, crashed and burned. The Beechcraft was not under control of, or in radio contact with any FAA traffic facility. Weather at the time of the collision was thin broken clouds with 6-7 miles visibility in haze. PROBABLE CAUSE: “The failure of the DC-9 crew to see and avoid the Beechcraft. Contributing to this cause were physiological and environmental conditions and the excessive speed of the DC-9 which reduced visual detection capabilities under an air traffic control system which was not designed or equipped to separate a mixture of controlled and uncontrolled traffic.”