Investigation
The National Transportation Safety Board (NTSB) was notified of the accident at 14:40 CST, and immediately dispatched an investigation team to the scene. Agents of the Federal Bureau of Investigation (FBI) were on the scene about 45 minutes after the crash, before any investigators from the NTSB.
The Flight Data Recorder on board the aircraft had become inoperative approximately 14 minutes before the crash. Fortunately, the ARTS-III (Automated Terminal Radar Services) system was in operation at the time of the accident, and that data was saved on tapes in the Midway Control Tower. Those tapes were analyzed extensively and compared to Boeing flight profile data, to develop the course, speed, rate of descent, and altitudes of the plane as it made its approach to Chicago Midway. The Cockpit Voice Recorder was working normally, and the tape in that "black box" was relatively undamaged. That enabled the NTSB to sequence it in time with the readings of ARTS-III. The NTSB then was able to determine the power output of the engines, at any given point in time, with CVR tape sound analysis. That correlation (CVR with ARTS-III) showed that the stick shaker (stall warning device attached to the pilots' control yoke) started 6 to 7 seconds after the plane leveled off at 1,000 ft MSL, and it continued until ground impact.
That ARTS-III system tracked the plane from a position of 55 miles east of its antennae site to the point when the plane was stalling at 380 ft. above ground level.
The official finding of the NTSB was that the probable cause of the accident was the stalling of the airplane (airspeed too low), because the captain failed to ensure that the flight remained within the required airspeed and altitude parameters for that non-precision approach profile. No evidence was ever found of sabotage or foul play.
From their performance study and simulator tests, the NTSB concluded that the spoilers must have been extended to at least the flight detent position during the rapid descent that preceded the stall. It was thus likely that when the Captain attempted to level off and then to go-around, the crew failed to immediately retract the spoilers. That made it all the more difficult to recover from the stall before ground impact. Although the spoilers were found to be fully retracted in the wreckage, it was possible that the spoilers could have retracted on their own, as a result of the impact forces and loss of hydraulic pressure.
The final mistake was inappropriate manipulation of the flaps, from 30 degrees to 15 degrees, while the plane was still at too low of an airspeed, with spoilers extended. That was the wrong thing to do while attempting to go-around with the stick shaker activating, because it decreased lift and therefore increased the stall speed.
Read more about this topic: United Airlines Flight 553