What happened
On the evening of the accident, a scheduled domestic flight operated by United Air Lines, consisting of a Boeing 727-22 registered as N7036U, was traveling from LaGuardia Airport in New York to O'Hare International Airport in Chicago. The aircraft had previously completed several legs across the United States before arriving at LaGuardia for its final scheduled segment.
The flight departed New York under instrument flight rules, initially cruising at 35,000 feet. As the aircraft approached the Chicago area, it was transitioned from the Chicago Air Route Traffic Control Center to Chicago Approach Control. During this phase of flight, the crew received several instructions to descend through various altitudes, including levels of 31,000 feet, 24,000 feet, and eventually down to 14,000 feet.
At approximately 21:19, while under the supervision of Chicago Approach Control, the aircraft was instructed to maintain an altitude of 6,000 feet. Shortly after this instruction, the crew engaged in a brief exchange regarding the altimeter setting for O'Hare. Following this communication, which concluded at 21:20, the aircraft disappeared from radar. The plane subsequently struck Lake Michigan and exploded upon impact. There were 30 fatalities and no survivors.
Findings
Investigators were unable to establish a definitive reason why the aircraft failed to level off at its assigned altitude of 6,000 feet, which was the failure to maintain the assigned altitude during descent.