Accident Sequence
The flight was cleared to an altitude of 6,000 feet MSL by air traffic control (ATC), but the plane never leveled off at 6,000 feet. Instead, it continued its descent, at an uninterrupted rate of approximately 2,000 feet per minute, until it hit the waters of Lake Michigan, which is 577 feet MSL.
The control tower at O'Hare lost radio contact with the plane as it approached the western shore of Lake Michigan. A tower crewman at O'Hare said the pilot had just received landing instructions and had replied "Roger" when communication with the plane failed. Wallace Whigam, a lifeguard for the Chicago Park District, reported from the North Avenue Beach House that he had seen an orange flash on the horizon. Three seconds later, he reported, there was a "thundering roar." Other reports of the crash flooded police and Coast Guard from the North Side and North Shore.
The Coast Guard reported that skin divers had assembled at the North Shore Yacht Club in Highland Park, which was used as an informal search base. After a search of several hours there were no signs of survivors, though the area was kept ready in case any were found. Hours after the crash, members of the Civil Aeronautics Board (the predecessor to the NTSB) were on scene to begin investigating the accident.
The most likely explanation is the pilots thought they were descending through 16,000 feet MSL when they were actually descending through only 6,000 feet MSL. Time and radar-image analyses indicated the plane was already down to an altitude of between 1,000 and 2,500 feet MSL when it was again given the 6,000-foot (1,800 m) clearance limit. That final clearance was acknowledged by the captain, and was the last communication with ATC prior to impact with the water.
The captain of a 707 which was 30 miles (48 km) behind the accident flight stated their descent was in instrument conditions until they broke out of the cloud layer at about 8,000 to 10,000 feet and approximately 15 to 20 miles (24 to 32 km) east of the shoreline. The night visibility was "fuzzy and unclear", and lights on the shoreline were the only ones visible.
Read more about this topic: United Airlines Flight 389
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