United Airlines Flight 389

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Diagram showing the face of the three-pointer altimeter design cited in the crash.

United Airlines Flight 389 was a scheduled flight from LaGuardia Airport, New York City, New York to O'Hare International Airport, Chicago, Illinois. On August 16, 1965 at approximately 21:21 EST it crashed into Lake Michigan 20 miles east of Fort Sheridan, Illinois near Lake Forest, Illinois, while descending from 35000 feet (mean sea level-MSL). All 30 people onboard perished, including the former president of the Air Line Pilots Association. There was no indication of any unusual problem prior to impact.

A definitive cause was not determined by NTSB investigators. However, it was believed that the crash was most likely the result of the pilots misreading their three-pointer (3p) altimeters by 10,000 feet.

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 ft. per minute, until it hit the water of Lake Michigan, which is 577 feet MSL.

The most likely explanation is the pilots thought they were descending through 16,000 feet MSL, when they were actually descending through 6,000 feet MSL. Time and radar image analysis indicated the plane was already down to an altitude of between 1,000 to 2,500 feet MSL, when it was given the 6,000 foot clearance limit. That final clearance was acknowledge by the Captain, and was the last communication with ATC, prior to impact with the water.

A study by the Naval Research Laboratory, published in January, 1965, found that of four different designs of pilot altimeters, the 3p design was the one most prone to misreading by pilots. In fact, that extensive study revealed that the 3p design was misread almost 8 times more often, than the best designed, of the four altimeters tested. It was also noted that it took the pilots considerably longer, to decipher the correct reading of the 3p, than was the case with the other altimeters.

The plane was traveling at a speed of approximately 200 knots (230 mph), when it impacted the water. The investigation was hampered by the fact that neither the Cockpit Voice Recorder (CVR), nor the Flight Data Recorder (FDR) were recovered from the wreckage, which was in 250 foot deep, muddy water.

The first proven case of a crash, caused by a pilot misreading the altimeter by 10,000 feet, was of a BEA Vickers Viscount, on April 28, 1958, outside of Ayr Scotland. The second proven case, was the crash of a BOAC Britiannia 312, near Christ Church, England, on December 24, 1958. Both flights carried only flight crews and survivors helped to pinpoint the cause.

United continues to use the flight number today on a Tulsa-Denver-Oakland route.