Boeing 377-10-34 construction number 15970, original registration N31230, operated by United Airlines as Mainliner Oahu This was the first Model 377 to be lost. On 12 September 1951, it took off from San Francisco International Airport for crew training exercises, using Oakland Municipal Airport for simulated ILS approaches. Two such approaches were carried out, as well as a missed approach procedure, but when a third was delayed because of other traffic, the captain returned to San Francisco. After getting clearance for an ILS approach there, 15970 stalled at an altitude of 300 ft and plunged into San Francisco Bay. There were no survivors from the flight crew of three on board.