United Air Lines’ Boeing Stratocruiser “Mainliner Oahu” was being used for a semi-annual instrument check of a captain. In addition, consideration had been given to investigating the feasibility of using the Oakland, California, Municipal Airport for certain phases of crew training in this type aircraft for a new class of pilots and flight engineers which was to convene that afternoon. The flight departed at 09:42 and reported on top of the broken scattered clouds four minutes later. The flight proceeded to Oakland, where a simulated ILS approach was carried out, followed by a missed approach procedure. The flight again reported on top at 10:36. Another simulated ILS approach was requested of the Oakland tower but the flight was advised that there would be a delay due to other traffic. In view of this, the pilot decided to return to San Francisco. At 10:39, the flight was cleared for an ILS approach to the San Francisco Airport. The aircraft, with No. 4 propeller feathered, stalled and abruptly dived from an altitude of approximately 300 feet and was demolished upon impact in San Francisco Bay. PROBABLE CAUSE: “An inadvertent stall at a low altitude from which recovery was not effected.”