Flight Background
The flight departed San Francisco at 1042, operating as United Trainer 7030. Captain Frederick S. Angstadt, the flight manager, served as pilot and was undergoing a semi-annual instrument check administered by Assistant Flight Manager Hugh C. Worthington, who acted as copilot. Flight Engineer Charles K. Brogden was the third crew member. The flight also aimed to evaluate the feasibility of using Oakland Municipal Airport for future crew training, as Captains Angstadt and Worthington were scheduled to supervise a new class of pilots and flight engineers that afternoon.
Prior to departure, Captain Angstadt reviewed the 0928 sequence weather reports. Local weather included stratus clouds with tops around 1,600 feet; San Francisco and Oakland reported an 800-foot overcast ceiling with visibility of three miles in haze and smoke, while Fresno and Sacramento had unlimited visibility. The forecast for the Bay area indicated scattered clouds by 1100 PST. Clearance was issued for local VFR flight within a 100-mile radius of San Francisco and below 10,000 feet. The aircraft's gross weight at takeoff was 114,886 pounds with 4,700 gallons of fuel, well under the maximum permissible limit.
Flight Events
After reporting on top of the broken scattered clouds to San Francisco tower at 1046, the flight proceeded to Oakland. The Oakland tower approved a simulated Instrument Landing System (ILS) approach, which was performed along with a missed approach procedure. The flight again reported on top at 1136. A second simulated ILS approach was requested but delayed due to other traffic, prompting the pilot to return to San Francisco. The flight switched from Oakland tower to San Francisco Approach Control at 1137.
At 1139, the flight was cleared for an ILS approach to San Francisco Airport but did not acknowledge on the 119.1 MHz frequency. The controller heard an unreadable call from the aircraft on 121.9 MHz, instructed the flight to listen on 119.1, and then directed it to hold VFR and stand by. The flight subsequently requested permission for a simulated ILS approach, which was granted with instructions to report upon leaving the outer marker inbound. This message and another were not acknowledged, and no emergency call was received. The crash was reported to the U.S. Coast Guard Air Station at 1046 by the manager of a nearby airport.
Accident and Investigation
The aircraft, with the No. 4 propeller feathered, stalled and abruptly dived from an altitude of approximately 300 feet, demolishing upon impact in San Francisco Bay. Engines 1, 2, and 3 were developing power at impact. The No. 4 propeller was feathered, with no evidence of structural failure or malfunction of that engine or propeller. The landing gear was extended, and wing flaps were down 10 degrees at impact. About 75 percent of the aircraft was recovered; no evidence of fire, fuel or electrical system malfunction, or structural failure in flight was found.
Probable Cause
The Board determined that the probable cause of the accident was an inadvertent stall at a low altitude from which recovery was not effected. The cause of the stall was not definitely determined.